Re: [PATCH 1/2] vmscan: shrink_slab() require number of lru_pages, not page order

From: KOSAKI Motohiro
Date: Sun Jun 27 2010 - 21:32:18 EST

> On Fri, 25 Jun 2010, KOSAKI Motohiro wrote:
> > Fix simple argument error. Usually 'order' is very small value than
> > lru_pages. then it can makes unnecessary icache dropping.
> This is going to reduce the delta that is added to shrinker->nr
> significantly thereby increasing the number of times that shrink_slab() is
> called.

Yup. But,

Smaller shrink -> only makes retry
Bigger shrink -> makes unnecessary icache/dcache drop. it can bring
mysterious low performance.

> What does the "lru_pages" parameter do in shrink_slab()? Looks
> like its only role is as a divison factor in a complex calculation of
> pages to be scanned.

scanned/lru_pages ratio define basic shrink_slab puressure strength.

So, If you intentionally need bigger slab pressure, bigger scanned parameter
passing is better rather than mysterious 'order' parameter.

> do_try_to_free_pages passes 0 as "lru_pages" to shrink_slab() when trying
> to do cgroup lru scans. Why is that?

When cgroup lru scans, do_try_to_free_pages() don't call shrink_slab().

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