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

[Mikulas Patocka]

On Thu, 3 Dec 2015, Baolin Wang wrote:

> On 3 December 2015 at 10:56, Baolin Wang <baolin.wang@xxxxxxxxxx> wrote:
> > On 3 December 2015 at 03:56, Alasdair G Kergon <agk@xxxxxxxxxx> 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.

Mikulas
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