Fix live-comm: merge TCP socket write-write sequence in a single write
authorJonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Mon, 24 Jul 2017 20:06:17 +0000 (16:06 -0400)
committerJérémie Galarneau <jeremie.galarneau@efficios.com>
Thu, 27 Jul 2017 19:41:56 +0000 (15:41 -0400)
commitb727abf5b1da0904c4321b534bd6d002ce4597f1
treed7dbc7bc4740152ce73fba78714fcf288ee09407
parent772ff7857761fb0fd12c5c158ae69460f9baf3dc
Fix live-comm: merge TCP socket write-write sequence in a single write

The live protocol implementation is often sending content
on TCP sockets in two separate writes. One to send a command header,
and the second one sending the command's payload. This was presumably
done under the assumption that it would not result in two separate
TCP packets being sent on the network (or that it would not matter).

Delayed ACK-induced delays were observed [1] on the second write of the
"write header, write payload" sequence and result in problematic
latency build-ups for live clients connected to moderately/highly
active sessions.

Fundamentaly, this problem arises due to the combination of Nagle's
algorithm and the delayed ACK mechanism which make write-write-read
sequences on TCP sockets problematic as near-constant latency is
expected when clients can keep-up with the event production rate.

In such a write-write-read sequence, the second write is held up until
the first write is acknowledged (TCP ACK). The solution implemented
by this patch bundles the writes into a single one [2].

[1] https://github.com/tbricks/wireshark-lttng-plugin
Basic Wireshark dissector for lttng-live by Anto Smyk from Itiviti
[2] https://lists.freebsd.org/pipermail/freebsd-net/2006-January/009527.html

Reported-by: Anton Smyk <anton.smyk@itiviti.com>
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
formats/lttng-live/lttng-live-comm.c
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