If the TSC is constant and non-stop, also set it reliable.
(We will turn this off in DMI quirks for multi-chassis systems)
The performance number on a 16-way Nehalem system running
32 tasks that context-switch between each other is significant:
sched_clock_stable=0 sched_clock_stable=1
.................... ....................
22.456925 million/sec 24.306972 million/sec [+8.2%]
lmbench's "lat_ctx -s 0 2" goes from 0.63 microseconds to
0.59 microseconds - a 6.7% increase in context-switching
performance.
Perfstat of 1 million pipe context switches between two tasks:
Performance counter stats for './pipe-test-1m':
[before] [after]
............ ............
37621.421089 36436.848378 task clock ticks (msecs)
0 0 CPU migrations (events)
2000274 2000189 context switches (events)
194 193 pagefaults (events)
8433799643 8171016416 CPU cycles (events) -3.21%
8370133368 8180999694 instructions (events) -2.31%
4158565 3895941 cache references (events) -6.74%
44312 46264 cache misses (events)
2349.287976 2279.362465 wall-time (msecs) -3.06%
The speedup comes straight from the reduction in the instruction
count. sched_clock_cpu() got simpler and the whole workload thus
executes faster.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
83ce400928