Messages in this thread | | | Date | Fri, 16 Nov 2007 04:48:42 +0100 | From | Eric Dumazet <> | Subject | Re: [NET]: rt_check_expire() can take a long time, add a cond_resched() |
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Arjan van de Ven a écrit : > On Thu, 15 Nov 2007 04:01:48 GMT > Linux Kernel Mailing List <linux-kernel@vger.kernel.org> wrote: > >> Gitweb: >> http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=d90bf5a976793edfa88d3bb2393f0231eb8ce1e5 >> Commit: d90bf5a976793edfa88d3bb2393f0231eb8ce1e5 Parent: >> 66ba886254edbbd9442d30f1eef6f6fb0145027d Author: Eric Dumazet >> <dada1@cosmosbay.com> AuthorDate: Wed Nov 14 16:14:05 2007 -0800 >> Committer: David S. Miller <davem@davemloft.net> >> CommitDate: Wed Nov 14 16:14:05 2007 -0800 >> >> [NET]: rt_check_expire() can take a long time, add a >> cond_resched() >> On commit 39c90ece7565f5c47110c2fa77409d7a9478bd5b: > >> When the IP route cache is big, rt_check_expire() can take a long >> time to run. (default settings : 20% of the hash table is scanned at >> each invocation) >> >> Adding cond_resched() helps giving cpu to higher priority tasks if >> necessary. >> >> Using a "if (need_resched())" test before calling >> "cond_resched();" is necessary to avoid spending too much time doing >> the resched check. > > int __sched cond_resched(void) > { > if (need_resched() && ..... > > somehow I wonder why the second if() is useful at all; it's another > spot for a branch predictor to miss... and a void function call is > really really cheap...
Its not that cheap. The ChangeLog included my own numbers, on a Pentium M machine. (i686, 1.6 GHz, 1.5 GB ram)
Without "if (need_resched())" (so calling need_resched() X.XXX.XXX times), each run takes 88ms
With the extra check (and *much* less function calls), each run takes 25ms
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