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

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Message-ID: <CAGqmi75Y9wbwBS0ZythcNF1gi6bW7g_XcuMDgLu=Nx4=pWC8Jw@mail.gmail.com>
Date:   Thu, 19 Oct 2017 00:31:18 +0300
From:   Timofey Titovets <nefelim4ag@...il.com>
To:     Srividya Desireddy <srividya.dr@...sung.com>
Cc:     "sjenning@...hat.com" <sjenning@...hat.com>,
        "ddstreet@...e.org" <ddstreet@...e.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "penberg@...nel.org" <penberg@...nel.org>,
        Dinakar Reddy Pathireddy <dinakar.p@...sung.com>,
        SHARAN ALLUR <sharan.allur@...sung.com>,
        RAJIB BASU <rajib.basu@...sung.com>,
        JUHUN KIM <juhunkim@...sung.com>,
        "srividya.desireddy@...il.com" <srividya.desireddy@...il.com>
Subject: Re: [PATCH] zswap: Same-filled pages handling

> +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.

> +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.