linux-kernel - Re: [RFC 0/2] nocopy bvec for direct IO

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Date:   Sun, 13 Dec 2020 22:03:44 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ming Lei <ming.lei@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC 0/2] nocopy bvec for direct IO

On 09/12/2020 16:53, Jens Axboe wrote:
> 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.

I'm curious, what block size did you use?

-- 
Pavel Begunkov