linux-kernel - Re: [PATCH -mm -v4 3/5] mm, swap: VMA based swap readahead

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Date:   Wed, 13 Sep 2017 10:40:19 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...hat.com>, Shaohua Li <shli@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH -mm -v4 3/5] mm, swap: VMA based swap readahead

On Mon, Aug 07, 2017 at 01:40:36PM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@...el.com>
> 
> The swap readahead is an important mechanism to reduce the swap in
> latency.  Although pure sequential memory access pattern isn't very
> popular for anonymous memory, the space locality is still considered
> valid.
> 
> In the original swap readahead implementation, the consecutive blocks
> in swap device are readahead based on the global space locality
> estimation.  But the consecutive blocks in swap device just reflect
> the order of page reclaiming, don't necessarily reflect the access
> pattern in virtual memory.  And the different tasks in the system may
> have different access patterns, which makes the global space locality
> estimation incorrect.
> 
> In this patch, when page fault occurs, the virtual pages near the
> fault address will be readahead instead of the swap slots near the
> fault swap slot in swap device.  This avoid to readahead the unrelated
> swap slots.  At the same time, the swap readahead is changed to work
> on per-VMA from globally.  So that the different access patterns of
> the different VMAs could be distinguished, and the different readahead
> policy could be applied accordingly.  The original core readahead
> detection and scaling algorithm is reused, because it is an effect
> algorithm to detect the space locality.

Andrew,

Every zram users like low-end android device has used 0 page-cluster
to disable swap readahead because it has no seek cost and works as
synchronous IO operation so if we do readahead multiple pages,
swap falut latency would be (4K * readahead window size). IOW,
readahead is meaningful only if it doesn't bother faulted page's
latency.

However, this patch introduces additional knob /sys/kernel/mm/swap/
vma_ra_max_order as well as page-cluster. It means existing users
has used disabled swap readahead doesn't work until they should be
aware of new knob and modification of their script/code to disable
vma_ra_max_order as well as page-cluster.

I say it's a *regression* and wanted to fix it but Huang's opinion
is that it's not a functional regression so userspace should be fixed
by themselves.
Please look into detail of discussion in
http://lkml.kernel.org/r/%3C1505183833-4739-4-git-send-email-minchan@kernel.org%3E

The discussion is never productive so it's time to follow maintainer's
opinion. Could you share your opinion?

Thanks.