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

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Message-ID: <4dd79f37-c874-59b2-c37e-193ff0696131@kernel.dk>
Date:   Sat, 30 Nov 2019 12:56:02 -0800
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: optimise bvec_iter_advance()

On 11/30/19 12:11 PM, Pavel Begunkov wrote:
> 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

Sweet thanks. Make sure it's green.

-- 
Jens Axboe