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

On Tue, May 25, 2021 at 9:06 PM Richard W.M. Jones <rjones redhat com> wrote:
> I ran perf as below.  Although nbdcopy and nbdkit themselves do not
> require root (and usually should _not_ be run as root), in this case
> perf must be run as root, so everything has to be run as root.
>   # perf record -a -g --call-graph=dwarf ./nbdkit -U - sparse-random size=1T --run "MALLOC_CHECK_= ../libnbd/run nbdcopy \$uri \$uri"

This uses 64 requests with a request size of 32m. In my tests using
--requests 16 --request-size 1048576 is faster. Did you try to profile

> Some things to explain:
>  * The output is perf.data in the local directory.  This file may be
>    huge (22GB for me!)
>  * I am running this from the nbdkit directory, so ./nbdkit runs the
>    locally compiled copy of nbdkit.  This allows me to make quick
>    changes to nbdkit and see the effects immediately.
>  * I am running nbdcopy using "../libnbd/run nbdcopy", so that's from
>    the adjacent locally compiled libnbd directory.  Again the reason
>    for this is so I can make changes, recompile libnbd, and see the
>    effect quickly.
>  * "MALLOC_CHECK_=" is needed because of complicated reasons to do
>    with how the nbdkit wrapper enables malloc-checking.  We should
>    probably provide a way to disable malloc-checking when benchmarking
>    because it adds overhead for no benefit, but I've not done that yet
>    (patches welcome!)

Why enable malloc checking in nbdkit when profiling nbdcopy?

>  * The test harness is nbdkit-sparse-random-plugin, documented here:
>    https://libguestfs.org/nbdkit-sparse-random-plugin.1.html

Does it create a similar pattern to real world images, or more like
the worst case?

In my tests using nbdkit memory and pattern plugins was way more
stable compared with real images via qemu-nbd/nbdkit, but real image
give more real results :-)

Maybe we can extract the extents from a real image, and add a plugin
accepting json extents and inventing data for the data extents?

>  * I'm using DWARF debugging info to generate call stacks, which is
>    more reliable than the default (frame pointers).

When I tried to use perf, I did not get proper call stacks, maybe this
was the reason.

>  * The -a option means I'm measuring events on the whole machine.  You
>    can read the perf manual to find out how to measure only a single
>    process (eg. just nbdkit or just nbdcopy).  But actually measuring
>    the whole machine gives a truer picture, I believe.

Why profile the whole machine? I would profile only nbdcopy or nbdkit,
depending on what we are trying to focus on.

Looking in the attached flame graph, if we focus on the nbdcopy worker_thread,
and sort by time:

poll_both_ends: 14.53% (58%)
malloc: 5.55% (22%)
nbd_ops_async_read: 4.34% (17%)
nbd_ops_get_extents: 0.52% (2%)

If we focus into into poll_both_ends:
send 10.17% (69%)
free 4.53% (31%)

So we have a lot of opportunities to optimize by allocating all buffers up front
as done in examples/libev-copy. But I'm not sure we will would see the same
picture when using smaller buffers (--request-size 1m).

nbd_ops_async_read is surprising - this is an async operation that should
consume no time. Why does it take 17% of the time?

Thanks for the info!


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