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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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