linux-kernel - Re: [PATCH] mm, vmacache: Add kconfig VMACACHE

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Date:	Fri, 23 Jan 2015 20:02:01 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	WANG Chao <chaowang@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>
CC:	Rik van Riel <riel@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, vmacache: Add kconfig VMACACHE_SHIFT

On 01/23/2015 12:14 AM, WANG Chao wrote:
> On 01/22/15 at 11:22am, Rik van Riel wrote:
>> > On 01/22/2015 11:19 AM, Davidlohr Bueso wrote:
>>> > > On Thu, 2015-01-22 at 15:57 +0800, WANG Chao wrote:
>>>> > >> Hi, Davidlohr
>>>> > >>
>>>> > >> On 01/21/15 at 11:46pm, Davidlohr Bueso wrote:
>>>>> > >>> On Thu, 2015-01-22 at 14:29 +0800, WANG Chao wrote:
>>>>>> > >>>> Add a new kconfig option VMACACHE_SHIFT (as a power of 2) to specify the
>>>>>> > >>>> number of slots vma cache has for each thread. Range is chosen 0-4 (1-16
>>>>>> > >>>> slots) to consider both overhead and performance penalty. Default is 2
>>>>>> > >>>> (4 slots) as it originally is, which provides good enough balance.
>>>>>> > >>>>
>>>>> > >>>
>>>>> > >>> Nack. I don't feel comfortable making scalability features of core code
>>>>> > >>> configurable.
>>>> > >>
>>>> > >> Out of respect, is this a general rule not making scalability features
>>>> > >> of core code configurable?
>>> > > 
>>> > > I doubt its a rule, just common sense. Users have no business
>>> > > configuring such low level details. The optimizations need to
>>> > > transparently work for everyone.
>> > 
>> > There may sometimes be a good reason for making this kind of
>> > thing configurable, but since there were no performance
>> > numbers in the changelog, I have not seen any such reason for
>> > this particular change :)
> True. I didn't run any kind of benchmark, thus no numbers here. This is
> purely hypothetical.
> 
> I'm glad to run some tests. For the sake of consistency, could you
> please show me a hint how do you measure at the first place? I can do
> hit-rate, but I don't know how you measure cpu cycles. Could you
> elaborate?

I don't think there's a need to look for problems where there are none.

Have you observed a performance issue that might be improved by changing
the shift here?


Thanks,
Sasha
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