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SubjectRe: [PATCH]: 1/4 batch mark_page_accessed()
Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
>
> Because the ordering of LRU pages should be enhanced in respect to locality,
> with the mark_page_accessed batching you group together tasks accessed pages
> and move them at once to the active list.
>
> You maintain better locality ordering, while decreasing the precision of aging/
> temporal locality.
>
> Which should enhance disk writeout performance.

I'll buy that explanation. Although I'm a bit sceptical that it is
measurable.

Was that particular workload actually performing significant amounts of
writeout in vmscan.c? (We should have direct+kswapd counters for that, but
we don't. /proc/vmstat:pgrotated will give us an idea).


> On the other hand, without batching you mix the locality up in LRU - the LRU becomes
> more precise in terms of "LRU aging", but less ordered in terms of sequential
> access pattern.
>
> The disk IO intensive reaim has very significant gain from the batching, its
> probably due to the enhanced LRU ordering (what Nikita says).
>
> The slowdown is probably due to the additional atomic_inc by page_cache_get().
>
> Is there no way to avoid such page_cache_get there (and in lru_cache_add also)?

Not really. The page is only in the pagevec at that time - if someone does
a put_page() on it the page will be freed for real, and will then be
spilled onto the LRU. Messy.
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