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SubjectRe: [NET]: rt_check_expire() can take a long time, add a cond_resched()
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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