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Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was sc
From: |
Rafael Aquini |
Subject: |
Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated |
Date: |
Fri, 21 Sep 2012 14:53:38 -0300 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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 <address@hidden>
> Acked-by: Rik van Riel <address@hidden>
> ---
Acked-by: Rafael Aquini <address@hidden>
- [Qemu-devel] [PATCH 7/9] Revert "mm: have order > 0 compaction start off where it left", (continued)
- [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Mel Gorman, 2012/09/21
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated,
Rafael Aquini <=
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Andrew Morton, 2012/09/22
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Mel Gorman, 2012/09/24
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Andrew Morton, 2012/09/24
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Mel Gorman, 2012/09/25
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Andrew Morton, 2012/09/25
- [Qemu-devel] [PATCH] mm: compaction: cache if a pageblock was scanned and no pages were isolated -fix2, Mel Gorman, 2012/09/27
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Mel Gorman, 2012/09/27
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Minchan Kim, 2012/09/25
- Re: [Qemu-devel] [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated, Mel Gorman, 2012/09/27
[Qemu-devel] [PATCH 9/9] mm: compaction: Restart compaction from near where it left off, Mel Gorman, 2012/09/21