linux-kernel - Re: [PATCH v4] mm: don't cap request size based on read-ahead setting

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Date:   Fri, 18 Nov 2016 08:09:55 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Hillf Danton <hillf.zj@...baba-inc.com>,
        'Andrew Morton' <akpm@...ux-foundation.org>,
        'Johannes Weiner' <hannes@...xchg.org>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        'Linus Torvalds' <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4] mm: don't cap request size based on read-ahead setting

On 11/17/2016 10:58 PM, Hillf Danton wrote:
> On Friday, November 18, 2016 5:23 AM Jens Axboe wrote:
>>
>> We ran into a funky issue, where someone doing 256K buffered reads saw
>> 128K requests at the device level. Turns out it is read-ahead capping
>> the request size, since we use 128K as the default setting. This doesn't
>> make a lot of sense - if someone is issuing 256K reads, they should see
>> 256K reads, regardless of the read-ahead setting, if the underlying
>> device can support a 256K read in a single command.
>>
>> To make matters more confusing, there's an odd interaction with the
>> fadvise hint setting. If we tell the kernel we're doing sequential IO on
>> this file descriptor, we can get twice the read-ahead size. But if we
>> tell the kernel that we are doing random IO, hence disabling read-ahead,
>> we do get nice 256K requests at the lower level. This is because
>> ondemand and forced read-ahead behave differently, with the latter doing
>> the right thing.
>
> As far as I read, forced RA is innocent but it is corrected below.
> And with RA disabled, we should drop care of ondemand.
>
> I'm scratching.

The changelog should have been updated. Forced read-ahead is also
affected, the patch is correct. We want to use the min of 'nr_to_read'
and the proper read-ahead request size, the latter being the max of
ra->ra_pages and bdi->io_pages.

-- 
Jens Axboe