Re: perf-stat changes after "Use hrtimers for event multiplexing"

From: Peter Zijlstra
Date: Sat Jan 04 2014 - 14:03:16 EST

On Thu, Jan 02, 2014 at 02:12:42PM +0800, fengguang.wu@xxxxxxxxx wrote:
> Greetings,
>
> We noticed many perf-stat changes between commit 9e6302056f ("perf: Use
> hrtimers for event multiplexing") and its parent commit ab573844e.
> Are these expected changes?
>
> ab573844e3058ee 9e6302056f8029f438e853432
> --------------- -------------------------
> 152917 +842.9% 1441897 TOTAL interrupts.0:IO-APIC-edge.timer
> 545996 +478.0% 3155637 TOTAL interrupts.LOC
> 182281 +12.3% 204718 TOTAL softirqs.SCHED
> 1.986e+08 -96.4% 7105919 TOTAL perf-stat.node-store-misses
> 107241719 -99.7% 317525 TOTAL perf-stat.node-prefetch-misses
> 1.938e+08 -90.7% 17930426 TOTAL perf-stat.node-load-misses
> 2590 +247.8% 9009 TOTAL vmstat.system.in
> 4.549e+12 +158.3% 1.175e+13 TOTAL perf-stat.stalled-cycles-backend
> 6.807e+12 +149.1% 1.696e+13 TOTAL perf-stat.stalled-cycles-frontend
> 1.753e+08 -50.8% 86339289 TOTAL perf-stat.node-prefetches
> 8.326e+11 +45.0% 1.207e+12 TOTAL perf-stat.cpu-cycles
> 37932143 +32.2% 50146025 TOTAL perf-stat.iTLB-load-misses
> 4.738e+11 +30.1% 6.165e+11 TOTAL perf-stat.iTLB-loads
> 2.56e+11 +30.1% 3.33e+11 TOTAL perf-stat.L1-icache-loads
> 4.951e+11 +24.6% 6.169e+11 TOTAL perf-stat.instructions
> 7.85e+08 +7.5% 8.439e+08 TOTAL perf-stat.LLC-prefetch-misses
> 1.891e+12 +22.8% 2.322e+12 TOTAL perf-stat.ref-cycles
> 4.344e+08 -20.3% 3.462e+08 TOTAL perf-stat.node-loads
> 2.836e+11 +17.4% 3.328e+11 TOTAL perf-stat.branch-loads
> 9.506e+10 +24.5% 1.183e+11 TOTAL perf-stat.branch-load-misses
> 2.803e+11 +18.4% 3.319e+11 TOTAL perf-stat.branch-instructions
> 7.988e+10 +20.9% 9.658e+10 TOTAL perf-stat.bus-cycles
> 2.041e+09 +22.2% 2.495e+09 TOTAL perf-stat.branch-misses
> 229145 -17.3% 189601 TOTAL perf-stat.cpu-migrations
> 1.782e+11 +17.9% 2.1e+11 TOTAL perf-stat.dTLB-loads
> 4.702e+08 -14.8% 4.006e+08 TOTAL perf-stat.LLC-load-misses
> 1.418e+11 +17.4% 1.666e+11 TOTAL perf-stat.L1-dcache-loads
> 1.838e+09 +16.1% 2.133e+09 TOTAL perf-stat.LLC-stores
> 2.428e+09 +11.3% 2.702e+09 TOTAL perf-stat.LLC-loads
> 2.788e+11 +8.6% 3.029e+11 TOTAL perf-stat.dTLB-stores
> 8.66e+08 +10.8% 9.594e+08 TOTAL perf-stat.LLC-prefetches
> 1.117e+09 +10.5% 1.234e+09 TOTAL perf-stat.dTLB-store-misses
> 1.705e+09 +5.3% 1.796e+09 TOTAL perf-stat.L1-dcache-store-misses
> 5.671e+09 +6.1% 6.015e+09 TOTAL perf-stat.L1-dcache-load-misses
> 8.794e+10 +3.6% 9.109e+10 TOTAL perf-stat.L1-dcache-stores
> 3.46e+09 +4.6% 3.618e+09 TOTAL perf-stat.cache-references
> 8.696e+08 +1.8% 8.849e+08 TOTAL perf-stat.cache-misses
> 1613129 +2.6% 1655724 TOTAL perf-stat.context-switches
>
> All of the changes happen in one of our test box, which has a DX58SO
> baseboard and 4-core CPU. The boot dmesg and kconfig are attached.
> We can test more boxes if necessary.

How do you run perf stat? Curious that you notice this now, its a fairly
old commit.

IIRC we did have a few wobbles with that, but I cannot remember much
detail.

The biggest difference between before and after that patch is that we'd
rotate while the core is 'idle'. So if you do something like 'perf stat
-a' and have significant idle time it does indeed make a difference.
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