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@xxxxxxx> > Acked-by: Rik van Riel <riel@xxxxxxxxxx> > --- Acked-by: Rafael Aquini <aquini@xxxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>