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

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 3.12.2015 v 11:36 Baolin Wang napsal(a):
> 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
>
>  From the data, we can find the bio based also can get the same
> performance as the request based. So if someone still don't like the
> request based things, I think we can optimize the bio based by
> expanding the scatterlists number. Thanks.
>


Hi

Do you see any performance impact if you use with cryptsetup options:

  --perf-same_cpu_crypt
  --perf-submit_from_crypt_cpus

with your regular unpatched kernel.

Zdenek