On Fri, 25 Feb 2011 20:04:58 +0000 Mel Gorman <mel@xxxxxxxxx> 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@xxxxxxxxxx: Fix initialisation of isolated] > Signed-off-by: Mel Gorman <mel@xxxxxxxxx> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxx> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>