• Christoph Lameter's avatar
    mm: page allocator: calculate a better estimate of NR_FREE_PAGES when memory... · aa454840
    Christoph Lameter authored
    mm: page allocator: calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake
    
    Ordinarily watermark checks are based on the vmstat NR_FREE_PAGES as it is
    cheaper than scanning a number of lists.  To avoid synchronization
    overhead, counter deltas are maintained on a per-cpu basis and drained
    both periodically and when the delta is above a threshold.  On large CPU
    systems, the difference between the estimated and real value of
    NR_FREE_PAGES can be very high.  If NR_FREE_PAGES is much higher than
    number of real free page in buddy, the VM can allocate pages below min
    watermark, at worst reducing the real number of pages to zero.  Even if
    the OOM killer kills some victim for freeing memory, it may not free
    memory if the exit path requires a new page resulting in livelock.
    
    This patch introduces a zone_page_state_snapshot() function (courtesy of
    Christoph) that takes a slightly more accurate view of an arbitrary vmstat
    counter.  It is used to read NR_FREE_PAGES while kswapd is awake to avoid
    the watermark being accidentally broken.  The estimate is not perfect and
    may result in cache line bounces but is expected to be lighter than the
    IPI calls necessary to continually drain the per-cpu counters while kswapd
    is awake.
    Signed-off-by: default avatarChristoph Lameter <cl@linux.com>
    Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    aa454840
mmzone.c 2.49 KB