On Sat, Aug 13, 2016 at 2:45 PM, Ingo Molnar <mi...@kernel.org> wrote:
>
> * Brian Gerst <brge...@gmail.com> wrote:
>
>> On Sat, Aug 13, 2016 at 1:16 PM, Linus Torvalds
>> <torva...@linux-foundation.org> wrote:
>> > On Sat, Aug 13, 2016 at 9:38 AM, Brian Gerst <brge...@gmail.com> wrote:
>> >> This patch set simplifies the switch_to() code, by moving the stack switch
>> >> code out of line into an asm stub before calling __switch_to(). This ends
>> >> up being more readable, and using the C calling convention instead of
>> >> clobbering all registers improves code generation. It also allows newly
>> >> forked processes to construct a special stack frame to seamlessly flow
>> >> to ret_from_fork, instead of using a test and branch, or an unbalanced
>> >> call/ret.
>> >
>> > Do you have performance numbers? Is it noticeable/measurable?
>>
>> How do I measure it? The perf documentation isn't easy to understand.
>
> Something like this:
>
> taskset 1 perf stat -a -e '{instructions,cycles}' --repeat 10 perf bench
> sched pipe
>
> ... will give a very good idea about the general impact of these changes on
> context switch overhead.
Before:
Performance counter stats for 'system wide' (10 runs):
12,010,932,128 instructions # 1.03 insn per
cycle ( +- 0.31% )
11,691,797,513 cycles
( +- 0.76% )
3.487329979 seconds time elapsed
( +- 0.78% )
After:
Performance counter stats for 'system wide' (10 runs):
12,097,706,506 instructions # 1.04 insn per
cycle ( +- 0.14% )
11,612,167,742 cycles
( +- 0.81% )
3.451278789 seconds time elapsed
( +- 0.82% )
The numbers with or without this patch series are roughly the same.
There is noticeable variation in the numbers each time I run it, so
I'm not sure how good of a benchmark this is.
--
Brian Gerst
