On 3 December 2015 at 23:49, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > 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. > OK. Thanks for your suggestions. > Mikulas -- Baolin.wang Best Regards -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
