* Andrew Morton ([EMAIL PROTECTED]) wrote: > On Fri, 1 Jun 2007 11:54:13 -0400 Mathieu Desnoyers <[EMAIL PROTECTED]> wrote: > > > * Andrew Morton ([EMAIL PROTECTED]) wrote: > > > On Wed, 30 May 2007 10:00:34 -0400 > > > Mathieu Desnoyers <[EMAIL PROTECTED]> 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.
(48.2691-16.0441)/16.0441 = 2.00 Which means that it is 200% slower to run this test while fetching the branch condition from main memory rather than using the load immediate. We could also put it like this : the speedup of the load immediate over the memory fetch is 3. 48.2691/16.0441 = 3.00 Is there a preferred way to present these results in the documentation ? -- 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 [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
