linux-kernel - Re: [PATCH 01/10] mm: allow swappiness that prefers anon over file

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Date:	Tue, 7 Jun 2016 10:18:18 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michal Hocko <mhocko@...e.cz>,
	Tim Chen <tim.c.chen@...ux.intel.com>, kernel-team@...com
Subject: Re: [PATCH 01/10] mm: allow swappiness that prefers anon over file

On Tue, Jun 07, 2016 at 09:25:50AM +0900, Minchan Kim wrote:
> Hi Johannes,
> 
> Thanks for the nice work. I didn't read all patchset yet but the design
> makes sense to me so it would be better for zram-based on workload
> compared to as is.

Thanks!

> On Mon, Jun 06, 2016 at 03:48:27PM -0400, Johannes Weiner wrote:
> > --- a/Documentation/sysctl/vm.txt
> > +++ b/Documentation/sysctl/vm.txt
> > @@ -771,14 +771,20 @@ with no ill effects: errors and warnings on these stats are suppressed.)
> >  
> >  swappiness
> >  
> > -This control is used to define how aggressive the kernel will swap
> > -memory pages.  Higher values will increase agressiveness, lower values
> > -decrease the amount of swap.  A value of 0 instructs the kernel not to
> > -initiate swap until the amount of free and file-backed pages is less
> > -than the high water mark in a zone.
> > +This control is used to define the relative IO cost of cache misses
> > +between the swap device and the filesystem as a value between 0 and
> > +200. At 100, the VM assumes equal IO cost and will thus apply memory
> > +pressure to the page cache and swap-backed pages equally. At 0, the
> > +kernel will not initiate swap until the amount of free and file-backed
> > +pages is less than the high watermark in a zone.
> 
> Generally, I agree extending swappiness value good but not sure 200 is
> enough to represent speed gap between file and swap sotrage in every
> cases. - Just nitpick.

How so? You can't give swap more weight than 100%. 200 is the maximum
possible value.

> Some years ago, I extended it to 200 like your patch and experimented it
> based on zram in our platform workload. At that time, it was terribly
> slow in app switching workload if swappiness is higher than 150.
> Although it was highly dependent on the workload, it's dangerous to
> recommend it before fixing balacing between file and anon, I think.
> IOW, I think this patch should be last one in this patchset.

Good point. I'll tone down the recommendations. But OTOH it's a fairly
trivial patch, so I wouldn't want it to close after the current 10/10.

> >  The default value is 60.
> >  
> > +On non-rotational swap devices, a value of 100 (or higher, depending
> > +on what's backing the filesystem) is recommended.
> > +
> > +For in-memory swap, like zswap, values closer to 200 are recommended.
> 
>                 maybe, like zram
> 
> I'm not sure it would be good suggestion for zswap because it ends up
> writing cached pages to swap device once it reaches threshold.
> Then, the cost is compression + decompression + write I/O which is
> heavier than normal swap device(i.e., write I/O). OTOH, zram have no
> (writeback I/O+ decompression) cost.

Oh, good catch. Yeah, I'll change that for v2.

Thanks for your input, Minchan