• Mel Gorman's avatar
    mm: page allocator: adjust the per-cpu counter threshold when memory is low · 88f5acf8
    Mel Gorman authored
    Commit aa454840 ("calculate a better estimate of NR_FREE_PAGES when memory
    is low") noted that watermarks were based on the vmstat NR_FREE_PAGES.  To
    avoid synchronization overhead, these counters are maintained on a per-cpu
    basis and drained both periodically and when a threshold is above a
    threshold.  On large CPU systems, the difference between the estimate and
    real value of NR_FREE_PAGES can be very high.  The system can get into a
    case where pages are allocated far below the min watermark potentially
    causing livelock issues.  The commit solved the problem by taking a better
    reading of NR_FREE_PAGES when memory was low.
    
    Unfortately, as reported by Shaohua Li this accurate reading can consume a
    large amount of CPU time on systems with many sockets due to cache line
    bouncing.  This patch takes a different approach.  For large machines
    where counter drift might be unsafe and while kswapd is awake, the per-cpu
    thresholds for the target pgdat are reduced to limit the level of drift to
    what should be a safe level.  This incurs a performance penalty in heavy
    memory pressure by a factor that depends on the workload and the machine
    but the machine should function correctly without accidentally exhausting
    all memory on a node.  There is an additional cost when kswapd wakes and
    sleeps but the event is not expected to be frequent - in Shaohua's test
    case, there was one recorded sleep and wake event at least.
    
    To ensure that kswapd wakes up, a safe version of zone_watermark_ok() is
    introduced that takes a more accurate reading of NR_FREE_PAGES when called
    from wakeup_kswapd, when deciding whether it is really safe to go back to
    sleep in sleeping_prematurely() and when deciding if a zone is really
    balanced or not in balance_pgdat().  We are still using an expensive
    function but limiting how often it is called.
    
    When the test case is reproduced, the time spent in the watermark
    functions is reduced.  The following report is on the percentage of time
    spent cumulatively spent in the functions zone_nr_free_pages(),
    zone_watermark_ok(), __zone_watermark_ok(), zone_watermark_ok_safe(),
    zone_page_state_snapshot(), zone_page_state().
    
    vanilla                      11.6615%
    disable-threshold            0.2584%
    
    David said:
    
    : We had to pull aa454840 "mm: page allocator: calculate a better estimate
    : of NR_FREE_PAGES when memory is low and kswapd is awake" from 2.6.36
    : internally because tests showed that it would cause the machine to stall
    : as the result of heavy kswapd activity.  I merged it back with this fix as
    : it is pending in the -mm tree and it solves the issue we were seeing, so I
    : definitely think this should be pushed to -stable (and I would seriously
    : consider it for 2.6.37 inclusion even at this late date).
    Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
    Reported-by: default avatarShaohua Li <shaohua.li@intel.com>
    Reviewed-by: default avatarChristoph Lameter <cl@linux.com>
    Tested-by: default avatarNicolas Bareil <nico@chdir.org>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Kyle McMartin <kyle@mcmartin.ca>
    Cc: <stable@kernel.org>		[2.6.37.1, 2.6.36.x]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    88f5acf8
mmzone.h 35.1 KB