linux-kernel - Re: [PATCH v1] tools/vm/page-types.c: remove memset() in walk

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Message-ID: <20160308055834.GA9987@hori1.linux.bs1.fc.nec.co.jp>
Date:	Tue, 8 Mar 2016 05:58:35 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
CC:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v1] tools/vm/page-types.c: remove memset() in walk_pfn()

On Tue, Mar 08, 2016 at 08:12:09AM +0300, Konstantin Khlebnikov wrote:
> On Tue, Mar 8, 2016 at 4:47 AM, Naoya Horiguchi
> <n-horiguchi@...jp.nec.com> wrote:
> > I found that page-types is very slow and my testing shows many timeout errors.
> > Here's an example with a simple program allocating 1000 thps.
> >
> >   $ time ./page-types -p $(pgrep -f test_alloc)
> >   ...
> >   real    0m17.201s
> >   user    0m16.889s
> >   sys     0m0.312s
> >
> >   $ time ./page-types.patched -p $(pgrep -f test_alloc)
> >   ...
> >   real    0m0.182s
> >   user    0m0.046s
> >   sys     0m0.135s
> >
> > Most of time is spent in memset(), which isn't necessary because we check
> > that the return of kpagecgroup_read() is equal to pages and uninitialized
> > memory is never used. So we can drop this memset().
> 
> These zeros are used in show_page_range() - for merging pages into ranges.

Hi Konstantin,

Thank you for the response. The below code does solve the problem, so that's fine.

But I don't understand how the zeros are used. show_page_range() is called
via add_page() which is called for i=0 to i=pages-1, and the buffer cgi is
already filled for the range [i, pages-1] by kpagecgroup_read(), so even if
without zero initialization, kpagecgroup_read() properly fills zeros, right?
IOW, is there any problem if we don't do this zero initialization?

Thanks,
Naoya Horiguchi

> You could add fast-path for count=1
> 
> @@ -633,7 +633,10 @@ static void walk_pfn(unsigned long voffset,
>         unsigned long pages;
>         unsigned long i;
> 
> -       memset(cgi, 0, sizeof cgi);
> +       if (count == 1)
> +               cgi[0] = 0;
> +       else
> +               memset(cgi, 0, sizeof cgi);
> 
>         while (count) {
>                 batch = min_t(unsigned long, count, KPAGEFLAGS_BATCH);
>