From: "Huang\, Ying" <ying.huang@intel.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, "Huang\,
Ying" <ying.huang@intel.com>, <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: Thu, 14 Sep 2017 20:01:30 +0800 [thread overview]
Message-ID: <87h8w5jxph.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <20170914075345.GA5533@bbox> (Minchan Kim's message of "Thu, 14 Sep 2017 16:53:45 +0900")
Minchan Kim <minchan@kernel.org> writes:
> On Wed, Sep 13, 2017 at 02:02:29PM -0700, Andrew Morton wrote:
>> On Wed, 13 Sep 2017 10:40:19 +0900 Minchan Kim <minchan@kernel.org> wrote:
>>
>> > 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
>>
>> hm, tricky problem. I do agree that linking the physical and virtual
>> readahead schemes in the proposed fashion is unfortunate. I also agree
>> that breaking existing setups (a bit) is also unfortunate.
>>
>> Would it help if, when page-cluster is written to zero, we do
>>
>> printk_once("physical readahead disabled, virtual readahead still
>> enabled. Disable virtual readhead via
>> /sys/kernel/mm/swap/vma_ra_max_order").
>>
>> Or something like that. It's pretty lame, but it should help alert the
>> zram-readahead-disabling people to the issue?
>
> It was my last resort. If we cannot find other ways after all, yes, it would
> be a minimum we should do. But it still breaks users don't/can't read/modify
> alert and program.
>
> How about this?
>
> Can't we make vma-based readahead config option?
> With that, users who no interest on readahead don't enable vma-based
> readahead. In this case, page-cluster works as expected "disable readahead
> completely" so it doesn't break anything.
Now. Users can choose between VMA based readahead and original
readahead via a knob as follow at runtime,
/sys/kernel/mm/swap/vma_ra_enabled
Best Regards,
Huang, Ying
> People who want to use upcoming vma-based readahead can enable the feature
> and we can say such unfortunate things in config/document description
> somewhere so upcoming users will be aware of that unforunate two knobs.
next prev parent reply other threads:[~2017-09-14 12:01 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
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 [this message]
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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