From a368ab67aa55615a03b2c9c00fb965bee3ebeaa4 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Tue, 7 Apr 2015 14:26:41 -0700 Subject: [PATCH] mm: move zone lock to a different cache line than order-0 free page lists 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 Signed-off-by: Mel Gorman Reported-by: Huang Ying Tested-by: Huang Ying Acked-by: David Rientjes Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mmzone.h | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index f279d9c158cd..2782df47101e 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -474,16 +474,15 @@ struct zone { unsigned long wait_table_bits; ZONE_PADDING(_pad1_) - - /* Write-intensive fields used from the page allocator */ - spinlock_t lock; - /* free areas of different sizes */ struct free_area free_area[MAX_ORDER]; /* zone flags, see below */ unsigned long flags; + /* Write-intensive fields used from the page allocator */ + spinlock_t lock; + ZONE_PADDING(_pad2_) /* Write-intensive fields used by page reclaim */ -- 2.30.2