Re: [PATCH] mm: Move zone lock to a different cache line than order-0 free page lists

[David Rientjes <rientjes@google.com>]

On Fri, 27 Mar 2015, Mel Gorman wrote:

> Huang Ying reported the following problem due to commit 3484b2de9499
> ("mm: rearrange zone fields into read-only, page alloc, statistics and
> page reclaim lines") from the Intel performance tests
> 
>     24b7e5819ad5cbef  3484b2de9499df23c4604a513b
>     ----------------  --------------------------
>              %stddev     %change         %stddev
>                  \          |                \
>         152288 \261  0%     -46.2%      81911 \261  0%  aim7.jobs-per-min
>            237 \261  0%     +85.6%        440 \261  0%  aim7.time.elapsed_time
>            237 \261  0%     +85.6%        440 \261  0%  aim7.time.elapsed_time.max
>          25026 \261  0%     +70.7%      42712 \261  0%  aim7.time.system_time
>        2186645 \261  5%     +32.0%    2885949 \261  4%  aim7.time.voluntary_context_switches
>        4576561 \261  1%     +24.9%    5715773 \261  0%  aim7.time.involuntary_context_switches
> 
> The problem is specific to very large machines under stress. It was not
> reproducible with the machines I had used to justify the original patch
> because large numbers of CPUs are required. When pressure is high enough,
> the cache line is bouncing between CPUs trying to acquire the lock and
> the holder of the lock adjusting free lists. The intention was that the
> acquirer of the lock would automatically have the cache line holding the
> free lists but according to Huang, this is not a universal win.
> 
> One possibility is to move the zone lock to its own cache line but it
> increases the size of the zone. This patch moves the lock to the other
> end of the free lists where they do not contend under high pressure. It
> does mean the page allocator paths now require more cache lines but Huang
> reports that it restores performance to previous levels on large machines
> 
>              %stddev     %change         %stddev
>                  \          |                \
>          84568 \261  1%     +94.3%     164280 \261  1%  aim7.jobs-per-min
>        2881944 \261  2%     -35.1%    1870386 \261  8%  aim7.time.voluntary_context_switches
>            681 \261  1%      -3.4%        658 \261  0%  aim7.time.user_time
>        5538139 \261  0%     -12.1%    4867884 \261  0%  aim7.time.involuntary_context_switches
>          44174 \261  1%     -46.0%      23848 \261  1%  aim7.time.system_time
>            426 \261  1%     -48.4%        219 \261  1%  aim7.time.elapsed_time
>            426 \261  1%     -48.4%        219 \261  1%  aim7.time.elapsed_time.max
>            468 \261  1%     -43.1%        266 \261  2%  uptime.boot
> 
> Reported-and-tested-by: Huang Ying <ying.huang@intel.com>
> Signed-off-by: Mel Gorman <mgorman@suse.de>

Acked-by: David Rientjes <rientjes@google.com>

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