linux-kernel - Re: [NET]: rt_check_expire() can take a long time, add a cond

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Date:	Fri, 16 Nov 2007 04:48:42 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	davem@...emloft.net
Subject: Re: [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@...r.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@...mosbay.com> AuthorDate: Wed Nov 14 16:14:05 2007 -0800
>> Committer:  David S. Miller <davem@...emloft.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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