Re: [RFC] mm: optimise generic_file_read_iter

From: Pavel Begunkov <asml.silence@gmail.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC] mm: optimise generic_file_read_iter
Date: Sat, 7 Aug 2021 11:30:48 +0100	[thread overview]
Message-ID: <d6d36192-4afa-c8a5-5bc0-43bb667b7694@gmail.com> (raw)
In-Reply-To: <YQ09tqMda2ke2qHy@zeniv-ca.linux.org.uk>

On 8/6/21 2:48 PM, 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?

Answering for both patches -- I haven't benchmarked it and don't expect
to find anything just from this one, considering variance between runs.
I took a loot at the assembly (gcc 11.1), it removes 2 reads to get
i_size, write+read that i_size from stack, because it stashed it on
the stack.

For example, we've squeezed several percents of throughput before on
the io_uring side just by cutting sheer number of not too expensive
individually instructions. IMHO, it's easier to do when you spotted
something by the way, than rediscovering the same during a performance
safari.

-- 
Pavel Begunkov