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

[Konstantin Khlebnikov <koct9i@gmail.com>]

On Tue, Mar 8, 2016 at 8:58 AM, Naoya Horiguchi
<n-horiguchi@ah.jp.nec.com> wrote:
> 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@ah.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?

kpagecgroup_read() reads only if kpagecgroup were opened,
/proc/kpagecgroup might even not exist. Probably it's better to fill
them with zeros here.
Pre-memset was an optimization - it fills buffer only once instead on
each kpagecgroup_read() call.

>
> 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);
>>

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