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

[Baolin Wang <baolin.wang@linaro.org>]

On 3 December 2015 at 23:49, Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
> On Thu, 3 Dec 2015, Baolin Wang wrote:
>
>> On 3 December 2015 at 10:56, Baolin Wang <baolin.wang@linaro.org> wrote:
>> > On 3 December 2015 at 03:56, Alasdair G Kergon <agk@redhat.com> wrote:
>> >> On Wed, Dec 02, 2015 at 08:46:54PM +0800, Baolin Wang wrote:
>> >>> These are the benchmarks for request based dm-crypt. Please check it.
>> >>
>> >> Now please put request-based dm-crypt completely to one side and focus
>> >> just on the existing bio-based code.  Why is it slower and what can be
>> >> adjusted to improve this?
>> >>
>> >
>> > OK. I think I find something need to be point out.
>> > 1. From the IO block size test in the performance report, for the
>> > request based, we can find it can not get the corresponding
>> > performance if we just expand the IO size. Because In dm crypt, it
>> > will map the data buffer of one request with scatterlists, and send
>> > all scatterlists of one request to the encryption engine to encrypt or
>> > decrypt.  I found if the scatterlist list number is small and each
>> > scatterlist length is bigger, it will improve the encryption speed,
>> > that helps the engine palys best performance. But a big IO size does
>> > not mean bigger scatterlists (maybe many scatterlists with small
>> > length), that's why we can not get the corresponding performance if we
>> > just expand the IO size I think.
>> >
>> > 2. Why bio based is slower?
>> > If you understand 1, you can obviously understand the crypto engine
>> > likes bigger scatterlists to improve the performance. But for bio
>> > based, it only send one scatterlist (the scatterlist's length is
>> > always '1 << SECTOR_SHIFT' = 512) to the crypto engine at one time. It
>> > means if the bio size is 1M, the bio based will send 2048 times (evey
>> > time the only one scatterlist length is 512 bytes) to crypto engine to
>> > handle, which is more time-consuming and ineffective for the crypto
>> > engine. But for request based, it can map the whole request with many
>> > scatterlists (not just one scatterlist), and send all the scatterlists
>> > to the crypto engine which can improve the performance, is it right?
>> >
>> > Another optimization solution I think is we can expand the scatterlist
>> > entry number for bio based.
>> >
>>
>> I did some testing about my assumption of expanding the scatterlist
>> entry number for bio based. I did some modification for the bio based
>> to support multiple scatterlists, then it will get the same
>> performance as the request based things.
>>
>> 1. bio based with expanding the scatterlist entry
>> time dd if=/dev/dm-0 of=/dev/null bs=64K count=16384 iflag=direct
>> 1073741824 bytes (1.1 GB) copied, 94.5458 s, 11.4 MB/s
>> real    1m34.562s
>> user    0m0.030s
>> sys     0m3.850s
>>
>> 2. Sequential read 1G with requset based:
>> time dd if=/dev/dm-0 of=/dev/null bs=64K count=16384 iflag=direct
>> 1073741824 bytes (1.1 GB) copied, 94.8922 s, 11.3 MB/s
>> real    1m34.908s
>> user    0m0.030s
>> sys     0m4.000s
>
> Measuring the system time this way is completely wrong because it doesn't
> account for the time spent in kernel threads.
>

OK. Thanks for your suggestions.

> Mikulas



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Baolin.wang
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