Reduce the number of calls to reset the keepalive timer
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Authored by jtl on Oct 13 2016, 6:44 PM.

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Reviewers

gallatin
rrs

Group Reviewers

transport

Commits

rS307319: The code currently resets the keepalive timer each time a packet is

Summary

The code currently resets the keepalive timer each time a packet is received on a TCP session that has entered the ESTABLISHED state. This results in a lot of calls to reset the keepalive timer.

This patch changes the behavior so we set the keepalive timer for the keepalive idle time (TP_KEEPIDLE). When the keepalive timer fires, it will first check to see if the session has been idle for TP_KEEPIDLE ticks. If not, it will reschedule the keepalive timer for the time the session will have been idle for TP_KEEPALIVE ticks.

For a session with regular communication, the keepalive timer should fire approximately once every TP_KEEPIDLE ticks. For sessions with irregular communication, the keepalive timer might fire more often. But, the disruption from a periodic keepalive timer should be less than the regular cost of resetting the keepalive timer on every packet.

MFC after: 2 weeks

Test Plan

(Not done yet.) Grab performance metrics from a live system.

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Repository

rS FreeBSD src repository - subversion

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Event Timeline

jtl updated this revision to Diff 21363.Oct 13 2016, 6:44 PM

jtl retitled this revision from to Reduce the number of calls to reset the keepalive timer.

jtl updated this object.

jtl edited the test plan for this revision. (Show Details)

jtl added reviewers: gallatin, rrs.

Herald added a reviewer: transport. · View Herald TranscriptOct 13 2016, 6:44 PM

Herald added a subscriber: imp. · View Herald Transcript

I think that a grand idea :)

This revision is now accepted and ready to land.Oct 14 2016, 2:08 PM

This seems like a great idea. I don't see any downside.

jch added a subscriber: jch.Oct 14 2016, 2:30 PM

Closed by commit rS307319: The code currently resets the keepalive timer each time a packet is (authored by jtl). · Explain WhyOct 14 2016, 2:57 PM

This revision was automatically updated to reflect the committed changes.

Like.

head/sys/netinet/tcp_timer.c
473	I'd suggest comment change. It should explain not only why we do this check, rather than why we don't reset. Suggested text to be checked by native speaker: "We don't reschedule the keepalive callout on every packet, because that costs a lot of CPU cycles. Cheaper to actually execute the callout once it expires and check against tp->t_rcvtime. If the check doesn't pass, schedule next keepalive and return."