2013-07-15 08:12:52

by Kirill A. Shutemov

Subject: RE: [PATCH] perf, tools, bench: Fix memcpy benchmark for large sizes

Andi Kleen wrote:
> From: Andi Kleen <[email protected]>
>
> glibc calloc() has an optimization to not explicitely memset()
> very large calloc allocations that just came from mmap(),
> because they are known to be zero.
>
> This could result in the perf memcpy benchmark reading only from
> the zero page, which gives unrealistic results.
>
> Always call memset explicitly on the source area to avoid this problem.
>
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Andi Kleen <[email protected]>

Acked-by: Kirill A. Shutemov <[email protected]>

> ---
> tools/perf/bench/mem-memcpy.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/tools/perf/bench/mem-memcpy.c b/tools/perf/bench/mem-memcpy.c
> index 93c83e3..690f75f 100644
> --- a/tools/perf/bench/mem-memcpy.c
> +++ b/tools/perf/bench/mem-memcpy.c
> @@ -117,6 +117,8 @@ static void alloc_mem(void **dst, void **src, size_t length)
> *src = zalloc(length);
> if (!src)
> die("memory allocation failed - maybe length is too large?\n");
> + /* Make sure to always replace the zero pages even if MMAP_THRESH is crossed */
> + memset(src, 0, length);
> }
>
> static u64 do_memcpy_cycle(memcpy_t fn, size_t len, bool prefault)
> --
> 1.8.1.4

--
Kirill A. Shutemov

2013-07-15 12:16:20

by Hitoshi Mitake

Subject: Re: [PATCH] perf, tools, bench: Fix memcpy benchmark for large sizes

At Mon, 15 Jul 2013 11:15:32 +0300 (EEST),
Kirill A. Shutemov wrote:
>
> Andi Kleen wrote:
> > From: Andi Kleen <[email protected]>
> >
> > glibc calloc() has an optimization to not explicitely memset()
> > very large calloc allocations that just came from mmap(),
> > because they are known to be zero.
> >
> > This could result in the perf memcpy benchmark reading only from
> > the zero page, which gives unrealistic results.
> >
> > Always call memset explicitly on the source area to avoid this problem.
> >
> > Cc: [email protected]
> > Cc: [email protected]
> > Signed-off-by: Andi Kleen <[email protected]>
>
> Acked-by: Kirill A. Shutemov <[email protected]>

Thanks for this fix, Andi.
Acked-by: Hitoshi Mitake <[email protected]>

>
> > ---
> > tools/perf/bench/mem-memcpy.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/tools/perf/bench/mem-memcpy.c b/tools/perf/bench/mem-memcpy.c
> > index 93c83e3..690f75f 100644
> > --- a/tools/perf/bench/mem-memcpy.c
> > +++ b/tools/perf/bench/mem-memcpy.c
> > @@ -117,6 +117,8 @@ static void alloc_mem(void **dst, void **src, size_t length)
> > *src = zalloc(length);
> > if (!src)
> > die("memory allocation failed - maybe length is too large?\n");
> > + /* Make sure to always replace the zero pages even if MMAP_THRESH is crossed */
> > + memset(src, 0, length);
> > }
> >
> > static u64 do_memcpy_cycle(memcpy_t fn, size_t len, bool prefault)
> > --
> > 1.8.1.4
>
> --
> Kirill A. Shutemov
> --
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