linux-kernel - RE: [PATCH] perf, tools, bench: Fix memcpy benchmark for large sizes

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Date:	Mon, 15 Jul 2013 11:15:32 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	acme@...radead.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.jf.intel.com>, h.mitake@...il.com,
	kirill.shutemov@...ux.jf.intel.com
Subject: RE: [PATCH] perf, tools, bench: Fix memcpy benchmark for large sizes

Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.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.mitake@...il.com
> Cc: kirill.shutemov@...ux.intel.com
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.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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