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    SubjectRe: [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

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