Re: [RFC] mm: optimise generic_file_read_iter

From: Jens Axboe <axboe@kernel.dk>
To: Al Viro <viro@zeniv.linux.org.uk>,
	Pavel Begunkov <asml.silence@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC] mm: optimise generic_file_read_iter
Date: Fri, 6 Aug 2021 11:18:28 -0600	[thread overview]
Message-ID: <cd31510a-2065-6078-468d-bbff816f818d@kernel.dk> (raw)
In-Reply-To: <YQ09tqMda2ke2qHy@zeniv-ca.linux.org.uk>

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