Nir Soffer
2021-May-26 13:49 UTC
[Libguestfs] FYI: perf commands I'm using to benchmark nbdcopy
On Wed, May 26, 2021 at 4:03 PM Richard W.M. Jones <rjones at redhat.com> wrote:> > On Wed, May 26, 2021 at 02:50:32PM +0300, Nir Soffer wrote: > > Basically all give very similar results. > > > > # hyperfine "./copy-libev $SRC $DST" "qemu-img convert -n -W -m 16 -S > > 1048576 $SRC $DST" "../copy/nbdcopy --sparse=1048576 > > --request-size=1048576 --flush --requests=16 --connections=1 $SRC > > $DST" > > Benchmark #1: ./copy-libev nbd+unix:///?socket=/tmp/src.sock > > nbd+unix:///?socket=/tmp/dst.sock > > Time (mean ? ?): 103.514 s ? 0.836 s [User: 7.153 s, System: 19.422 s] > > Range (min ? max): 102.265 s ? 104.824 s 10 runs > > > > Benchmark #2: qemu-img convert -n -W -m 16 -S 1048576 > > nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock > > Time (mean ? ?): 103.104 s ? 0.899 s [User: 2.897 s, System: 25.204 s] > > Range (min ? max): 101.958 s ? 104.499 s 10 runs > > > > Benchmark #3: ../copy/nbdcopy --sparse=1048576 --request-size=1048576 > > --flush --requests=16 --connections=1 > > nbd+unix:///?socket=/tmp/src.sock nbd+unix:///?socket=/tmp/dst.sock > > Time (mean ? ?): 104.085 s ? 0.977 s [User: 7.188 s, System: 19.965 s] > > Range (min ? max): 102.314 s ? 105.153 s 10 runs > > In my testing, nbdcopy is a clear 4x faster than qemu-img convert, with > 4 also happening to be the default number of connections/threads. > Why use nbdcopy --connections=1? That completely disables threads in > nbdcopy.Because qemu-nbd does not report multicon when writing, so practically you get one nbd handle for writing.> Also I'm not sure if --flush is fair (it depends on what > qemu-img does, which I don't know).qemu is flushing at the end of the operation. Not flushing is cheating :-)> The other interesting things are the qemu-img convert flags you're using: > > -m 16 number of coroutines, default is 8We use 8 in RHV since the difference is very small, and when running concurrent copies it does not matter. Since we use up to 64 concurrent requests in nbdcopy, it is useful to compare similar setup in qemu.> -W out of order writes, but the manual says "This is only recommended > for preallocated devices like host devices or other raw block > devices" which is a very unclear recommendation to me. > What's special about host devices versus (eg) files or > qcow2 files which means -W wouldn't always be recommended?This is how RHV use qemu-img convert when copying to raw preallocated volumes. Using -W can be up to 6x times faster. We use the same for imageio for any type of disk. This is the reason I tested this way. -W is equivalent to the nbdocpy multithreaded copy using a single connection. qemu-img does N concurrent reads. If you don't specify -W, it writes the data in the right order (based on offset). If a read did not finish, the copy loops waits until the read complets before writing. This ensure exactly one concurrent write, and it is much slower.> Anyway I tried various settings to see if I could improve the > performance of qemu-img convert vs nbdcopy using the sparse-random > test harness. The results seem to confirm what has been said in this > thread so far. > > libnbd-1.7.11-1.fc35.x86_64 > nbdkit-1.25.8-2.fc35.x86_64 > qemu-img-6.0.0-1.fc35.x86_64 > > $ hyperfine 'nbdkit -U - sparse-random size=100G --run "qemu-img convert \$uri \$uri"' 'nbdkit -U - sparse-random size=100G --run "qemu-img convert -m 16 -W \$uri \$uri"' 'nbdkit -U - sparse-random size=100G --run "nbdcopy \$uri \$uri"' 'nbdkit -U - sparse-random size=100G --run "nbdcopy --request-size=1048576 --requests=16 \$uri \$uri"' > Benchmark #1: nbdkit -U - sparse-random size=100G --run "qemu-img convert \$uri \$uri" > Time (mean ? ?): 17.245 s ? 1.004 s [User: 28.611 s, System: 7.219 s] > Range (min ? max): 15.711 s ? 18.930 s 10 runs > > Benchmark #2: nbdkit -U - sparse-random size=100G --run "qemu-img convert -m 16 -W \$uri \$uri" > Time (mean ? ?): 8.618 s ? 0.266 s [User: 33.091 s, System: 7.331 s] > Range (min ? max): 8.130 s ? 8.943 s 10 runs > > Benchmark #3: nbdkit -U - sparse-random size=100G --run "nbdcopy \$uri \$uri" > Time (mean ? ?): 5.227 s ? 0.153 s [User: 34.299 s, System: 30.136 s] > Range (min ? max): 5.049 s ? 5.439 s 10 runs > > Benchmark #4: nbdkit -U - sparse-random size=100G --run "nbdcopy --request-size=1048576 --requests=16 \$uri \$uri" > Time (mean ? ?): 4.198 s ? 0.197 s [User: 32.105 s, System: 24.562 s] > Range (min ? max): 3.868 s ? 4.474 s 10 runs > > Summary > 'nbdkit -U - sparse-random size=100G --run "nbdcopy --request-size=1048576 --requests=16 \$uri \$uri"' ran > 1.25 ? 0.07 times faster than 'nbdkit -U - sparse-random size=100G --run "nbdcopy \$uri \$uri"' > 2.05 ? 0.12 times faster than 'nbdkit -U - sparse-random size=100G --run "qemu-img convert -m 16 -W \$uri \$uri"' > 4.11 ? 0.31 times faster than 'nbdkit -U - sparse-random size=100G --run "qemu-img convert \$uri \$uri"'This shows that nbdcopy works better when the latency is (practically) zero, copying data from memory to memory. This is useful for minimizing overhead in nbdcopy, but when copying real images with real storage with direct I/O the time to write the data to storage hides everything else. Would it be useful to add latency in the sparse-random plugin, so it behaves more like real storage? (or maybe it is already possible with a filter?)> > ## Compare nbdcopy request size with 16 requests and one connection > > This is testing 4 connections I think? Or is the destination not > advertising multi-conn? > > > ## Compare number of requests with multiple connections > > > > To enable multiple connections to the destination, I hacked nbdcopy > > to ignore the the destination can_multicon always use multiple > > connections. This is how we use qemu-nbd with imageio in RHV. > > So qemu-nbd doesn't advertise multi-conn? I'd prefer if we fixed qemu-nbd.Right, but according to Eric, advertising multi-conn is not safe with current code. But he confirmed that using multiple writers writing to distinct ranges is safe, so nbdcopy can use this now for extra 10% performance in real copies. Nir
Richard W.M. Jones
2021-May-26 14:15 UTC
[Libguestfs] FYI: perf commands I'm using to benchmark nbdcopy
On Wed, May 26, 2021 at 04:49:50PM +0300, Nir Soffer wrote:> On Wed, May 26, 2021 at 4:03 PM Richard W.M. Jones <rjones at redhat.com> wrote: > > In my testing, nbdcopy is a clear 4x faster than qemu-img convert, with > > 4 also happening to be the default number of connections/threads. > > Why use nbdcopy --connections=1? That completely disables threads in > > nbdcopy. > > Because qemu-nbd does not report multicon when writing, so practically > you get one nbd handle for writing.Let's see if we can fix that. Crippling nbdcopy because of a missing feature in qemu-nbd isn't right. I wonder what Eric's reasoning for multi-conn not being safe is.> > Also I'm not sure if --flush is fair (it depends on what > > qemu-img does, which I don't know). > > qemu is flushing at the end of the operation. Not flushing is cheating :-)That's fair enough. I will add that flag to my future tests. I also pushed these commits to disable malloc checking outside tests: https://gitlab.com/nbdkit/libnbd/-/commit/88e72dcb1631b315957f5f98e3cdfcdd1fd0fe29 https://gitlab.com/nbdkit/nbdkit/-/commit/6039780f3bb0617650fa1fa4c1399b0d3f1dcb26> > The other interesting things are the qemu-img convert flags you're using: > > > > -m 16 number of coroutines, default is 8 > > We use 8 in RHV since the difference is very small, and when running > concurrent copies it does not matter. Since we use up to 64 concurrent > requests in nbdcopy, it is useful to compare similar setup in qemu.I'm not really clear on the relationship (in qemu-img) between number of coroutines, number of pthreads and number of requests in flight. At this rate I'm going to have to look at the source :-)> > -W out of order writes, but the manual says "This is only recommended > > for preallocated devices like host devices or other raw block > > devices" which is a very unclear recommendation to me. > > What's special about host devices versus (eg) files or > > qcow2 files which means -W wouldn't always be recommended? > > This is how RHV use qemu-img convert when copying to raw preallocated > volumes. Using -W can be up to 6x times faster. We use the same for imageio > for any type of disk. This is the reason I tested this way. > > -W is equivalent to the nbdocpy multithreaded copy using a single connection. > > qemu-img does N concurrent reads. If you don't specify -W, it writes > the data in the right order (based on offset). If a read did not > finish, the copy loops waits until the read complets before > writing. This ensure exactly one concurrent write, and it is much > slower.Thanks - interesting. Still not sure why you wouldn't want to use this flag all the time. See also: https://lists.nongnu.org/archive/html/qemu-discuss/2021-05/msg00070.html ...> This shows that nbdcopy works better when the latency is > (practically) zero, copying data from memory to memory. This is > useful for minimizing overhead in nbdcopy, but when copying real > images with real storage with direct I/O the time to write the data > to storage hides everything else. > > Would it be useful to add latency in the sparse-random plugin, so it > behaves more like real storage? (or maybe it is already possible > with a filter?)We could use one of these filters: https://libguestfs.org/nbdkit-delay-filter.1.html https://libguestfs.org/nbdkit-rate-filter.1.html Something like "--filter=delay wdelay=1ms" might be more realistic. To simulate NVMe we might need to be able to specify microseconds there. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/