Re: [PATCH] block: optimise bvec_iter_advance()

[Jens Axboe <axboe@kernel.dk>]

On 11/30/19 10:56 AM, Arvind Sankar wrote:
> On Sat, Nov 30, 2019 at 12:22:27PM +0300, Pavel Begunkov wrote:
>> 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!
>>
> 
> You don't find listing 1 simpler than listing 2? It does save one
> register, as it doesn't have to keep track of done independently from
> bytes. This is always going to be the case unless the compiler can
> eliminate done by transforming Listing 2 into Listing 1. Unfortunately,
> even if it gets much smarter, it's unlikely to be able to do that,
> because they're equivalent only if there is no overflow, so it would
> need to know that bytes + iter->bi_bvec_done cannot overflow, and that
> iter->bi_bvec_done must be smaller than cur->bv_len initially.
> 
> Listing 1:
> 
> 	bytes += iter->bi_bvec_done;
> 	while (bytes) {
> 		const struct bio_vec *cur = bv + idx;
> 
> 		if (bytes < cur->bv_len)
> 			break;
> 		bytes -= cur->bv_len;
> 		idx++;
> 	}
> 
> 	iter->bi_idx = idx;
> 	iter->bi_bvec_done = bytes;
> 
> Listing 2:
> 
> 	while (bytes) {
> 		const struct bio_vec *cur = bv + idx;
> 		unsigned int len = min(bytes, cur->bv_len - done);
> 
> 		bytes -= len;
> 		done += len;
> 		if (done == cur->bv_len) {
> 			idx++;
> 			done = 0;
> 		}
> 	}
> 
> 	iter->bi_idx = idx;
> 	iter->bi_bvec_done = done;

Have yet to take a closer look (and benchmark) and the patches and
the generated code, but fwiw I do agree that case #1 is easier to
read.

-- 
Jens Axboe