Date: Sun, 30 Jan 2005 18:59:20 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Colin Percival <cperciva@freebsd.org> Cc: current@freebsd.org Subject: Re: tcp_isn_tick() / dummynet() callout madness ? Message-ID: <Pine.NEB.3.96L.1050130185829.15336L-100000@fledge.watson.org> In-Reply-To: <41FD24C2.5070700@freebsd.org>
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On Sun, 30 Jan 2005, Colin Percival wrote: > Robert Watson wrote: > > since the callout_reset() is one of the more > > expensive parts of this code, Colin has been looking at some locking > > optimizations to lower the cost. > > To elaborate somewhat: I think I can avoid the spinlock cost when > callouts reset themselves (which is the case here). However, while this > will reduce the time spent in the callouts themselves, it's really only > a 50% solution -- softclock locks and unlocks the callout spin lock each > time it launches a callout. If we're spending 5% of our cpu time in > these two callouts, then they're actually responsible for using 10% of > our cpu time; I think I can cut that in half, but in the end we can't > avoid the cost of a mtx_lock_spin / mtx_unlock_spin pair (in softclock) > for each callout. On some further iteration, it transpired that Poul-Henning's configuration included WITNESS, and without that things look a bit more reasonable. We should still run the callout less often, if we can, and I think the optimization is useful. Robert N M Watson
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