process_id_max(5)process_id_max(5)NAMEprocess_id_max - limit the maximum value for process IDs (PIDs)
VALUES
Failsafe
Default
Minimum
Maximum
must be greater than or equal to If the difference between and inclu‐
sive is less than is effectively limited to this difference.
DESCRIPTION
The tunable allows the administrator to select a potential value range
for process IDs (PIDs) as generated by (see fork(2)). It allows the
administrator to select a balance between compatibility, capacity, and
aesthetics.
Warning: Some programs cannot tolerate PID values up to the maximum.
If such programs exist and are critical, the maximum PID should be
appropriately constrained. For more details on these concerns, see
below.
Who is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Do not increase the maximum PID if there are critical applications
which assume that PIDs fit into a restricted range. (See below).
The value of can be increased at any time, and it takes effect immedi‐
ately. (However, its effect may not be noticed until a sufficient num‐
ber of new processes have been created to cause the system to utilize
the available higher values.)
A decrease in the value of also takes effect immediately. However any
existing processes with PIDs that are higher than the new value are not
affected. The decrease will be in full effect for all processes only
after a reboot.
When Should the Value of This Tunable Be Raised?
Increase the maximum PID if the range of PIDs defined by the and tun‐
ables needs to be increased to allow the creation of more simultaneous
processes. See nproc(5) for limits on the number of processes.
Increase the maximum PID in systems which have many active processes
(for example, >25,000). The larger range may increase the efficiency
of creating of new processes (because it may take less work to find
available PIDs).
If it is desired to validate that software programs execute properly in
environments where PID values may be large, increase the tunable along
with the tunable to force all new process IDs to take on large values.
(See process_id_min(5) for more information.)
Do not increase the maximum PID if there are critical applications
which assume that PIDs fit into a restricted range. (See below.)
What are the Side Effects of Raising the Value?
If the difference between and tunables is less than the number of pro‐
cesses allowed to exist simultaneously is limited to that difference.
When Should the Value of This Tunable Be Lowered?
Lower the maximum PID if critical applications make assumptions that
the PID range is restricted.
What are the Side Effects of Lowering the Value?
If the difference between and tunables is less than the number of pro‐
cesses allowed to exist simultaneously is limited to that difference.
What Other Tunable Values Should Be Changed at the Same Time?
It may be desirable to change For program development and validation, a
change in the tunable may also be needed.
Potential Application Issues
The range of PID values has, in the past, been restricted to 0..30,000.
Some programs have built-in assumptions about this range. This section
briefly describes some of those assumptions.
Some application programs have a built-in assumption that a PID does
not exceed 30,000 (which was the old value of the (undocumented) and
constants). They could fail if PIDs exceed this maximum.
Some application programs store PIDs in 16-bit variables (type in C).
Such programs could fail if the maximum PID exceeds 32,767.
Some programs provide output formats which can be sensitive to the num‐
ber of digits in the PID. Such programs may produce aesthetically dis‐
pleasing output if PIDs exceed 5 digits (exceed 99,999). In some cases
automatic expansion of output fields can disturb column alignment. In
some other cases, adjacent fields could run together, making the output
incomprehensible.
Some programs or scripts parse the outputs of other programs which con‐
tain PID values. Some such programs have built-in assumptions that a
PID will not exceed five character positions. Such a program could
fail if the range exceeds 99,999.
Because session IDs (SIDs) and process group IDs (PGIDs) are the same
as the process ID of the session or group leader, an increase in the
maximum PID also increases the maximum SID and PGID. Though much less
likely, the same application issues may exist for SIDs and PGIDs.
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parame‐
ter may be removed or have its meaning changed in future releases of
HP-UX.
The HP-UX kernel may silently round the selected values for and/or
(e.g., to the nearest power of 2) in order to accommodate the PID gen‐
eration algorithm.
Do not increase the maximum PID if there are critical applications
which assume that PIDs fit into a restricted range. See the previous
section, for more details on such programmatic assumptions. The
default maximum (30,000) has been selected to provide compatibility
with all such programs. This value should be used if program sensitiv‐
ity to larger PID values is unknown. See process_id_min(5) for infor‐
mation about how large PID values can be selected for software valida‐
tion purposes.
Increasing the PID range does not increase the maximum number of pro‐
cesses in the system.
Installation of optional kernel software, from HP or other vendors, may
cause changes to tunable parameter values. After installation, some
tunable parameters may no longer be at the default or recommended val‐
ues. For information about the effects of installation on tunable val‐
ues, consult the documentation for the kernel software being installed.
For information about optional kernel software that was factory
installed on your system, see at
AUTHOR
was developed by HP.
SEE ALSOfork(2), nproc(5), process_id_min(5).
whitepaper, available on
Tunable Kernel Parameters process_id_max(5)
