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

Mathieu Desnoyers
2007-06-01T15:54:25Z

* 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.

(I am adding these results to the documentation)

Mathieu