2013-07-16 10:08:11

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]>
> ---
> 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);

It should be memset(*src, 0, length) instead.

There's fix for wrong memory allocation fail check in v3.11-rc1.

--
Kirill A. Shutemov

2013-07-16 13:14:58

by Hitoshi Mitake

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

At Tue, 16 Jul 2013 13:11:00 +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]>
> > ---
> > 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);
>
> It should be memset(*src, 0, length) instead.
>
> There's fix for wrong memory allocation fail check in v3.11-rc1.

I couldn't catch it, thanks. The memset() should be fixed.

Thanks,
Hitoshi

2013-07-16 14:20:19

by Andi Kleen

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

> > @@ -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);
>
> It should be memset(*src, 0, length) instead.

Thanks for catching. I wonder why it fixed the problem for me.

> There's fix for wrong memory allocation fail check in v3.11-rc1.

Doubt it matters.

-Andi

--
[email protected] -- Speaking for myself only