Andi Kleen wrote:
> From: Andi Kleen <a...@linux.intel.com>
>
> 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: h.mit...@gmail.com
> Cc: kirill.shute...@linux.intel.com
> Signed-off-by: Andi Kleen <a...@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shute...@linux.intel.com>
> ---
> 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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