[Libguestfs] FYI: perf commands I'm using to benchmark nbdcopy

Wed May 26 13:49:50 UTC 2021

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 ± σ):     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 ± σ):     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 ± σ):     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 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.

>  -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 ± σ):     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 ± σ):      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 ± σ):      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 ± σ):      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