Messages in this thread | | | Date | Fri, 1 Jun 2007 09:19:09 -0700 | From | Andrew Morton <> | Subject | Re: [patch 9/9] Scheduler profiling - Use conditional calls |
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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)
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