* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Mon, 20 Aug 2012 16:52:29 +0300 > "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > > > Clearing a 2MB huge page will typically blow away several levels of CPU > > caches. To avoid this only cache clear the 4K area around the fault > > address and use a cache avoiding clears for the rest of the 2MB area. > > > > This patchset implements cache avoiding version of clear_page only for > > x86. If an architecture wants to provide cache avoiding version of > > clear_page it should to define ARCH_HAS_USER_NOCACHE to 1 and implement > > clear_page_nocache() and clear_user_highpage_nocache(). > > Patchset looks nice to me, but the changelogs are terribly > short of performance measurements. For this sort of change I > do think it is important that pretty exhaustive testing be > performed, and that the results (or a readable summary of > them) be shown. And that testing should be designed to probe > for slowdowns, not just the speedups! That is my general impression as well. Firstly, doing before/after "perf stat --repeat 3 ..." runs showing a statistically significant effect on a workload that is expected to win from this, and on a workload expected to be hurting from this would go a long way towards convincing me. Secondly, if you can find some user-space simulation of the intended positive (and negative) effects then a 'perf bench' testcase designed to show weakness of any such approach, running the very kernel assembly code in user-space would also be rather useful. See: comet:~/tip> git grep x86 tools/perf/bench/ | grep inclu tools/perf/bench/mem-memcpy-arch.h:#include "mem-memcpy-x86-64-asm-def.h" tools/perf/bench/mem-memcpy-x86-64-asm.S:#include "../../../arch/x86/lib/memcpy_64.S" tools/perf/bench/mem-memcpy.c:#include "mem-memcpy-x86-64-asm-def.h" tools/perf/bench/mem-memset-arch.h:#include "mem-memset-x86-64-asm-def.h" tools/perf/bench/mem-memset-x86-64-asm.S:#include "../../../arch/x86/lib/memset_64.S" tools/perf/bench/mem-memset.c:#include "mem-memset-x86-64-asm-def.h" that code uses the kernel-side assembly code and runs it in user-space. Although obviously clearing pages on page faults needs some care to properly simulate in user-space. Without repeatable hard numbers such code just gets into the kernel and bitrots there as new CPU generations come in - a few years down the line the original decisions often degrade to pure noise. We've been there, we've done that, we don't want to repeat it. Thanks, Ingo
