bufmod

- STREAMS Buffer Module

Synopsis

#include <sys/bufmod.h>

ioctl(fd, I_PUSH, "bufmod");

Description

bufmod 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 optionally 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 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, bufmod 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 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 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 accumulating 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 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 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 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 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 forcing 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), 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.

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.