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

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Date:   Wed, 18 Oct 2017 13:43:10 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Srividya Desireddy <srividya.dr@...sung.com>
Cc:     "sjenning\@redhat.com" <sjenning@...hat.com>,
        "ddstreet\@ieee.org" <ddstreet@...e.org>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "penberg\@kernel.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\@gmail.com" <srividya.desireddy@...il.com>
Subject: Re: [PATCH] zswap: Same-filled pages handling

Srividya Desireddy <srividya.dr@...sung.com> writes:
>
> On a ARM Quad Core 32-bit device with 1.5GB RAM by launching and
> relaunching different applications, out of ~64000 pages stored in
> zswap, ~11000 pages were same-value filled pages (including zero-filled
> pages) and ~9000 pages were zero-filled pages.

What are the values for the non zero cases?

> +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;
> +	}

So on 32bit it checks for 32bit repeating values and on 64bit
for 64bit repeating values. Does that make sense?

Did you test the patch on a 64bit system?

Overall I would expect this extra pass to be fairly expensive. It may
be better to add some special check to the compressor, and let
it abort if it sees a string of same values, and only do the check
then.

-Andi