bufmod(7M)
NAME
bufmod - STREAMS Buffer Module
SYNOPSIS
ioctl(fd, I_PUSH, "bufmod");
DESCRIPTION
bufmod is a STREAMS module that buffers incoming messages,
reducing the number of system calls and the associated over-
head 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 optionally con-
verts M_PROTO messages to M_DATA and adds them to chunks as
well. It also optionally adds to each message a header con-
taining a timestamp, and a cumulative count of messages
dropped on the stream read side due to resource exhaustion
or flow control. Thedefault settings 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, buf-
mod is capable of truncating upstream messages to a fixed,
programmable length.
When a message arrives, bufmod processes it in several
steps. The following 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 message type field and leaving the con-
tents 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 truncation in bytes. The sbh_msglen field gives the
length in bytes of the message after the truncation has been
done. sbh_totlen gives the distance 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
alignment 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 exhaustion. In the current
implementation message dropping due to flow control 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_timestamp 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 overflow, the module passes it
upward in an over-size chunk of its own. Otherwise, the
module concatenates the message to the end of the current
chunk.
To ensure that messages do not languish forever in an accu-
mulating chunk, bufmod maintains a read timeout. Whenever
this timeout expires, the module closes off the current
chunk and passes it upward. The module restarts the timeout
period when it receives a read side data message 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 messages during slack intervals, but avoids
any timeout overhead when there is no activity.
bufmod handles other message types as follows. Upon receiv-
ing 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 messages for internal
synchronization and does not pass them through.) bufmod
passes all other messages through unaltered to its upper
neighbor, maintaining message order for non high priority
messages by passing 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 win-
dow.
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 unal-
tered to its lower neighbor. If SB_SEND_ON_WRITE is set,
message arrival on the writer side suffices to close and
transmit the current read side chunk.
IOCTLS
bufmod 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
values 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 ALSO
dlpi(7P), le(7D), pfmod(7M)
NOTES
Older versions of bufmod did not support the behavioral
flexibility controlled by the SBIOCSFLAGS ioctl. Applica-
tions 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.
BUGS
bufmod does not defend itself against allocation failures,
so that it is possible, although very unlikely, for the
stream head to use inappropriate high and low water marks
after the chunk size or snapshot length have changed.
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