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

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Date:	Mon, 15 Jul 2013 21:16:07 +0900
From:	Hitoshi Mitake <mitake.hitoshi@...il.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Andi Kleen <andi@...stfloor.org>, 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

At Mon, 15 Jul 2013 11:15:32 +0300 (EEST),
Kirill A. Shutemov wrote:
> 
> 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>

Thanks for this fix, Andi.
Acked-by: Hitoshi Mitake <mitake.hitoshi@....ntt.co.jp>

> 
> > ---
> >  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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