Re: [patch 4/4 v4]swap: make cluster allocation per-cpu

[Shaohua Li <shli@kernel.org>]

On Wed, Jun 12, 2013 at 03:22:31PM -0700, Andrew Morton wrote:
> On Tue, 26 Mar 2013 13:38:43 +0800 Shaohua Li <shli@kernel.org> wrote:
> 
> > swap cluster allocation is to get better request merge to improve performance.
> > But the cluster is shared globally, if multiple tasks are doing swap, this will
> > cause interleave disk access. While multiple tasks swap is quite common, for
> > example, each numa node has a kswapd thread doing swap or multiple
> > threads/processes do direct page reclaim.
> > 
> > We makes the cluster allocation per-cpu here. The interleave disk access issue
> > goes away. All tasks will do sequential swap.
> 
> Why per-cpu rather than, say, per-mm or per-task?

Good question. per-cpu is easier to implement :). per-mm or per-task is likely
to be more readahead friendly for sequential workload. per-cpu is better in
swapout side for random workload. Not sure which is more important. I choose to
solve the swapout issue. Definitely this should be revisit if we want a better
swapin readahead. But if you dislike this patch, I'm ok to drop it in next
post.

> > If one CPU can't get its per-cpu cluster, it will fallback to scan swap_map.
> 
> Under what circumstances can a cpu "not get its per-cpu cluster"?  A
> cpu can always "get" its per-cpu data, by definition (unless perhaps
> interrupts are involved).  Perhaps this description needs some
> expanding upon.

the circumstance is there is no free cluster. I'll rewrite the description.

Thanks,
Shaohua

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