linux-kernel - Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value

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Date:	Thu, 3 Dec 2015 13:14:11 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value

On Thu, Nov 26, 2015 at 10:52:52AM +0900, Joonsoo Kim wrote:
> On Wed, Nov 25, 2015 at 10:04:44AM -0600, Christoph Lameter wrote:
> > > Although vmstat values aren't designed for accuracy, these are already
> > > used by some sensitive places so it is better to be more accurate.
> > 
> > The design is to sacrifice accuracy and the time the updates occur for
> > performance reasons. This is not the purpose the counters were designed
> > for. If you put these demands on the vmstat then you will get complex
> > convoluted code and compromise performance.
> 
> I understand design decision, but, it is better to get value as much
> as accurate if there is no performance problem. My patch would not
> cause much performance degradation because it is just adding one
> this_cpu_read().
> 
> Consider about following example. Current implementation returns
> interesting output if someone do following things.
> 
> v1 = zone_page_state(XXX);
> mod_zone_page_state(XXX, 1);
> v2 = zone_page_state(XXX);
> 
> v2 would be same with v1 in most of cases even if we already update
> it.
> 
> This situation could occurs in page allocation path and others. If
> some task try to allocate many pages, then watermark check returns
> same values until updating vmstat even if some freepage are allocated.
> There are some adjustments for this imprecision but why not do it become
> accurate? I think that this change is reasonable trade-off.
> 
Christoph, any comment?

Thanks.
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