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

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Message-ID: <20150123051422.GC8670@dhcp-129-179.nay.redhat.com>
Date:	Fri, 23 Jan 2015 13:14:22 +0800
From:	WANG Chao <chaowang@...hat.com>
To:	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 01/22/15 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?

Thanks
WANG Chao
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