Re: [PATCH]: 1/4 batch mark_page_accessed()

[Andrew Morton <akpm@osdl.org>]

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.