Re: [PATCH] zswap: Same-filled pages handling

[Dan Streetman]

On Wed, Oct 18, 2017 at 5:31 PM, Timofey Titovets <nefelim4ag@xxxxxxxxx> wrote:
>> +static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
>> +{
>> +       unsigned int pos;
>> +       unsigned long *page;
>> +
>> +       page = (unsigned long *)ptr;
>> +       for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) {
>> +               if (page[pos] != page[0])
>> +                       return 0;
>> +       }
>> +       *value = page[0];
>> +       return 1;
>> +}
>> +
>
> In theory you can speedup that check by memcmp(),
> And do something like first:
> memcmp(ptr, ptr + PAGE_SIZE/sizeof(*page)/2, PAGE_SIZE/2);
> After compare 1/4 with 2/4
> Then 1/8 with 2/8.
> And after do you check with pattern, only on first 512 bytes.
>
> Just because memcmp() on fresh CPU are crazy fast.
> That can easy make you check less expensive.

I did check this, and it is actually significantly worse; keep in mind
that doing it ^ way may is a smaller loop, but is actually doing more
memory comparisons.

>
>> +static void zswap_fill_page(void *ptr, unsigned long value)
>> +{
>> +       unsigned int pos;
>> +       unsigned long *page;
>> +
>> +       page = (unsigned long *)ptr;
>> +       if (value == 0)
>> +               memset(page, 0, PAGE_SIZE);
>> +       else {
>> +               for (pos = 0; pos < PAGE_SIZE / sizeof(*page); pos++)
>> +                       page[pos] = value;
>> +       }
>> +}
>
> Same here, but with memcpy().
>
> P.S.
> I'm just too busy to make fast performance test in user space,
> but my recent experience with that CPU commands, show what that make a sense:
> KSM patch: https://patchwork.kernel.org/patch/9980803/
> User space tests: https://github.com/Nefelim4ag/memcmpe
> PAGE_SIZE: 65536, loop count: 1966080
> memcmp:  -28                    time: 3216 ms,  th: 40064.644611 MiB/s
> memcmpe: -28, offset: 62232     time: 3588 ms,  th: 35902.462390 MiB/s
> memcmpe: -28, offset: 62232     time: 71 ms,    th: 1792233.164286 MiB/s
>
> IIRC, with code like our, you must see ~2.5GiB/s
>
> Thanks.
> --
> Have a nice day,
> Timofey.