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Subject: Re: [PATCH] block: optimise bvec_iter_advance()
Message-ID: <84a065b0-956c-460a-3575-260df7117fb8@gmail.com>
Date:   Sat, 30 Nov 2019 23:11:57 +0300
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From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, Arvind Sankar <nivedita@alum.mit.edu>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Message-ID: <84a065b0-956c-460a-3575-260df7117fb8@gmail.com>
Subject: Re: [PATCH] block: optimise bvec_iter_advance()
References: <cover.1574974574.git.asml.silence@gmail.com>
 <06b1b796b8d9bcaa6d5b325668525b7a5663035b.1574974574.git.asml.silence@gmail.com>
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 <20191130185634.GA1848835@rani.riverdale.lan>
 <42c183af-2fcf-7d72-70e1-a7a31ab541e5@kernel.dk>
In-Reply-To: <42c183af-2fcf-7d72-70e1-a7a31ab541e5@kernel.dk>

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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 inlin=
e,
>>>>> 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 advanc=
e
>>>> 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 t=
he
>>>> 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:
>>
>> =C2=A0=C2=A0=C2=A0=C2=A0bytes +=3D iter->bi_bvec_done;
>> =C2=A0=C2=A0=C2=A0=C2=A0while (bytes) {
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 const struct bio_vec *cur =3D=
 bv + idx;
>>
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (bytes < cur->bv_len)
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bre=
ak;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bytes -=3D cur->bv_len;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 idx++;
>> =C2=A0=C2=A0=C2=A0=C2=A0}
>>
>> =C2=A0=C2=A0=C2=A0=C2=A0iter->bi_idx =3D idx;
>> =C2=A0=C2=A0=C2=A0=C2=A0iter->bi_bvec_done =3D bytes;
>>
>> Listing 2:
>>
>> =C2=A0=C2=A0=C2=A0=C2=A0while (bytes) {
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 const struct bio_vec *cur =3D=
 bv + idx;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 unsigned int len =3D min(by=
tes, cur->bv_len - done);
>>
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 bytes -=3D len;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 done +=3D len;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (done =3D=3D cur->bv_len=
) {
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 idx=
++;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 don=
e =3D 0;
>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
>> =C2=A0=C2=A0=C2=A0=C2=A0}
>>
>> =C2=A0=C2=A0=C2=A0=C2=A0iter->bi_idx =3D idx;
>> =C2=A0=C2=A0=C2=A0=C2=A0iter->bi_bvec_done =3D done;
>=20
> 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.
>=20
Ok, ok, I'm not keen on bike-shedding. I'll resend a simplified version

--=20
Pavel Begunkov


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