Re: [PATCH] block: optimise bvec_iter_advance()

[Jens Axboe]

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