linux-kernel - Re: [RFC PATCH] mm: let the bdi_writeout fraction respond more quickly

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Date:	Thu, 17 Jun 2010 13:41:19 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Richard Kennedy <richard@....demon.co.uk>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC PATCH] mm: let the bdi_writeout fraction respond more  quickly

On 2010-06-17 13:39, Richard Kennedy wrote:
> On Wed, 2010-06-16 at 20:54 +0200, Peter Zijlstra wrote:
>> On Mon, 2010-06-14 at 15:44 +0100, Richard Kennedy wrote:
>>>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>>>> index 2fdda90..315dd04 100644
>>>> --- a/mm/page-writeback.c
>>>> +++ b/mm/page-writeback.c
>>>> @@ -144,7 +144,7 @@ static int calc_period_shift(void)
>>>>       else
>>>>               dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
>>>>                               100;
>>>> -     return 2 + ilog2(dirty_total - 1);
>>>> +     return ilog2(dirty_total - 1) - 4;
>>>>  } 
>>
>> IIRC I suggested similar things in the past and all we needed to do was
>> find people doing the measurements on different bits of hardware or so..
>>
>> I don't have any problems with the approach, all we need to make sure is
>> that we never return 0 or a negative number (possibly ensure a minimum
>> positive shift value).
> 
> Yep that sounds reasonable. would minimum shift of 4 be ok ?
> 
> something like
> 
> 	max ( (ilog2(dirty_total - 1)- 4) , 4);
> 
> Unfortunately volunteers don't seem to be leaping out of the woodwork,
> maybe Andrew could be persuaded to try this in his tree for a while and
> see if any one squeaks ?

I'm pretty sure that most volunteers are curious what to actually test,
so they shy away from it. If you added a good explanation of an easy way
to test the before and after, then it would be more approachable.

I'll give it a spin here.

-- 
Jens Axboe

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