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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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