Date: Sun, 30 Jan 2005 15:23:34 -0600 (CST) From: Mike Silbersack <silby@silby.com> To: Colin Percival <cperciva@freebsd.org> Cc: current@freebsd.org Subject: Re: tcp_isn_tick() / dummynet() callout madness ? Message-ID: <20050130151427.U59844@odysseus.silby.com> In-Reply-To: <41FD24C2.5070700@freebsd.org> References: <Pine.NEB.3.96L.1050130112410.15336A-100000@fledge.watson.org> <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. > > Colin Percival Is there any way to get around that cost with some relatively simple change to the callout API? Just a few places in the kernel account for most of the use of callouts, so even if a rewrite of those would be necessary, it should pay off. Or, potentially crazy idea here; instead of incurring the cost of a spinlock to remove a callout entry from a bucket, could you do some atomic operation to mark that entry as done, and then only remove those entries once and a while? I guess if spinlocks weren't so expensive, this wouldn't be a big deal... why do they need to be spinlocks? :) Mike "Silby" Silbersack
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