Re: [patch 9/9] Scheduler profiling - Use conditional calls

Andrew Morton
2007-06-01T16:19:25Z

On Fri, 1 Jun 2007 11:54:13 -0400 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> * Andrew Morton (akpm@linux-foundation.org) wrote:
> > On Wed, 30 May 2007 10:00:34 -0400
> > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> >
> > > @@ -2990,7 +2991,8 @@
> > > print_irqtrace_events(prev);
> > > dump_stack();
> > > }
> > > - profile_hit(SCHED_PROFILING, __builtin_return_address(0));
> > > + cond_call(profile_on,
> > > + profile_hit(SCHED_PROFILING, __builtin_return_address(0)));
> > >
> >
> > That's looking pretty neat. Do you have any before-and-after performance
> > figures for i386 and for a non-optimised architecture?
>
> Sure, here is the result of a small test comparing:
> 1 - Branch depending on a cache miss (has to fetch in memory, caused by a 128
> bytes stride)). This is the test that is likely to look like what
> side-effect the original profile_hit code was causing, under the
> assumption that the kernel is already using L1 and L2 caches at
> their full capacity and that a supplementary data load would cause
> cache trashing.
> 2 - Branch depending on L1 cache hit. Just for comparison.
> 3 - Branch depending on a load immediate in the instruction stream.
>
> It has been compiled with gcc -O2. Tests done on a 3GHz P4.
>
> In the first test series, the branch is not taken:
>
> number of tests : 1000
> number of branches per test : 81920
> memory hit cycles per iteration (mean) : 48.252
> L1 cache hit cycles per iteration (mean) : 16.1693
> instruction stream based test, cycles per iteration (mean) : 16.0432
>
>
> In the second test series, the branch is taken and an integer is
> incremented within the block:
>
> number of tests : 1000
> number of branches per test : 81920
> memory hit cycles per iteration (mean) : 48.2691
> L1 cache hit cycles per iteration (mean) : 16.396
> instruction stream based test, cycles per iteration (mean) : 16.0441
>
> Therefore, the memory fetch based test seems to be 200% slower than the
> load immediate based test.

Confused. From what did you calculate that 200%?

> (I am adding these results to the documentation)

Good, thanks.