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Subject: Re: [RFC 0/2] nocopy bvec for direct IO
To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Ming Lei <ming.lei@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>
References: <cover.1607477897.git.asml.silence@gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <b165cd42-be79-69ed-ae06-a3f3ff633c62@kernel.dk>
Date:   Wed, 9 Dec 2020 09:53:49 -0700
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On 12/8/20 7:19 PM, Pavel Begunkov wrote:
> The idea is to avoid copying, merging, etc. bvec from iterator to bio
> in direct I/O and use the one we've already got. Hook it up for io_uring.
> Had an eye on it for a long, and it also was brought up by Matthew
> just recently. Let me know if I forgot or misplaced some tags.
> 
> A benchmark got me 430KIOPS vs 540KIOPS, or +25% on bare metal. And perf
> shows that bio_iov_iter_get_pages() was taking ~20%. The test is pretty
> silly, but still imposing. I'll redo it closer to reality for next
> iteration, anyway need to double check some cases.
> 
> If same applied to iomap, common chunck can be moved from block_dev
> into bio_iov_iter_get_pages(), but if there any benefit for filesystems,
> they should explicitly opt in with ITER_BVEC_FLAG_FIXED.

Ran this on a real device, and I get a 10% bump in performance with it.
That's pretty amazing! So please do pursue this one and pull it to
completion.

-- 
Jens Axboe