Messages in this thread | | | Date | Fri, 1 Jun 2007 11:54:13 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 9/9] Scheduler profiling - Use conditional calls |
| |
* 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
-- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |