bufmod(7M) STREAMS Modules bufmod(7M)NAMEbufmod - STREAMS Buffer Module
SYNOPSIS
#include <sys/bufmod.h>
ioctl(fd, I_PUSH, "bufmod");
DESCRIPTIONbufmod is a STREAMS module that buffers incoming messages, reducing the
number of system calls and the associated overhead required to read and
process them. Although bufmod was originally designed to be used in
conjunction with STREAMS-based networking device drivers, the version
described here is general purpose so that it can be used anywhere
STREAMS input buffering is required.
Read-side Behavior
The behavior of bufmod depends on various parameters and flags that can
be set and queried as described below under IOCTLS. bufmod collects
incoming M_DATA messages into chunks, passing each chunk upstream when
the chunk becomes full or the current read timeout expires. It option‐
ally converts M_PROTO messages to M_DATA and adds them to chunks as
well. It also optionally adds to each message a header containing a
timestamp, and a cumulative count of messages dropped on the stream
read side due to resource exhaustion or flow control. Thedefault set‐
tings of bufmod allow it to drop messages when flow control sets in or
resources are exhausted; disabling headers and explicitly requesting no
drops makes bufmod pass all messages through. Finally, bufmod is capa‐
ble of truncating upstream messages to a fixed, programmable length.
When a message arrives, bufmod processes it in several steps. The fol‐
lowing paragraphs discuss each step in turn.
Upon receiving a message from below, if the SB_NO_HEADER flag is not
set, bufmod immediately timestamps it and saves the current time value
for later insertion in the header described below.
Next, if SB_NO_PROTO_CVT is not set, bufmod converts all leading
M_PROTO blocks in the message to M_DATA blocks, altering only the mes‐
sage type field and leaving the contents alone.
It then truncates the message to the current snapshot length, which is
set with the SBIOCSSNAP ioctl described below.
Afterwards, if SB_NO_HEADER is not set, bufmod prepends a header to the
converted message. This header is defined as follows.
struct sb_hdr {
uint_t sbh_origlen;
uint_t sbh_msglen;
uint_t sbh_totlen;
uint_t sbh_drops;
#if defined(_LP64) || defined(_I32LPx)
struct timeval32 sbh_timestamp;
#else
struct timeval sbh_timestamp;
#endif /* !_LP64 */
};
The sbh_origlen field gives the message's original length before trun‐
cation in bytes. The sbh_msglen field gives the length in bytes of the
message after the truncation has been done. sbh_totlen gives the dis‐
tance in bytes from the start of the truncated message in the current
chunk (described below) to the start of the next message in the chunk;
the value reflects any padding necessary to insure correct data align‐
ment for the host machine and includes the length of the header itself.
sbh_drops reports the cumulative number of input messages that this
instance of bufmod has dropped due to flow control or resource exhaus‐
tion. In the current implementation message dropping due to flow con‐
trol can occur only if the SB_NO_DROPS flag is not set. (Note: this
accounts only for events occurring within bufmod, and does not count
messages dropped by downstream or by upstream modules.) The sbh_time‐
stamp field contains the message arrival time expressed as a struct
timeval.
After preparing a message, bufmod attempts to add it to the end of the
current chunk, using the chunk size and timeout values to govern the
addition. The chunk size and timeout values are set and inspected using
the ioctl() calls described below. If adding the new message would make
the current chunk grow larger than the chunk size, bufmod closes off
the current chunk, passing it up to the next module in line, and starts
a new chunk. If adding the message would still make the new chunk over‐
flow, the module passes it upward in an over-size chunk of its own.
Otherwise, the module concatenates the message to the end of the cur‐
rent chunk.
To ensure that messages do not languish forever in an accumulating
chunk, bufmod maintains a read timeout. Whenever this timeout expires,
the module closes off the current chunk and passes it upward. The mod‐
ule restarts the timeout period when it receives a read side data mes‐
sage and a timeout is not currently active. These two rules insure that
bufmod minimizes the number of chunks it produces during periods of
intense message activity and that it periodically disposes of all mes‐
sages during slack intervals, but avoids any timeout overhead when
there is no activity.
bufmod handles other message types as follows. Upon receiving an
M_FLUSH message specifying that the read queue be flushed, the module
clears the currently accumulating chunk and passes the message on to
the module or driver above. (Note: bufmod uses zero length M_CTL mes‐
sages for internal synchronization and does not pass them through.)
bufmod passes all other messages through unaltered to its upper neigh‐
bor, maintaining message order for non high priority messages by pass‐
ing up any accumulated chunk first.
If the SB_DEFER_CHUNK flag is set, buffering does not begin until the
second message is received within the timeout window.
If the SB_SEND_ON_WRITE flag is set, bufmod passes up the read side any
buffered data when a message is received on the write side.
SB_SEND_ON_WRITE and SB_DEFER_CHUNK are often used together.
Write-side Behavior
bufmod intercepts M_IOCTL messages for the ioctls described below. The
module passes all other messages through unaltered to its lower neigh‐
bor. If SB_SEND_ON_WRITE is set, message arrival on the writer side
suffices to close and transmit the current read side chunk.
IOCTLSbufmod responds to the following ioctls.
SBIOCSTIME Set the read timeout value to the value referred to by
the struct timeval pointer given as argument. Setting
the timeout value to zero has the side-effect of forc‐
ing the chunk size to zero as well, so that the module
will pass all incoming messages upward immediately upon
arrival. Negative values are rejected with an EINVAL
error.
SBIOCGTIME Return the read timeout in the struct timeval pointed
to by the argument. If the timeout has been cleared
with the SBIOCCTIME ioctl, return with an ERANGE error.
SBIOCCTIME Clear the read timeout, effectively setting its value
to infinity. This results in no timeouts being active
and the chunk being delivered when it is full.
SBIOCSCHUNK Set the chunk size to the value referred to by the
uint_t pointer given as argument. See Notes for a
description of effect on stream head high water mark.
SBIOCGCHUNK Return the chunk size in the uint_t pointed to by the
argument.
SBIOCSSNAP Set the current snapshot length to the value given in
the uint_t pointed to by the ioctl's final argument.
bufmod interprets a snapshot length value of zero as
meaning infinity, so it will not alter the message. See
Notes for a description of effect on stream head high
water mark.
SBIOCGSNAP Returns the current snapshot length in the uint_t
pointed to by the ioctl's final argument.
SBIOCSFLAGS Set the current flags to the value given in the uint_t
pointed to by the ioctl's final argument. Possible val‐
ues are a combination of the following.
SB_SEND_ON_WRITE Transmit the read side chunk on
arrival of a message on the write
side.
SB_NO_HEADER Do not add headers to read side
messages.
SB_NO_DROPS Do not drop messages due to flow
control upstream.
SB_NO_PROTO_CVT Do not convert M_PROTO messages
into M_DATA.
SB_DEFER_CHUNK Begin buffering on arrival of the
second read side message in a
timeout interval.
SBIOCGFLAGS Returns the current flags in the uint_t pointed to by
the ioctl's final argument.
SEE ALSOdlpi(7P), pfmod(7M)NOTES
Older versions of bufmod did not support the behavioral flexibility
controlled by the SBIOCSFLAGS ioctl. Applications that wish to take
advantage of this flexibility can guard themselves against old versions
of the module by invoking the SBIOCGFLAGS ioctl and checking for an
EINVAL error return.
When buffering is enabled by issuing an SBIOCSCHUNK ioctl to set the
chunk size to a non zero value, bufmod sends a SETOPTS message to
adjust the stream head high and low water marks to accommodate the
chunked messages.
When buffering is disabled by setting the chunk size to zero, message
truncation can have a significant influence on data traffic at the
stream head and therefore the stream head high and low water marks are
adjusted to new values appropriate for the smaller truncated message
sizes.
BUGSbufmod does not defend itself against allocation failures, so that it
is possible, although very unlikely, for the stream head to use inap‐
propriate high and low water marks after the chunk size or snapshot
length have changed.
SunOS 5.11 11 Nov 1997 bufmod(7M)
