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@xxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
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