Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated

[Rafael Aquini <aquini@redhat.com>]

On Fri, Sep 21, 2012 at 11:46:22AM +0100, Mel Gorman wrote:
> When compaction was implemented it was known that scanning could potentially
> be excessive. The ideal was that a counter be maintained for each pageblock
> but maintaining this information would incur a severe penalty due to a
> shared writable cache line. It has reached the point where the scanning
> costs are an serious problem, particularly on long-lived systems where a
> large process starts and allocates a large number of THPs at the same time.
> 
> Instead of using a shared counter, this patch adds another bit to the
> pageblock flags called PG_migrate_skip. If a pageblock is scanned by
> either migrate or free scanner and 0 pages were isolated, the pageblock
> is marked to be skipped in the future. When scanning, this bit is checked
> before any scanning takes place and the block skipped if set.
> 
> The main difficulty with a patch like this is "when to ignore the cached
> information?" If it's ignored too often, the scanning rates will still
> be excessive. If the information is too stale then allocations will fail
> that might have otherwise succeeded. In this patch
> 
> o CMA always ignores the information
> o If the migrate and free scanner meet then the cached information will
>   be discarded if it's at least 5 seconds since the last time the cache
>   was discarded
> o If there are a large number of allocation failures, discard the cache.
> 
> The time-based heuristic is very clumsy but there are few choices for a
> better event. Depending solely on multiple allocation failures still allows
> excessive scanning when THP allocations are failing in quick succession
> due to memory pressure. Waiting until memory pressure is relieved would
> cause compaction to continually fail instead of using reclaim/compaction
> to try allocate the page. The time-based mechanism is clumsy but a better
> option is not obvious.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> Acked-by: Rik van Riel <riel@redhat.com>
> ---

Acked-by: Rafael Aquini <aquini@redhat.com>