erl(1) User Commands erl(1)NAMEerl - The Erlang Emulator
DESCRIPTION
The erl program starts an Erlang runtime system. The exact details (for
example, whether erl is a script or a program and which other programs
it calls) are system-dependent.
Windows users probably wants to use the werl program instead, which
runs in its own window with scrollbars and supports command-line edit‐
ing. The erl program on Windows provides no line editing in its shell,
and on Windows 95 there is no way to scroll back to text which has
scrolled off the screen. The erl program must be used, however, in
pipelines or if you want to redirect standard input or output.
Note:
As of ERTS version 5.9 (OTP-R15B) the runtime system will by default
not bind schedulers to logical processors. For more information see
documentation of the +sbt system flag.
EXPORTSerl <arguments>
Starts an Erlang runtime system.
The arguments can be divided into emulator flags, flags and
plain arguments:
* Any argument starting with the character + is interpreted as
an emulator flag.
As indicated by the name, emulator flags controls the behav‐
ior of the emulator.
* Any argument starting with the character - (hyphen) is
interpreted as a flag which should be passed to the Erlang
part of the runtime system, more specifically to the init
system process, see init(3erl).
The init process itself interprets some of these flags, the
init flags. It also stores any remaining flags, the user
flags. The latter can be retrieved by calling init:get_argu‐
ment/1.
It can be noted that there are a small number of "-" flags
which now actually are emulator flags, see the description
below.
* Plain arguments are not interpreted in any way. They are
also stored by the init process and can be retrieved by
calling init:get_plain_arguments/0. Plain arguments can
occur before the first flag, or after a -- flag. Addition‐
ally, the flag -extra causes everything that follows to
become plain arguments.
Example:
% erl +W w -sname arnie +R 9 -s my_init -extra +bertie
(arnie@host)1> init:get_argument(sname).
{ok,[["arnie"]]}
(arnie@host)2> init:get_plain_arguments().
["+bertie"]
Here +W w and +R 9 are emulator flags. -s my_init is an init
flag, interpreted by init. -sname arnie is a user flag, stored
by init. It is read by Kernel and will cause the Erlang runtime
system to become distributed. Finally, everything after -extra
(that is, +bertie) is considered as plain arguments.
% erl-myflag 1
1> init:get_argument(myflag).
{ok,[["1"]]}
2> init:get_plain_arguments().
[]
Here the user flag -myflag 1 is passed to and stored by the init
process. It is a user defined flag, presumably used by some user
defined application.
FLAGS
In the following list, init flags are marked (init flag). Unless other‐
wise specified, all other flags are user flags, for which the values
can be retrieved by calling init:get_argument/1. Note that the list of
user flags is not exhaustive, there may be additional, application spe‐
cific flags which instead are documented in the corresponding applica‐
tion documentation.
--(init flag):
Everything following -- up to the next flag (-flag or +flag) is
considered plain arguments and can be retrieved using
init:get_plain_arguments/0.
-Application Par Val:
Sets the application configuration parameter Par to the value Val
for the application Application, see app(5) and application(3erl).
-args_file FileName:
Command line arguments are read from the file FileName. The argu‐
ments read from the file replace the '-args_file FileName' flag on
the resulting command line.
The file FileName should be a plain text file and may contain com‐
ments and command line arguments. A comment begins with a # charac‐
ter and continues until next end of line character. Backslash (\\)
is used as quoting character. All command line arguments accepted
by erl are allowed, also the -args_file FileName flag. Be careful
not to cause circular dependencies between files containing the
-args_file flag, though.
The -extra flag is treated specially. Its scope ends at the end of
the file. Arguments following an -extra flag are moved on the com‐
mand line into the -extra section, i.e. the end of the command line
following after an -extra flag.
-async_shell_start:
The initial Erlang shell does not read user input until the system
boot procedure has been completed (Erlang 5.4 and later). This flag
disables the start synchronization feature and lets the shell start
in parallel with the rest of the system.
-boot File:
Specifies the name of the boot file, File.boot, which is used to
start the system. See init(3erl). Unless File contains an absolute
path, the system searches for File.boot in the current and
$ROOT/bin directories.
Defaults to $ROOT/bin/start.boot.
-boot_var Var Dir:
If the boot script contains a path variable Var other than $ROOT,
this variable is expanded to Dir. Used when applications are
installed in another directory than $ROOT/lib, see sys‐
tools:make_script/1,2.
-code_path_cache:
Enables the code path cache of the code server, see code(3erl).
-compile Mod1 Mod2 ...:
Compiles the specified modules and then terminates (with non-zero
exit code if the compilation of some file did not succeed). Implies
-noinput. Not recommended - use erlc instead.
-config Config:
Specifies the name of a configuration file, Config.config, which is
used to configure applications. See app(5) and application(3erl).
-connect_all false:
If this flag is present, global will not maintain a fully connected
network of distributed Erlang nodes, and then global name registra‐
tion cannot be used. See global(3erl).
-cookie Cookie:
Obsolete flag without any effect and common misspelling for -set‐
cookie. Use -setcookie instead.
-detached:
Starts the Erlang runtime system detached from the system console.
Useful for running daemons and backgrounds processes. Implies
-noinput.
-emu_args:
Useful for debugging. Prints out the actual arguments sent to the
emulator.
-env Variable Value:
Sets the host OS environment variable Variable to the value Value
for the Erlang runtime system. Example:
% erl-env DISPLAY gin:0
In this example, an Erlang runtime system is started with the DIS‐
PLAY environment variable set to gin:0.
-eval Expr(init flag):
Makes init evaluate the expression Expr, see init(3erl).
-extra(init flag):
Everything following -extra is considered plain arguments and can
be retrieved using init:get_plain_arguments/0.
-heart:
Starts heart beat monitoring of the Erlang runtime system. See
heart(3erl).
-hidden:
Starts the Erlang runtime system as a hidden node, if it is run as
a distributed node. Hidden nodes always establish hidden connec‐
tions to all other nodes except for nodes in the same global group.
Hidden connections are not published on either of the connected
nodes, i.e. neither of the connected nodes are part of the result
from nodes/0 on the other node. See also hidden global groups,
global_group(3erl).
-hosts Hosts:
Specifies the IP addresses for the hosts on which Erlang boot
servers are running, see erl_boot_server(3erl). This flag is manda‐
tory if the -loader inet flag is present.
The IP addresses must be given in the standard form (four decimal
numbers separated by periods, for example "150.236.20.74". Hosts
names are not acceptable, but a broadcast address (preferably lim‐
ited to the local network) is.
-id Id:
Specifies the identity of the Erlang runtime system. If it is run
as a distributed node, Id must be identical to the name supplied
together with the -sname or -name flag.
-init_debug:
Makes init write some debug information while interpreting the boot
script.
-instr(emulator flag):
Selects an instrumented Erlang runtime system (virtual machine) to
run, instead of the ordinary one. When running an instrumented run‐
time system, some resource usage data can be obtained and analysed
using the module instrument. Functionally, it behaves exactly like
an ordinary Erlang runtime system.
-loader Loader:
Specifies the method used by erl_prim_loader to load Erlang modules
into the system. See erl_prim_loader(3erl). Two Loader methods are
supported, efile and inet. efile means use the local file system,
this is the default. inet means use a boot server on another
machine, and the -id, -hosts and -setcookie flags must be specified
as well. If Loader is something else, the user supplied Loader port
program is started.
-make:
Makes the Erlang runtime system invoke make:all() in the current
working directory and then terminate. See make(3erl). Implies
-noinput.
-man Module:
Displays the manual page for the Erlang module Module. Only sup‐
ported on Unix.
-mode interactive | embedded:
Indicates if the system should load code dynamically (interactive),
or if all code should be loaded during system initialization
(embedded), see code(3erl). Defaults to interactive.
-name Name:
Makes the Erlang runtime system into a distributed node. This flag
invokes all network servers necessary for a node to become distrib‐
uted. See net_kernel(3erl). It is also ensured that epmd runs on
the current host before Erlang is started. See epmd(1).
The name of the node will be Name@Host, where Host is the fully
qualified host name of the current host. For short names, use the
-sname flag instead.
-noinput:
Ensures that the Erlang runtime system never tries to read any
input. Implies -noshell.
-noshell:
Starts an Erlang runtime system with no shell. This flag makes it
possible to have the Erlang runtime system as a component in a
series of UNIX pipes.
-nostick:
Disables the sticky directory facility of the Erlang code server,
see code(3erl).
-oldshell:
Invokes the old Erlang shell from Erlang 3.3. The old shell can
still be used.
-pa Dir1 Dir2 ...:
Adds the specified directories to the beginning of the code path,
similar to code:add_pathsa/1. See code(3erl). As an alternative to
-pa, if several directories are to be prepended to the code and the
directories have a common parent directory, that parent directory
could be specified in the ERL_LIBS environment variable. See
code(3erl).
-pz Dir1 Dir2 ...:
Adds the specified directories to the end of the code path, similar
to code:add_pathsz/1. See code(3erl).
-remsh Node:
Starts Erlang with a remote shell connected to Node.
-rsh Program:
Specifies an alternative to rsh for starting a slave node on a
remote host. See slave(3erl).
-run Mod [Func [Arg1, Arg2, ...]](init flag):
Makes init call the specified function. Func defaults to start. If
no arguments are provided, the function is assumed to be of arity
0. Otherwise it is assumed to be of arity 1, taking the list
[Arg1,Arg2,...] as argument. All arguments are passed as strings.
See init(3erl).
-s Mod [Func [Arg1, Arg2, ...]](init flag):
Makes init call the specified function. Func defaults to start. If
no arguments are provided, the function is assumed to be of arity
0. Otherwise it is assumed to be of arity 1, taking the list
[Arg1,Arg2,...] as argument. All arguments are passed as atoms. See
init(3erl).
-setcookie Cookie:
Sets the magic cookie of the node to Cookie, see
erlang:set_cookie/2.
-shutdown_time Time:
Specifies how long time (in milliseconds) the init process is
allowed to spend shutting down the system. If Time ms have elapsed,
all processes still existing are killed. Defaults to infinity.
-sname Name:
Makes the Erlang runtime system into a distributed node, similar to
-name, but the host name portion of the node name Name@Host will be
the short name, not fully qualified.
This is sometimes the only way to run distributed Erlang if the DNS
(Domain Name System) is not running. There can be no communication
between nodes running with the -sname flag and those running with
the -name flag, as node names must be unique in distributed Erlang
systems.
-smp [enable|auto|disable]:
-smp enable and -smp starts the Erlang runtime system with SMP sup‐
port enabled. This may fail if no runtime system with SMP support
is available. -smp auto starts the Erlang runtime system with SMP
support enabled if it is available and more than one logical pro‐
cessor are detected. -smp disable starts a runtime system without
SMP support.
NOTE: The runtime system with SMP support will not be available on
all supported platforms. See also the +S flag.
-version(emulator flag):
Makes the emulator print out its version number. The same as erl
+V.
EMULATOR FLAGSerl invokes the code for the Erlang emulator (virtual machine), which
supports the following flags:
+a size:
Suggested stack size, in kilowords, for threads in the async-thread
pool. Valid range is 16-8192 kilowords. The default suggested stack
size is 16 kilowords, i.e, 64 kilobyte on 32-bit architectures.
This small default size has been chosen since the amount of async-
threads might be quite large. The default size is enough for driv‐
ers delivered with Erlang/OTP, but might not be sufficiently large
for other dynamically linked in drivers that use the driver_async()
functionality. Note that the value passed is only a suggestion, and
it might even be ignored on some platforms.
+A size:
Sets the number of threads in async thread pool, valid range is
0-1024. If thread support is available, the default is 10.
+B [c | d | i]:
The c option makes Ctrl-C interrupt the current shell instead of
invoking the emulator break handler. The d option (same as specify‐
ing +B without an extra option) disables the break handler. The i
option makes the emulator ignore any break signal.
If the c option is used with oldshell on Unix, Ctrl-C will restart
the shell process rather than interrupt it.
Note that on Windows, this flag is only applicable for werl, not
erl (oldshell). Note also that Ctrl-Break is used instead of Ctrl-C
on Windows.
+c:
Disable compensation for sudden changes of system time.
Normally, erlang:now/0 will not immediately reflect sudden changes
in the system time, in order to keep timers (including receive-
after) working. Instead, the time maintained by erlang:now/0 is
slowly adjusted towards the new system time. (Slowly means in one
percent adjustments; if the time is off by one minute, the time
will be adjusted in 100 minutes.)
When the +c option is given, this slow adjustment will not take
place. Instead erlang:now/0 will always reflect the current system
time. Note that timers are based on erlang:now/0. If the system
time jumps, timers then time out at the wrong time.
NOTE: You can check whether the adjustment is enabled or disabled
by calling erlang:system_info(tolerant_timeofday).
+d:
If the emulator detects an internal error (or runs out of memory),
it will by default generate both a crash dump and a core dump. The
core dump will, however, not be very useful since the content of
process heaps is destroyed by the crash dump generation.
The +d option instructs the emulator to only produce a core dump
and no crash dump if an internal error is detected.
Calling erlang:halt/1 with a string argument will still produce a
crash dump. On Unix systems, sending an emulator process a SIGUSR1
signal will also force a crash dump.
+e Number:
Set max number of ETS tables.
+ec:
Force the compressed option on all ETS tables. Only intended for
test and evaluation.
+fnl:
The VM works with file names as if they are encoded using the ISO-
latin-1 encoding, disallowing Unicode characters with codepoints
beyond 255.
See STDLIB User's Guide for more infomation about unicode file
names. Note that this value also applies to command-line parameters
and environment variables (see STDLIB User's Guide).
+fnu[{w|i|e}]:
The VM works with file names as if they are encoded using UTF-8 (or
some other system specific Unicode encoding). This is the default
on operating systems that enforce Unicode encoding, i.e. Windows
and MacOS X.
The +fnu switch can be followed by w, i, or e to control the way
wrongly encoded file names are to be reported. w means that a warn‐
ing is sent to the error_logger whenever a wrongly encoded file
name is "skipped" in directory listings, i means that those wrongly
encoded file names are silently ignored and e means that the API
function will return an error whenever a wrongly encoded file (or
directory) name is encountered. w is the default. Note that
file:read_link/1 will always return an error if the link points to
an invalid file name.
See STDLIB User's Guide for more infomation about unicode file
names. Note that this value also applies to command-line parameters
and environment variables (see STDLIB User's Guide).
+fna[{w|i|e}]:
Selection between +fnl and +fnu is done based on the current locale
settings in the OS, meaning that if you have set your terminal for
UTF-8 encoding, the filesystem is expected to use the same encoding
for file names. This is default on all operating systems except
MacOS X and Windows.
The +fna switch can be followed by w, i, or e. This will have
effect if the locale settings cause the behavior of +fnu to be
selected. See the description of +fnu above. If the locale settings
cause the behavior of +fnl to be selected, then w, i, or e will not
have any effect.
See STDLIB User's Guide for more infomation about unicode file
names. Note that this value also applies to command-line parameters
and environment variables (see STDLIB User's Guide).
+hms Size:
Sets the default heap size of processes to the size Size.
+hmbs Size:
Sets the default binary virtual heap size of processes to the size
Size.
+K true | false:
Enables or disables the kernel poll functionality if the emulator
supports it. Default is false (disabled). If the emulator does not
support kernel poll, and the +K flag is passed to the emulator, a
warning is issued at startup.
+l:
Enables auto load tracing, displaying info while loading code.
+L:
Don't load information about source file names and line numbers.
This will save some memory, but exceptions will not contain infor‐
mation about the file names and line numbers.
+MFlag Value:
Memory allocator specific flags, see erts_alloc(3erl) for further
information.
+n Behavior:
Control behavior of signals to ports.
As of OTP-R16 signals to ports are truly asynchronously delivered.
Note that signals always have been documented as asynchronous. The
underlying implementation has, however, previously delivered these
signals synchronously. Correctly written Erlang programs should be
able to handle this without any issues. Bugs in existing Erlang
programs that make false assumptions about signals to ports may,
however, be tricky to find. This switch has been introduced in
order to at least make it easier to compare behaviors during a
transition period. Note that this flag is deprecated as of its
introduction, and is scheduled for removal in OTP-R17. Behavior
should be one of the following characters:
d:
The default. Asynchronous signals. A process that sends a signal
to a port may continue execution before the signal has been
delivered to the port.
s:
Synchronous signals. A processes that sends a signal to a port
will not continue execution until the signal has been delivered.
Should only be used for testing and debugging.
a:
Asynchronous signals. As the default, but a processes that sends
a signal will even more frequently continue execution before the
signal has been delivered to the port. Should only be used for
testing and debugging.
+pc Range:
Sets the range of characters that the system will consider print‐
able in heuristic detection of strings. This typically affects the
shell, debugger and io:format functions (when ~tp is used in the
format string).
Currently two values for the Range are supported:
latin1:
The default. Only characters in the ISO-latin-1 range can be
considered printable, which means that a character with a code
point > 255 will never be considered printable and that lists
containing such characters will be displayed as lists of inte‐
gers rather than text strings by tools.
unicode:
All printable Unicode characters are considered when determin‐
ing if a list of integers is to be displayed in string syntax.
This may give unexpected results if for example your font does
not cover all Unicode characters.
Se also io:printable_range/0.
+P Number|legacy:
Sets the maximum number of simultaneously existing processes for
this system if a Number is passed as value. Valid range for Number
is [1024-134217727]
NOTE: The actual maximum chosen may be much larger than the Number
passed. Currently the runtime system often, but not always, chooses
a value that is a power of 2. This might, however, be changed in
the future. The actual value chosen can be checked by calling
erlang:system_info(process_limit).
The default value is 262144
If legacy is passed as value, the legacy algorithm for allocation
of process identifiers will be used. Using the legacy algorithm,
identifiers will be allocated in a strictly increasing fashion
until largest possible identifier has been reached. Note that this
algorithm suffers from performance issues and can under certain
circumstances be extremely expensive. The legacy algoritm is depre‐
cated, and the legacy option is scheduled for removal in OTP-R18.
+Q Number|legacy:
Sets the maximum number of simultaneously existing ports for this
system if a Number is passed as value. Valid range for Number is
[1024-134217727]
NOTE: The actual maximum chosen may be much larger than the actual
Number passed. Currently the runtime system often, but not always,
chooses a value that is a power of 2. This might, however, be
changed in the future. The actual value chosen can be checked by
calling erlang:system_info(port_limit).
The default value used is normally 65536. However, if the runtime
system is able to determine maximum amount of file descriptors that
it is allowed to open and this value is larger than 65536, the cho‐
sen value will increased to a value larger or equal to the maximum
amount of file descriptors that can be opened.
On Windows the default value is set to 8196 because the normal OS
limitations are set higher than most machines can handle.
Previously the environment variable ERL_MAX_PORTS was used for set‐
ting the maximum number of simultaneously existing ports. This
environment variable is deprecated, and scheduled for removal in
OTP-R17, but can still be used.
If legacy is passed as value, the legacy algorithm for allocation
of port identifiers will be used. Using the legacy algorithm, iden‐
tifiers will be allocated in a strictly increasing fashion until
largest possible identifier has been reached. Note that this algo‐
rithm suffers from performance issues and can under certain circum‐
stances be extremely expensive. The legacy algoritm is deprecated,
and the legacy option is scheduled for removal in OTP-R18.
+R ReleaseNumber:
Sets the compatibility mode.
The distribution mechanism is not backwards compatible by default.
This flags sets the emulator in compatibility mode with an earlier
Erlang/OTP release ReleaseNumber. The release number must be in the
range <current release>-2..<current release>. This limits the emu‐
lator, making it possible for it to communicate with Erlang nodes
(as well as C- and Java nodes) running that earlier release.
Note: Make sure all nodes (Erlang-, C-, and Java nodes) of a dis‐
tributed Erlang system is of the same Erlang/OTP release, or from
two different Erlang/OTP releases X and Y, where all Y nodes have
compatibility mode X.
+r:
Force ets memory block to be moved on realloc.
+rg ReaderGroupsLimit:
Limits the amount of reader groups used by read/write locks opti‐
mized for read operations in the Erlang runtime system. By default
the reader groups limit equals 64.
When the amount of schedulers is less than or equal to the reader
groups limit, each scheduler has its own reader group. When the
amount of schedulers is larger than the reader groups limit, sched‐
ulers share reader groups. Shared reader groups degrades read lock
and read unlock performance while a large amount of reader groups
degrades write lock performance, so the limit is a tradeoff between
performance for read operations and performance for write opera‐
tions. Each reader group currently consumes 64 byte in each
read/write lock. Also note that a runtime system using shared
reader groups benefits from binding schedulers to logical proces‐
sors, since the reader groups are distributed better between sched‐
ulers.
+S Schedulers:SchedulerOnline:
Sets the number of scheduler threads to create and scheduler
threads to set online when SMP support has been enabled. The maxi‐
mum for both values is 1024. If the Erlang runtime system is able
to determine the amount of logical processors configured and logi‐
cal processors available, Schedulers will default to logical pro‐
cessors configured, and SchedulersOnline will default to logical
processors available; otherwise, the default values will be 1.
Schedulers may be omitted if :SchedulerOnline is not and vice
versa. The number of schedulers online can be changed at run time
via erlang:system_flag(schedulers_online, SchedulersOnline).
If Schedulers or SchedulersOnline is specified as a negative num‐
ber, the value is subtracted from the default number of logical
processors configured or logical processors available, respec‐
tively.
Specifying the value 0 for Schedulers or SchedulersOnline resets
the number of scheduler threads or scheduler threads online respec‐
tively to its default value.
This option is ignored if the emulator doesn't have SMP support
enabled (see the -smp flag).
+SP SchedulersPercentage:SchedulersOnlinePercentage:
Similar to +S but uses percentages to set the number of scheduler
threads to create, based on logical processors configured, and
scheduler threads to set online, based on logical processors avail‐
able, when SMP support has been enabled. Specified values must be
greater than 0. For example, +SP 50:25 sets the number of scheduler
threads to 50% of the logical processors configured and the number
of scheduler threads online to 25% of the logical processors avail‐
able. SchedulersPercentage may be omitted if :SchedulersOnlinePer‐
centage is not and vice versa. The number of schedulers online can
be changed at run time via erlang:system_flag(schedulers_online,
SchedulersOnline).
This option interacts with +S settings. For example, on a system
with 8 logical cores configured and 8 logical cores available, the
combination of the options +S 4:4 +SP 50:25 (in either order)
results in 2 scheduler threads (50% of 4) and 1 scheduler thread
online (25% of 4).
This option is ignored if the emulator doesn't have SMP support
enabled (see the -smp flag).
+SDcpu DirtyCPUSchedulers:DirtyCPUSchedulersOnline:
Sets the number of dirty CPU scheduler threads to create and dirty
CPU scheduler threads to set online when threading support has been
enabled. The maximum for both values is 1024, and each value is
further limited by the settings for normal schedulers: the number
of dirty CPU scheduler threads created cannot exceed the number of
normal scheduler threads created, and the number of dirty CPU
scheduler threads online cannot exceed the number of normal sched‐
uler threads online (see the +S and +SP flags for more details). By
default, the number of dirty CPU scheduler threads created equals
the number of normal scheduler threads created, and the number of
dirty CPU scheduler threads online equals the number of normal
scheduler threads online. DirtyCPUSchedulers may be omitted if
:DirtyCPUSchedulersOnline is not and vice versa. The number of
dirty CPU schedulers online can be changed at run time via
erlang:system_flag(dirty_cpu_schedulers_online, DirtyCPUScheduler‐
sOnline).
This option is ignored if the emulator doesn't have threading sup‐
port enabled. Currently, this option is experimental and is sup‐
ported only if the emulator was configured and built with support
for dirty schedulers enabled (it's disabled by default).
+SDPcpu DirtyCPUSchedulersPercentage:DirtyCPUSchedulersOnlinePercent‐
age:
Similar to +SDcpu but uses percentages to set the number of dirty
CPU scheduler threads to create and number of dirty CPU scheduler
threads to set online when threading support has been enabled.
Specified values must be greater than 0. For example, +SDPcpu 50:25
sets the number of dirty CPU scheduler threads to 50% of the logi‐
cal processors configured and the number of dirty CPU scheduler
threads online to 25% of the logical processors available. DirtyC‐
PUSchedulersPercentage may be omitted if :DirtyCPUSchedulersOn‐
linePercentage is not and vice versa. The number of dirty CPU
schedulers online can be changed at run time via erlang:sys‐
tem_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).
This option interacts with +SDcpu settings. For example, on a sys‐
tem with 8 logical cores configured and 8 logical cores available,
the combination of the options +SDcpu 4:4 +SDPcpu 50:25 (in either
order) results in 2 dirty CPU scheduler threads (50% of 4) and 1
dirty CPU scheduler thread online (25% of 4).
This option is ignored if the emulator doesn't have threading sup‐
port enabled. Currently, this option is experimental and is sup‐
ported only if the emulator was configured and built with support
for dirty schedulers enabled (it's disabled by default).
+SDio IOSchedulers:
Sets the number of dirty I/O scheduler threads to create when
threading support has been enabled. The valid range is 0-1024. By
default, the number of dirty I/O scheduler threads created is 10,
same as the default number of threads in the async thread pool .
This option is ignored if the emulator doesn't have threading sup‐
port enabled. Currently, this option is experimental and is sup‐
ported only if the emulator was configured and built with support
for dirty schedulers enabled (it's disabled by default).
+sFlag Value:
Scheduling specific flags.
+sbt BindType:
Set scheduler bind type.
Schedulers can also be bound using the +stbt flag. The only dif‐
ference between these two flags is how the following errors are
handled:
* Binding of schedulers is not supported on the specific plat‐
form.
* No available CPU topology. That is the runtime system was not
able to automatically detected the CPU topology, and no user
defined CPU topology was set.
If any of these errors occur when +sbt has been passed, the run‐
time system will print an error message, and refuse to start. If
any of these errors occur when +stbt has been passed, the runtime
system will silently ignore the error, and start up using unbound
schedulers.
Currently valid BindTypes:
u:
unbound - Schedulers will not be bound to logical processors,
i.e., the operating system decides where the scheduler threads
execute, and when to migrate them. This is the default.
ns:
no_spread - Schedulers with close scheduler identifiers will be
bound as close as possible in hardware.
ts:
thread_spread - Thread refers to hardware threads (e.g. Intel's
hyper-threads). Schedulers with low scheduler identifiers, will
be bound to the first hardware thread of each core, then sched‐
ulers with higher scheduler identifiers will be bound to the
second hardware thread of each core, etc.
ps:
processor_spread - Schedulers will be spread like
thread_spread, but also over physical processor chips.
s:
spread - Schedulers will be spread as much as possible.
nnts:
no_node_thread_spread - Like thread_spread, but if multiple
NUMA (Non-Uniform Memory Access) nodes exists, schedulers will
be spread over one NUMA node at a time, i.e., all logical pro‐
cessors of one NUMA node will be bound to schedulers in
sequence.
nnps:
no_node_processor_spread - Like processor_spread, but if multi‐
ple NUMA nodes exists, schedulers will be spread over one NUMA
node at a time, i.e., all logical processors of one NUMA node
will be bound to schedulers in sequence.
tnnps:
thread_no_node_processor_spread - A combination of
thread_spread, and no_node_processor_spread. Schedulers will be
spread over hardware threads across NUMA nodes, but schedulers
will only be spread over processors internally in one NUMA node
at a time.
db:
default_bind - Binds schedulers the default way. Currently the
default is thread_no_node_processor_spread (which might change
in the future).
Binding of schedulers is currently only supported on newer Linux,
Solaris, FreeBSD, and Windows systems.
If no CPU topology is available when the +sbt flag is processed
and BindType is any other type than u, the runtime system will
fail to start. CPU topology can be defined using the +sct flag.
Note that the +sct flag may have to be passed before the +sbt
flag on the command line (in case no CPU topology has been auto‐
matically detected).
The runtime system will by default not bind schedulers to logical
processors.
NOTE: If the Erlang runtime system is the only operating system
process that binds threads to logical processors, this improves
the performance of the runtime system. However, if other operat‐
ing system processes (as for example another Erlang runtime sys‐
tem) also bind threads to logical processors, there might be a
performance penalty instead. In some cases this performance
penalty might be severe. If this is the case, you are advised to
not bind the schedulers.
How schedulers are bound matters. For example, in situations when
there are fewer running processes than schedulers online, the
runtime system tries to migrate processes to schedulers with low
scheduler identifiers. The more the schedulers are spread over
the hardware, the more resources will be available to the runtime
system in such situations.
NOTE: If a scheduler fails to bind, this will often be silently
ignored. This since it isn't always possible to verify valid log‐
ical processor identifiers. If an error is reported, it will be
reported to the error_logger. If you want to verify that the
schedulers actually have bound as requested, call erlang:sys‐
tem_info(scheduler_bindings).
+sbwt none|very_short|short|medium|long|very_long:
Set scheduler busy wait threshold. Default is medium. The thresh‐
old determines how long schedulers should busy wait when running
out of work before going to sleep.
NOTE: This flag may be removed or changed at any time without
prior notice.
+scl true|false:
Enable or disable scheduler compaction of load. By default sched‐
uler compaction of load is enabled. When enabled, load balancing
will strive for a load distribution which causes as many sched‐
uler threads as possible to be fully loaded (i.e., not run out of
work). This is accomplished by migrating load (e.g. runnable pro‐
cesses) into a smaller set of schedulers when schedulers fre‐
quently run out of work. When disabled, the frequency with which
schedulers run out of work will not be taken into account by the
load balancing logic.
+scl false is similar to +sub true with the difference that +sub
true also will balance scheduler utilization between schedulers.
+sct CpuTopology:
* <Id> = integer(); when 0 =< <Id> =< 65535
* <IdRange> = <Id>-<Id>
* <IdOrIdRange> = <Id> | <IdRange>
* <IdList> = <IdOrIdRange>,<IdOrIdRange> | <IdOrIdRange>
* <LogicalIds> = L<IdList>
* <ThreadIds> = T<IdList> | t<IdList>
* <CoreIds> = C<IdList> | c<IdList>
* <ProcessorIds> = P<IdList> | p<IdList>
* <NodeIds> = N<IdList> | n<IdList>
* <IdDefs> = <LogicalIds><ThreadIds><CoreIds><Proces‐
sorIds><NodeIds> | <LogicalIds><ThreadIds><Cor‐
eIds><NodeIds><ProcessorIds>
* CpuTopology = <IdDefs>:<IdDefs> | <IdDefs>
Set a user defined CPU topology. The user defined CPU topology
will override any automatically detected CPU topology. The CPU
topology is used when binding schedulers to logical processors.
Upper-case letters signify real identifiers and lower-case let‐
ters signify fake identifiers only used for description of the
topology. Identifiers passed as real identifiers may be used by
the runtime system when trying to access specific hardware and if
they are not correct the behavior is undefined. Faked logical CPU
identifiers are not accepted since there is no point in defining
the CPU topology without real logical CPU identifiers. Thread,
core, processor, and node identifiers may be left out. If left
out, thread id defaults to t0, core id defaults to c0, processor
id defaults to p0, and node id will be left undefined. Either
each logical processor must belong to one and only one NUMA node,
or no logical processors must belong to any NUMA nodes.
Both increasing and decreasing <IdRange>s are allowed.
NUMA node identifiers are system wide. That is, each NUMA node on
the system have to have a unique identifier. Processor identi‐
fiers are also system wide. Core identifiers are processor wide.
Thread identifiers are core wide.
The order of the identifier types imply the hierarchy of the CPU
topology. Valid orders are either <LogicalIds><ThreadIds><Cor‐
eIds><ProcessorIds><NodeIds>, or <LogicalIds><ThreadIds><Cor‐
eIds><NodeIds><ProcessorIds>. That is, thread is part of a core
which is part of a processor which is part of a NUMA node, or
thread is part of a core which is part of a NUMA node which is
part of a processor. A cpu topology can consist of both processor
external, and processor internal NUMA nodes as long as each logi‐
cal processor belongs to one and only one NUMA node. If <Proces‐
sorIds> is left out, its default position will be before
<NodeIds>. That is, the default is processor external NUMA nodes.
If a list of identifiers is used in an <IdDefs>:
* <LogicalIds> have to be a list of identifiers.
* At least one other identifier type apart from <LogicalIds> also
have to have a list of identifiers.
* All lists of identifiers have to produce the same amount of
identifiers.
A simple example. A single quad core processor may be described
this way:
% erl +sct L0-3c0-3
1> erlang:system_info(cpu_topology).
[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,2}},
{core,{logical,3}}]}]
A little more complicated example. Two quad core processors. Each
processor in its own NUMA node. The ordering of logical proces‐
sors is a little weird. This in order to give a better example of
identifier lists:
% erl +sct L0-1,3-2c0-3p0N0:L7,4,6-5c0-3p1N1
1> erlang:system_info(cpu_topology).
[{node,[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,3}},
{core,{logical,2}}]}]},
{node,[{processor,[{core,{logical,7}},
{core,{logical,4}},
{core,{logical,6}},
{core,{logical,5}}]}]}]
As long as real identifiers are correct it is okay to pass a CPU
topology that is not a correct description of the CPU topology.
When used with care this can actually be very useful. This in
order to trick the emulator to bind its schedulers as you want.
For example, if you want to run multiple Erlang runtime systems
on the same machine, you want to reduce the amount of schedulers
used and manipulate the CPU topology so that they bind to differ‐
ent logical CPUs. An example, with two Erlang runtime systems on
a quad core machine:
% erl +sct L0-3c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname one
% erl +sct L3-0c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname two
In this example each runtime system have two schedulers each
online, and all schedulers online will run on different cores. If
we change to one scheduler online on one runtime system, and
three schedulers online on the other, all schedulers online will
still run on different cores.
Note that a faked CPU topology that does not reflect how the real
CPU topology looks like is likely to decrease the performance of
the runtime system.
For more information, see erlang:system_info(cpu_topology).
+secio true|false:
Enable or disable eager check I/O scheduling. The default is cur‐
rently false, but will most likely be changed to true in OTP 18.
The behaviour before this flag was introduced corresponds to
+secio false.
The flag effects when schedulers will check for I/O operations
possible to execute, and when such I/O operations will execute.
As the name of the parameter implies, schedulers will be more
eager to check for I/O when true is passed. This however also
implies that execution of outstanding I/O operation will not be
prioritized to the same extent as when false is passed.
erlang:system_info(eager_check_io) returns the value of this
parameter used when starting the VM.
+sfwi Interval:
Set scheduler forced wakeup interval. All run queues will be
scanned each Interval milliseconds. While there are sleeping
schedulers in the system, one scheduler will be woken for each
non-empty run queue found. An Interval of zero disables this fea‐
ture, which also is the default.
This feature has been introduced as a temporary workaround for
lengthy executing native code, and native code that do not bump
reductions properly in OTP. When these bugs have be fixed the
+sfwi flag will be removed.
+stbt BindType:
Try to set scheduler bind type. The same as the +sbt flag with
the exception of how some errors are handled. For more informa‐
tion, see the documentation of the +sbt flag.
+sub true|false:
Enable or disable scheduler utilization balancing of load. By
default scheduler utilization balancing is disabled and instead
scheduler compaction of load is enabled which will strive for a
load distribution which causes as many scheduler threads as pos‐
sible to be fully loaded (i.e., not run out of work). When sched‐
uler utilization balancing is enabled the system will instead try
to balance scheduler utilization between schedulers. That is,
strive for equal scheduler utilization on all schedulers.
+sub true is only supported on systems where the runtime system
detects and use a monotonically increasing high resolution clock.
On other systems, the runtime system will fail to start.
+sub true implies +scl false. The difference between +sub true
and +scl false is that +scl false will not try to balance the
scheduler utilization.
+swct very_eager|eager|medium|lazy|very_lazy:
Set scheduler wake cleanup threshold. Default is medium. This
flag controls how eager schedulers should be requesting wake up
due to certain cleanup operations. When a lazy setting is used,
more outstanding cleanup operations can be left undone while a
scheduler is idling. When an eager setting is used, schedulers
will more frequently be woken, potentially increasing CPU-uti‐
lization.
NOTE: This flag may be removed or changed at any time without
prior notice.
+sws default|legacy:
Set scheduler wakeup strategy. Default strategy changed in
erts-5.10/OTP-R16A. This strategy was previously known as pro‐
posal in OTP-R15. The legacy strategy was used as default from
R13 up to and including R15.
NOTE: This flag may be removed or changed at any time without
prior notice.
+swt very_low|low|medium|high|very_high:
Set scheduler wakeup threshold. Default is medium. The threshold
determines when to wake up sleeping schedulers when more work
than can be handled by currently awake schedulers exist. A low
threshold will cause earlier wakeups, and a high threshold will
cause later wakeups. Early wakeups will distribute work over mul‐
tiple schedulers faster, but work will more easily bounce between
schedulers.
NOTE: This flag may be removed or changed at any time without
prior notice.
+spp Bool:
Set default scheduler hint for port parallelism. If set to true,
the VM will schedule port tasks when doing so will improve paral‐
lelism in the system. If set to false, the VM will try to perform
port tasks immediately, improving latency at the expense of par‐
allelism. If this flag has not been passed, the default scheduler
hint for port parallelism is currently false. The default used
can be inspected in runtime by calling erlang:sys‐
tem_info(port_parallelism). The default can be overriden on port
creation by passing the parallelism option to open_port/2.
+sss size:
Suggested stack size, in kilowords, for scheduler threads. Valid
range is 4-8192 kilowords. The default stack size is OS depen‐
dent.
+t size:
Set the maximum number of atoms the VM can handle. Default is
1048576.
+T Level:
Enables modified timing and sets the modified timing level. Cur‐
rently valid range is 0-9. The timing of the runtime system will
change. A high level usually means a greater change than a low
level. Changing the timing can be very useful for finding timing
related bugs.
Currently, modified timing affects the following:
Process spawning:
A process calling spawn, spawn_link, spawn_monitor, or spawn_opt
will be scheduled out immediately after completing the call. When
higher modified timing levels are used, the caller will also
sleep for a while after being scheduled out.
Context reductions:
The amount of reductions a process is a allowed to use before
being scheduled out is increased or reduced.
Input reductions:
The amount of reductions performed before checking I/O is
increased or reduced.
NOTE: Performance will suffer when modified timing is enabled. This
flag is only intended for testing and debugging. Also note that
return_to and return_from trace messages will be lost when tracing
on the spawn BIFs. This flag may be removed or changed at any time
without prior notice.
+V:
Makes the emulator print out its version number.
+v:
Verbose.
+W w | i:
Sets the mapping of warning messages for error_logger. Messages
sent to the error logger using one of the warning routines can be
mapped either to errors (default), warnings (+W w), or info reports
(+W i). The current mapping can be retrieved using error_log‐
ger:warning_map/0. See error_logger(3erl) for further information.
+zFlag Value:
Miscellaneous flags.
+zdbbl size:
Set the distribution buffer busy limit (dist_buf_busy_limit) in
kilobytes. Valid range is 1-2097151. Default is 1024.
A larger buffer limit will allow processes to buffer more outgo‐
ing messages over the distribution. When the buffer limit has
been reached, sending processes will be suspended until the buf‐
fer size has shrunk. The buffer limit is per distribution chan‐
nel. A higher limit will give lower latency and higher throughput
at the expense of higher memory usage.
ENVIRONMENT VARIABLES
ERL_CRASH_DUMP:
If the emulator needs to write a crash dump, the value of this
variable will be the file name of the crash dump file. If the vari‐
able is not set, the name of the crash dump file will be
erl_crash.dump in the current directory.
ERL_CRASH_DUMP_NICE:
Unix systems: If the emulator needs to write a crash dump, it will
use the value of this variable to set the nice value for the
process, thus lowering its priority. The allowable range is 1
through 39 (higher values will be replaced with 39). The highest
value, 39, will give the process the lowest priority.
ERL_CRASH_DUMP_SECONDS:
Unix systems: This variable gives the number of seconds that the
emulator will be allowed to spend writing a crash dump. When the
given number of seconds have elapsed, the emulator will be termi‐
nated by a SIGALRM signal.
If the environment variable is not set or it is set to zero sec‐
onds, ERL_CRASH_DUMP_SECONDS=0, the runtime system will not even
attempt to write the crash dump file. It will just terminate.
If the environment variable is set to negative valie, e.g.
ERL_CRASH_DUMP_SECONDS=-1, the runtime system will wait indefi‐
nitely for the crash dump file to be written.
This environment variable is used in conjuction with heart if heart
is running:
ERL_CRASH_DUMP_SECONDS=0:
Suppresses the writing a crash dump file entirely, thus rebooting
the runtime system immediately. This is the same as not setting
the environment variable.
ERL_CRASH_DUMP_SECONDS=-1:
Setting the environment variable to a negative value will cause
the termination of the runtime system to wait until the crash
dump file has been completly written.
ERL_CRASH_DUMP_SECONDS=S:
Will wait for S seconds to complete the crash dump file and then
terminate the runtime system.
ERL_AFLAGS:
The content of this environment variable will be added to the
beginning of the command line for erl.
The -extra flag is treated specially. Its scope ends at the end of
the environment variable content. Arguments following an -extra
flag are moved on the command line into the -extra section, i.e.
the end of the command line following after an -extra flag.
ERL_ZFLAGS and ERL_FLAGS:
The content of these environment variables will be added to the end
of the command line for erl.
The -extra flag is treated specially. Its scope ends at the end of
the environment variable content. Arguments following an -extra
flag are moved on the command line into the -extra section, i.e.
the end of the command line following after an -extra flag.
ERL_LIBS:
This environment variable contains a list of additional library
directories that the code server will search for applications and
add to the code path. See code(3erl).
ERL_EPMD_ADDRESS:
This environment variable may be set to a comma-separated list of
IP addresses, in which case the epmd daemon will listen only on the
specified address(es) and on the loopback address (which is implic‐
itly added to the list if it has not been specified).
ERL_EPMD_PORT:
This environment variable can contain the port number to use when
communicating with epmd. The default port will work fine in most
cases. A different port can be specified to allow nodes of indepen‐
dent clusters to co-exist on the same host. All nodes in a cluster
must use the same epmd port number.
CONFIGURATION
The standard Erlang/OTP system can be re-configured to change the
default behavior on start-up.
The .erlang Start-up File:
When Erlang/OTP is started, the system searches for a file named
.erlang in the directory where Erlang/OTP is started. If not found,
the user's home directory is searched for an .erlang file.
If an .erlang file is found, it is assumed to contain valid Erlang
expressions. These expressions are evaluated as if they were input
to the shell.
A typical .erlang file contains a set of search paths, for example:
io:format("executing user profile in HOME/.erlang\n",[]).
code:add_path("/home/calvin/test/ebin").
code:add_path("/home/hobbes/bigappl-1.2/ebin").
io:format(".erlang rc finished\n",[]).
user_default and shell_default:
Functions in the shell which are not prefixed by a module name are
assumed to be functional objects (Funs), built-in functions (BIFs),
or belong to the module user_default or shell_default.
To include private shell commands, define them in a module
user_default and add the following argument as the first line in
the .erlang file.
code:load_abs("..../user_default").
erl:
If the contents of .erlang are changed and a private version of
user_default is defined, it is possible to customize the Erlang/OTP
environment. More powerful changes can be made by supplying command
line arguments in the start-up script erl. Refer to erl(1) and
init(3erl) for further information.
SEE ALSOinit(3erl), erl_prim_loader(3erl), erl_boot_server(3erl), code(3erl),
application(3erl), heart(3erl), net_kernel(3erl), auth(3erl),
make(3erl), epmd(1), erts_alloc(3erl)Ericsson AB erts 6.3 erl(1)
