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

Ryan Mallon
2013-11-26T01:56:36Z

On 25/11/13 10:38, Johannes Weiner wrote:
> The VM maintains cached filesystem pages on two types of lists. One
> list holds the pages recently faulted into the cache, the other list
> holds pages that have been referenced repeatedly on that first list.
> The idea is to prefer reclaiming young pages over those that have
> shown to benefit from caching in the past. We call the recently used
> list "inactive list" and the frequently used list "active list".
>
> Currently, the VM aims for a 1:1 ratio between the lists, which is the
> "perfect" trade-off between the ability to *protect* frequently used
> pages and the ability to *detect* frequently used pages. This means
> that working set changes bigger than half of cache memory go
> undetected and thrash indefinitely, whereas working sets bigger than
> half of cache memory are unprotected against used-once streams that
> don't even need caching.
>
> Historically, every reclaim scan of the inactive list also took a
> smaller number of pages from the tail of the active list and moved
> them to the head of the inactive list. This model gave established
> working sets more gracetime in the face of temporary use-once streams,
> but ultimately was not significantly better than a FIFO policy and
> still thrashed cache based on eviction speed, rather than actual
> demand for cache.
>
> This patch solves one half of the problem by decoupling the ability to
> detect working set changes from the inactive list size. By
> maintaining a history of recently evicted file pages it can detect
> frequently used pages with an arbitrarily small inactive list size,
> and subsequently apply pressure on the active list based on actual
> demand for cache, not just overall eviction speed.
>
> Every zone maintains a counter that tracks inactive list aging speed.
> When a page is evicted, a snapshot of this counter is stored in the
> now-empty page cache radix tree slot. On refault, the minimum access
> distance of the page can be assesed, to evaluate whether the page
> should be part of the active list or not.
>
> This fixes the VM's blindness towards working set changes in excess of
> the inactive list. And it's the foundation to further improve the
> protection ability and reduce the minimum inactive list size of 50%.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---

<snip>

> + * fault ------------------------+
> + * |
> + * +--------------+ | +-------------+
> + * reclaim <- | inactive | <-+-- demotion | active | <--+
> + * +--------------+ +-------------+ |
> + * | |
> + * +-------------- promotion ------------------+
> + *
> + *
> + * Access frequency and refault distance
> + *
> + * A workload is trashing when its pages are frequently used but they

"thrashing".

~Ryan