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    SubjectRE: [RFC][PATCH] avoid swapping out with swappiness==0
    Hello Kosaki-san,

    On 03/07/2012 01:18 PM, Satoru Moriya wrote:
    > On 03/07/2012 12:19 PM, KOSAKI Motohiro wrote:
    >> Thank you. I brought back to memory it. Unfortunately DB folks are
    >> still mainly using RHEL5 generation distros. At that time,
    >> swapiness=0 doesn't mean disabling swap.
    >> They want, "don't swap as far as kernel has any file cache page". but
    >> linux don't have such feature. then they used swappiness for emulate
    >> it. So, I think this patch clearly make userland harm. Because of, we
    >> don't have an alternative way.

    As I wrote in the previous mail(see below), with this patch
    the kernel begins to swap out when the sum of free pages and
    filebacked pages reduces less than watermark_high.

    So the kernel reclaims pages like following.

    nr_free + nr_filebacked >= watermark_high: reclaim only filebacked pages
    nr_free + nr_filebacked < watermark_high: reclaim only anonymous pages

    Do you think this behavior satisfies DB users' requirement?

    > If they expect the behavior that "don't swap as far as kernel has any
    > file cache page", this patch definitely helps them because if we set
    > swappiness==0, kernel does not swap out
    > *until* nr_free + nr_filebacked < high watermark in the zone.
    > It means kernel begins to swap out when nr_free + nr_filebacked
    > becomes less than high watermark.
    > But, yes, this patch actually changes the behavior with swappiness==0
    > and so it may make userland harm.
    > How about introducing new value e.g -1 to avoid swap and maintain
    > compatibility?


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