linux-kernel - Re: [PATCH] block: optimise bvec_iter

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Date:   Sat, 30 Nov 2019 12:22:27 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...onical.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: optimise bvec_iter_advance()

On 30/11/2019 02:24, Arvind Sankar wrote:
> On Sat, Nov 30, 2019 at 01:47:16AM +0300, Pavel Begunkov wrote:
>> On 30/11/2019 01:17, Arvind Sankar wrote:
>>>
>>> The loop can be simplified a bit further, as done has to be 0 once we go
>>> beyond the current bio_vec. See below for the simplified version.
>>>
>>
>> Thanks for the suggestion! I thought about it, and decided to not
>> for several reasons. I prefer to not fine-tune and give compilers
>> more opportunity to do their job. And it's already fast enough with
>> modern architectures (MOVcc, complex addressing, etc).
>>
>> Also need to consider code clarity and the fact, that this is inline,
>> so should be brief and register-friendly.
>>
> 
> It should be more register-friendly, as it uses fewer variables, and I
> think it's easier to see what the loop is doing, i.e. that we advance
> one bio_vec per iteration: in the existing code, it takes a bit of
> thinking to see that we won't spend more than one iteration within the
> same bio_vec.

Yeah, may be. It's more the matter of preference then. I don't think
it's simpler, and performance is entirely depends on a compiler and 
input. But, that's rather subjective and IMHO not worth of time.

Anyway, thanks for thinking this through!

> 
> I don't see this as fine-tuning, rather simplifying the code. I do agree
> that it's not going to make much difference for performance of the loop
> itself, as the most common case I think is that we either stay in the
> current bio_vec or advance by one.
> 
>>
>>> I also check if bi_size became zero so we can skip the rest of the
>>> calculations in that case. If we want to preserve the current behavior of
>>> updating iter->bi_idx and iter->bi_bvec_done even if bi_size is going to
>>> become zero, the loop condition should change to
>>>
>>> 		while (bytes && bytes >= cur->bv_len)
>>
>> Probably, it's better to leave it in a consistent state. Shouldn't be
>> a problem, but never know when and who will screw it up. 
>>
> 
> The WARN_ONCE case does leave it inconsistent, though that's not
> supposed to happen, so less of a pitfall there.
> 

But I hope, this WARN_ONCE won't ever happen, but I wouldn't be
suprised by code like:

last_page = (bv + iter->idx - 1)->page. 


-- 
Pavel Begunkov



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