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    SubjectRe: [PATCH 1/2] mm: compaction: Minimise the time IRQs are disabled while isolating free pages
    On Fri, Feb 25, 2011 at 08:04:58PM +0000, Mel Gorman wrote:
    > compaction_alloc() isolates free pages to be used as migration targets.
    > While its scanning, IRQs are disabled on the mistaken assumption the scanning
    > should be short. Analysis showed that IRQs were in fact being disabled for
    > substantial time. A simple test was run using large anonymous mappings with
    > transparent hugepage support enabled to trigger frequent compactions. A
    > monitor sampled what the worst IRQ-off latencies were and a post-processing
    > tool found the following;
    >
    > Total sampled time IRQs off (not real total time): 22355
    > Event compaction_alloc..compaction_alloc 8409 us count 1
    > Event compaction_alloc..compaction_alloc 7341 us count 1
    > Event compaction_alloc..compaction_alloc 2463 us count 1
    > Event compaction_alloc..compaction_alloc 2054 us count 1
    > Event shrink_inactive_list..shrink_zone 1864 us count 1
    > Event shrink_inactive_list..shrink_zone 88 us count 1
    > Event save_args..call_softirq 36 us count 1
    > Event save_args..call_softirq 35 us count 2
    > Event __make_request..__blk_run_queue 24 us count 1
    > Event __alloc_pages_nodemask..__alloc_pages_nodemask 6 us count 1
    >
    > i.e. compaction is disabled IRQs for a prolonged period of time - 8ms in
    > one instance. The full report generated by the tool can be found at
    > http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-vanilla-micro.report .
    > This patch reduces the time IRQs are disabled by simply disabling IRQs
    > at the last possible minute. An updated IRQs-off summary report then
    > looks like;
    >
    > Total sampled time IRQs off (not real total time): 5493
    > Event shrink_inactive_list..shrink_zone 1596 us count 1
    > Event shrink_inactive_list..shrink_zone 1530 us count 1
    > Event shrink_inactive_list..shrink_zone 956 us count 1
    > Event shrink_inactive_list..shrink_zone 541 us count 1
    > Event shrink_inactive_list..shrink_zone 531 us count 1
    > Event split_huge_page..add_to_swap 232 us count 1
    > Event save_args..call_softirq 36 us count 1
    > Event save_args..call_softirq 35 us count 2
    > Event __wake_up..__wake_up 1 us count 1
    >
    > A full report is again available at
    > http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-minimiseirq-free-v1r4-micro.report .
    > . As should be obvious, IRQ disabled latencies due to compaction are
    > almost elimimnated for this particular test.
    >
    > [aarcange@redhat.com: Fix initialisation of isolated]
    > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    Reviewed-by: Minchan Kim <minchan.kim@gmail.com>

    --
    Kind regards,
    Minchan Kim


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