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From: Pavel Begunkov <asml.silence@gmail.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec
Date: Fri, 20 Nov 2020 01:39:05 +0000	[thread overview]
Message-ID: <35d5db17-f6f6-ec32-944e-5ecddcbcb0f1@gmail.com> (raw)
In-Reply-To: <20201120012017.GJ29991@casper.infradead.org>

On 20/11/2020 01:20, Matthew Wilcox wrote:
> On Thu, Nov 19, 2020 at 11:24:38PM +0000, Pavel Begunkov wrote:
>> The block layer spends quite a while in iov_iter_npages(), but for the
>> bvec case the number of pages is already known and stored in
>> iter->nr_segs, so it can be returned immediately as an optimisation
> 
> Er ... no, it doesn't.  nr_segs is the number of bvecs.  Each bvec can
> store up to 4GB of contiguous physical memory.

Ah, really, missed min() with PAGE_SIZE in bvec_iter_len(), then it's a
stupid statement. Thanks!

Are there many users of that? All these iterators are a huge burden,
just to count one 4KB page in bvec it takes 2% of CPU time for me.

-- 
Pavel Begunkov

  reply	other threads:[~2020-11-20  1:42 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-19 23:24 [PATCH v2 0/2] optimise iov_iter Pavel Begunkov
2020-11-19 23:24 ` [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec Pavel Begunkov
2020-11-20  1:20   ` Matthew Wilcox
2020-11-20  1:39     ` Pavel Begunkov [this message]
2020-11-20  1:49       ` Matthew Wilcox
2020-11-20  1:56         ` Pavel Begunkov
2020-11-20  2:06           ` Matthew Wilcox
2020-11-20  2:08             ` Pavel Begunkov
2020-11-20  2:24             ` Ming Lei
2020-11-20 17:22               ` Pavel Begunkov
2020-11-20 17:23                 ` Pavel Begunkov
2020-11-20  2:22       ` Ming Lei
2020-11-20  2:25         ` Pavel Begunkov
2020-11-20  2:54           ` Matthew Wilcox
2020-11-20  8:14             ` Christoph Hellwig
2020-11-20 12:39               ` Matthew Wilcox
2020-11-20 13:00                 ` Pavel Begunkov
2020-11-20 13:13                   ` Matthew Wilcox
2020-11-20  9:57             ` Pavel Begunkov
2020-11-20 13:29   ` David Laight
2020-11-19 23:24 ` [PATCH v2 2/2] iov_iter: optimise iter type checking Pavel Begunkov

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