linux-kernel - Re: [PATCH 0/2] Introduce the request handling for dm-crypt

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Message-ID: <CAMz4kuJ6x=Ygmr2JBL=PAxoQCCFggFFLa57Aw3Q3EDkKCu5Bvg@mail.gmail.com>
Date:	Fri, 13 Nov 2015 11:25:37 +0800
From:	Baolin Wang <baolin.wang@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <axboe@...nel.dk>,
	Alasdair G Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
	neilb@...e.com, tj@...nel.org, jmoyer@...hat.com,
	keith.busch@...el.com, bart.vanassche@...disk.com,
	linux-raid@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	"Garg, Dinesh" <dineshg@...cinc.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Introduce the request handling for dm-crypt

On 12 November 2015 at 20:57, Arnd Bergmann <arnd@...db.de> wrote:
> On Thursday 12 November 2015 20:51:10 Baolin Wang wrote:
>> On 12 November 2015 at 20:24, Jan Kara <jack@...e.cz> wrote:
>> > On Thu 12-11-15 19:46:26, Baolin Wang wrote:
>> >> On 12 November 2015 at 19:06, Jan Kara <jack@...e.cz> wrote:
>> >> > Well, one question is "can handle" and other question is how big gain in
>> >> > throughput it will bring compared to say 1M chunks. I suppose there's some
>> >> > constant overhead to issue a request to the crypto hw and by the time it is
>> >> > encrypting 1M it may be that this overhead is well amortized by the cost of
>> >> > the encryption itself which is in principle linear in the size of the
>> >> > block. That's why I'd like to get idea of the real numbers...
>> >>
>> >> Please correct me if I misunderstood your point. Let's suppose the AES
>> >> engine can handle 16M at one time. If we give the size of data is less
>> >> than 16M, the engine can handle it at one time. But if the data size
>> >> is 20M (more than 16M), the engine driver will split the data with 16M
>> >> and 4M to deal with. I can not say how many numbers, but I think the
>> >> engine is like to big chunks than small chunks which is the hardware
>> >> engine's advantage.
>> >
>> > No, I meant something different. I meant that if HW can encrypt 1M in say
>> > 1.05 ms and it can encrypt 16M in 16.05 ms, then although using 16 M blocks
>> > gives you some advantage it becomes diminishingly small.
>> >
>>
>> But if it encrypts 16M with 1M one by one, it will be much more than
>> 16.05ms (should be consider the SW submits bio one by one).
>
> The example that Jan gave was meant to illustrate the case where it's not
> much more than 16.05ms, just slightly more.
>
> The point is that we need real numbers to show at what size we stop
> getting significant returns from increased block sizes.
>

Got it. Thanks.

>> >> >> > You mentioned that you use requests because of size limitations on bios - I
>> >> >> > had a look and current struct bio can easily describe 1MB requests (that's
>> >> >> > assuming 64-bit architecture, 4KB pages) when we have 1 page worth of
>> >> >> > struct bio_vec. Is that not enough?
>> >> >>
>> >> >> Usually one bio does not always use the full 1M, maybe some 1k/2k/8k
>> >> >> or some other small chunks. But request can combine some sequential
>> >> >> small bios to be a big block and it is better than bio at least.
>> >> >
>> >> > As Christoph mentions 4.3 should be better in submitting larger bios. Did
>> >> > you check it?
>> >>
>> >> I'm sorry I didn't check it. What's the limitation of one bio on 4.3?
>> >
>> > On 4.3 it is 1 MB (which should be enough because requests are limited to
>> > 512 KB by default anyway). Previously the maximum bio size depended on the
>> > queue parameters such as max number of segments etc.
>>
>> But it maybe not enough for HW engine which can handle maybe 10M/20M
>> at one time.
>
> Given that you have already done measurements, can you find out how much
> you lose in overall performance with your existing patch if you artificially
> limit the maximum size to sizes like 256kb, 1MB, 4MB, ...?
>

Cause my board AES engine throughput is 1M, I just did a simple dd
test with small chunks. Results are in last email.

>         Arnd



-- 
Baolin.wang
Best Regards
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