Messages in this thread | | | Date | Fri, 16 Nov 2007 05:12:21 +0100 | From | Eric Dumazet <> | Subject | Re: [NET]: rt_check_expire() can take a long time, add a cond_resched() |
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Eric Dumazet a écrit : > 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 >
Looking at cond_resched(), I think the extra cost comes from "mov %esp,%edx ; and $0xffffe000,%edx" (current_thread_info())
I dont have oprofile numbers yet, but I suspect CPU may have some delays to compute this pointer value, since %esp is probably 'busy' because of the preceding "call"
(In the case the "if (need_resched())" is done in rt_check_expire(), compiler moves this pointer computation (current_thread_info()) out of the loop)
c055f926 <cond_resched>: c055f926: 89 e2 mov %esp,%edx c055f928: 81 e2 00 e0 ff ff and $0xffffe000,%edx c055f92e: 8b 42 08 mov 0x8(%edx),%eax c055f931: a8 04 test $0x4,%al c055f933: 74 1a je c055f94f <cond_resched+0x29> c055f935: f6 42 17 10 testb $0x10,0x17(%edx) c055f939: 75 14 jne c055f94f <cond_resched+0x29> c055f93b: 83 3d 00 80 7c c0 01 cmpl $0x1,0xc07c8000 c055f942: 75 0b jne c055f94f <cond_resched+0x29> c055f944: e8 2b 80 bb ff call c0117974 <__cond_resched> c055f949: b8 01 00 00 00 mov $0x1,%eax c055f94e: c3 ret c055f94f: 31 c0 xor %eax,%eax c055f951: c3 ret - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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