linux-kernel - Re: [RFC][PATCH] mm: percpu pages: up batch size to fix arithmetic?? errror

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Date:	Wed, 11 Sep 2013 16:21:46 -0700
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Dave Hansen <dave@...1.net>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org, cl@...ux.com
Subject: Re: [RFC][PATCH] mm: percpu pages: up batch size to fix arithmetic??
 errror

On 09/11/2013 04:08 PM, Cody P Schafer wrote:
> On 09/11/2013 03:08 PM, Dave Hansen wrote:
>> I really don't know where the:
>>
>>     batch /= 4;             /* We effectively *= 4 below */
>>     ...
>>     batch = rounddown_pow_of_two(batch + batch/2) - 1;
>>
>> came from.  The round down code at *MOST* does a *= 1.5, but
>> *averages* out to be just under 1.
>>
>> On a system with 128GB in a zone, this means that we've got
>> (you can see in /proc/zoneinfo for yourself):
>>
>>                high:  186 (744kB)
>>                batch: 31  (124kB)
>>
>> That 124kB is almost precisely 1/4 of the "1/2 of a meg" that we
>> were shooting for.  We're under-sizing the batches by about 4x.
>> This patch kills the /=4.
>>
>> ---
>> diff -puN mm/page_alloc.c~debug-pcp-sizes-1 mm/page_alloc.c
>> --- linux.git/mm/page_alloc.c~debug-pcp-sizes-1    2013-09-11
>> 14:41:08.532445664 -0700
>> +++ linux.git-davehans/mm/page_alloc.c    2013-09-11
>> 15:03:47.403912683 -0700
>> @@ -4103,7 +4103,6 @@ static int __meminit zone_batchsize(stru
>>       batch = zone->managed_pages / 1024;
>>       if (batch * PAGE_SIZE > 512 * 1024)
>>           batch = (512 * 1024) / PAGE_SIZE;
>> -    batch /= 4;        /* We effectively *= 4 below */
>>       if (batch < 1)
>>           batch = 1;
>>
>> _
>>
>
> Looking back at the first git commit (way before my time), it appears
> that the percpu pagesets initially had a ->high and ->low (now removed),
> set to batch*6 and batch*2 respectively. I assume the idea was to keep
> the number of pages in the percpu pagesets around batch*4, hence the
> comment.
>
> So we have this variable called "batch", and the code is trying to store
> the _average_ number of pcp pages we want into it (not the batchsize),
> and then we divide our "average" goal by 4 to get a batchsize. All the
> comments refer to the size of the pcp pagesets, not to the pcp pageset
> batchsize.
>
> Looking further, in current code we don't refill the pcp pagesets unless
> they are completely empty (->low was removed a while ago), and then we
> only add ->batch pages.
>
> Has anyone looked at what type of average pcp sizing the current code
> results in?

Also, we may want to consider shrinking pcp->high down from 6*pcp->batch 
given that the original "6*" choice was based upon ->batch actually 
being 1/4th of the average pageset size, where now it appears closer to 
being the average.

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