linux-kernel - Re: [PATCH v2 1/2] iov_iter: optimise iov_iter

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Message-ID: <35d5db17-f6f6-ec32-944e-5ecddcbcb0f1@gmail.com>
Date:   Fri, 20 Nov 2020 01:39:05 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

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