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    SubjectRe: [PATCH]vmscan: add block plug for page reclaim
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    On Thu, 2011-07-28 at 07:45 +0800, Andrew Morton wrote:
    > On Sat, 23 Jul 2011 20:49:10 +0200
    > Jens Axboe <jaxboe@fusionio.com> wrote:
    >
    > > > I can observe the average request size changes. Before the patch, the
    > > > average request size is about 90k from iostat (but the variation is
    > > > big). With the patch, the request size is about 100k and variation is
    > > > small.
    > >
    > > That's a good win right there, imho.
    >
    > yup. Reduced CPU consumption on that path isn't terribly exciting IMO,
    > but improved request size is significant.
    >
    > Using an additional 44 bytes of stack on that path is also
    > significant(ly bad). But we need to fix that problem anyway. One way
    > we could improve things in mm/vmscan.c is to move the blk_plug into
    > scan_control then get the scan_control off the stack in some manner.
    > That's easy for kswapd: allocate one scan_control per kswapd at
    > startup. Doing it for direct-reclaim would be a bit trickier...
    unfortunately, the direct-reclaim case is what cares about stack.

    BTW, the scan_control can be dieted. may_unmap/may_swap/may_writepage
    can be a bit. swappiness < 100, so can be a char. order <= 11, can be a
    char. should I do it to cut the size?

    > And I have the usual maintainability whine. If someone comes up to
    > vmscan.c and sees it calling blk_start_plug(), how are they supposed to
    > work out why that call is there? They go look at the blk_start_plug()
    > definition and it is undocumented. I think we can do better than this?
    the block plug is a little tricky, we definitely should document it.
    Jens, if you don't mind, I'll add comments there.

    Thanks,
    Shaohua



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