netdev - RE: [PATCH] sctp: avoid refreshing heartbeat timer too often

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Date:	Thu, 31 Mar 2016 11:16:52 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	Neil Horman <nhorman@...driver.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>
Subject: RE: [PATCH] sctp: avoid refreshing heartbeat timer too often

From: Marcelo Ricardo Leitner
> Sent: 30 March 2016 13:13
> Em 30-03-2016 06:37, David Laight escreveu:
> > From: Marcelo Ricardo Leitner
> >> Sent: 29 March 2016 14:42
> >>
> >> Currently on high rate SCTP streams the heartbeat timer refresh can
> >> consume quite a lot of resources as timer updates are costly and it
> >> contains a random factor, which a) is also costly and b) invalidates
> >> mod_timer() optimization for not editing a timer to the same value.
> >> It may even cause the timer to be slightly advanced, for no good reason.
> >
> > Interesting thoughts:
> > 1) Is it necessary to use a different 'random factor' until the timer actually
> >     expires?
> 
> I don't understand you fully here, but we have to have a random factor
> on timer expire. As noted by Daniel Borkmann on his commit 8f61059a96c2
> ("net: sctp: improve timer slack calculation for transport HBs"):

When a HEARTBEAT chunk is sent determine the new interval, use that
interval until the timer actually expires when a new interval is
calculated. So the random number is only generated once per heartbeat.

>      RFC4960, section 8.3 says:
> 
>        On an idle destination address that is allowed to heartbeat,
>        it is recommended that a HEARTBEAT chunk is sent once per RTO
>        of that destination address plus the protocol parameter
>        'HB.interval', with jittering of +/- 50% of the RTO value,
>        and exponential backoff of the RTO if the previous HEARTBEAT
>        is unanswered.
> 
> Previous to his commit, it was using a random factor based on jiffies.
> 
> This patch then assumes that random_A+2 is just as random as random_B as
> long as it is within the allowed range, avoiding the unnecessary updates.
> 
> > 2) It might be better to allow the heartbeat timer to expire, on expiry work
> >     out the new interval based on when the last 'refresh' was done.
> 
> Cool, I thought about this too. It would introduce some extra complexity
> that is not really worth I think, specially because now we may be doing
> more timer updates even with this patch but it's not triggering any wake
> ups and we would need at least 2 wake ups then: one for the first
> timeout event, and then re-schedule the timer for the next updated one,
> and maybe again, and again.. less timer updates but more wake ups, one
> at every heartbeat interval even on a busy transport. Seems it's cheaper
> to just update the timer then.

One wakeup per heartbeat interval on a busy connection is probably noise.
Probably much less than the 1000s of timer updates that would otherwise happen.

A further optimisation would be to restart the timer if more than (say) 80%
of the way through the timeout period.

Similarly the HEARTBEAT could be sent if the 2nd wakeup would be almost immediate.

	David