From: Minchan Kim <minchan@kernel.org>
To: "Huang, Ying" <ying.huang@intel.com>,
Andrew Morton <akpm@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Johannes Weiner <hannes@cmpxchg.org>,
Rik van Riel <riel@redhat.com>, Shaohua Li <shli@kernel.org>,
Hugh Dickins <hughd@google.com>,
Fengguang Wu <fengguang.wu@intel.com>,
Tim Chen <tim.c.chen@intel.com>,
Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH -mm -v4 3/5] mm, swap: VMA based swap readahead
Date: Wed, 13 Sep 2017 10:40:19 +0900 [thread overview]
Message-ID: <20170913014019.GB29422@bbox> (raw)
In-Reply-To: <20170807054038.1843-4-ying.huang@intel.com>
On Mon, Aug 07, 2017 at 01:40:36PM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@intel.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.
next prev parent reply other threads:[~2017-09-13 1:40 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-07 5:40 [PATCH -mm -v4 0/5] mm, swap: VMA based swap readahead Huang, Ying
2017-08-07 5:40 ` [PATCH -mm -v4 1/5] mm, swap: Add swap readahead hit statistics Huang, Ying
2017-08-09 21:50 ` Andrew Morton
2017-08-09 23:17 ` Huang, Ying
2017-08-07 5:40 ` [PATCH -mm -v4 2/5] mm, swap: Fix swap readahead marking Huang, Ying
2017-08-07 5:40 ` [PATCH -mm -v4 3/5] mm, swap: VMA based swap readahead Huang, Ying
2017-09-13 1:40 ` Minchan Kim [this message]
2017-09-13 21:02 ` Andrew Morton
2017-09-14 0:53 ` Huang, Ying
2017-09-14 8:15 ` Minchan Kim
2017-09-14 7:53 ` Minchan Kim
2017-09-14 12:01 ` Huang, Ying
2017-09-14 13:14 ` Minchan Kim
2017-09-14 21:21 ` Andrew Morton
2017-09-15 3:15 ` Huang, Ying
2017-09-15 3:42 ` Minchan Kim
2017-09-15 4:46 ` Huang, Ying
2017-08-07 5:40 ` [PATCH -mm -v4 4/5] mm, swap: Add sysfs interface for " Huang, Ying
2017-08-07 5:40 ` [PATCH -mm -v4 5/5] mm, swap: Don't use VMA based swap readahead if HDD is used as swap Huang, Ying
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