Re: [PATCH] block: optimise bvec_iter_advance()

[Pavel Begunkov <asml.silence@gmail.com>]

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On 30/11/2019 21:57, Jens Axboe wrote:
> 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.
> 
Ok, ok, I'm not keen on bike-shedding. I'll resend a simplified version

-- 
Pavel Begunkov


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