Re: [patch 7/9] mm: thrash detection-based file cache sizing

Andrew Morton
2013-11-25T23:50:16Z

On Sun, 24 Nov 2013 18:38:26 -0500 Johannes Weiner <hannes@cmpxchg.org> wrote:

> ...
>
> + * Access frequency and refault distance
> + *
> + * A workload is trashing when its pages are frequently used but they
> + * are evicted from the inactive list every time before another access
> + * would have promoted them to the active list.
> + *
> + * In cases where the average access distance between thrashing pages
> + * is bigger than the size of memory there is nothing that can be
> + * done - the thrashing set could never fit into memory under any
> + * circumstance.
> + *
> + * However, the average access distance could be bigger than the
> + * inactive list, yet smaller than the size of memory. In this case,
> + * the set could fit into memory if it weren't for the currently
> + * active pages - which may be used more, hopefully less frequently:
> + *
> + * +-memory available to cache-+
> + * | |
> + * +-inactive------+-active----+
> + * a b | c d e f g h i | J K L M N |
> + * +---------------+-----------+

So making the inactive list smaller will worsen this problem?

If so, don't we have a conflict with this objective:

> Right now we have a fixed ratio (50:50) between inactive and active
> list but we already have complaints about working sets exceeding half
> of memory being pushed out of the cache by simple streaming in the
> background. Ultimately, we want to adjust this ratio and allow for a
> much smaller inactive list.

?

> + * It is prohibitively expensive to accurately track access frequency
> + * of pages. But a reasonable approximation can be made to measure
> + * thrashing on the inactive list, after which refaulting pages can be
> + * activated optimistically to compete with the existing active pages.
> + *
> + * Approximating inactive page access frequency - Observations:
> + *
> + * 1. When a page is accesed for the first time, it is added to the

"accessed"

> + * head of the inactive list, slides every existing inactive page
> + * towards the tail by one slot, and pushes the current tail page
> + * out of memory.
> + *
>
> ...
>