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

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Message-ID: <3dc0b17d-b907-d829-bfec-eab96a6f4c30@gmail.com>
Date:   Fri, 20 Nov 2020 01:56:22 +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 01:49, Matthew Wilcox wrote:
> On Fri, Nov 20, 2020 at 01:39:05AM +0000, Pavel Begunkov wrote:
>> 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.
> 
> __bio_try_merge_page() will create multipage BIOs, and that's
> called from a number of places including
> bio_try_merge_hw_seg(), bio_add_page(), and __bio_iov_iter_get_pages()

I get it that there are a lot of places, more interesting how often
it's actually triggered and if that's performance critical for anybody.
Not like I'm going to change it, just out of curiosity, but bvec.h
can be nicely optimised without it.

> 
> so ... yeah, it's used a lot.
> 

-- 
Pavel Begunkov