linux-kernel - Re: [RFC] mm: optimise generic_file_read

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Message-ID: <cd31510a-2065-6078-468d-bbff816f818d@kernel.dk>
Date:   Fri, 6 Aug 2021 11:18:28 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Pavel Begunkov <asml.silence@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm: optimise generic_file_read_iter

On 8/6/21 7:48 AM, Al Viro wrote:
> On Fri, Aug 06, 2021 at 12:42:43PM +0100, Pavel Begunkov wrote:
>> Unless direct I/O path of generic_file_read_iter() ended up with an
>> error or a short read, it doesn't use inode. So, load inode and size
>> later, only when they're needed. This cuts two memory reads and also
>> imrpoves code generation, e.g. loads from stack.
> 
> ... and the same question here.
> 
>> NOTE: as a side effect, it reads inode->i_size after ->direct_IO(), and
>> I'm not sure whether that's valid, so would be great to get feedback
>> from someone who knows better.
> 
> Ought to be safe, I think, but again, how much effect have you observed
> from the patch?

Ran a quick test here, doing polled IO (~3.3M IOPS) and we reduce the
overhead of generic_file_read_iter() from 1.5% of the runtime to 1.2%.
Noticeable. Will improve once we stop digging into the inode on the
io_uring side.

Anyway, just one data point, perhaps Pavel has some too.


-- 
Jens Axboe