linux-kernel - Re: [PATCH 1/2] mm: compaction: Minimise the time IRQs are disabled while isolating free pages

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Date:	Fri, 25 Feb 2011 23:34:44 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Arthur Marsh <arthur.marsh@...ernode.on.net>,
	Clemens Ladisch <cladisch@...glemail.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [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@...hat.com: Fix initialisation of isolated]
> Signed-off-by: Mel Gorman <mel@....ul.ie>

Acked-by: Johannes Weiner <hannes@...xchg.org>
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