Re: [PATCH v2 04/12] i386: Register feature bit properties as class properties

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Dec 15, 2020 at 03:11:06PM +0100, Markus Armbruster wrote:
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
> > Class properties make QOM introspection simpler and easier, as
> > they don't require an object to be instantiated.
> >
> > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> 
> This is significantly more than just "simpler and easier".
> 
> The other day, I played with the QMP core to reduce its appetite for
> malloc.  I came up with patches that should approximately halve it, and
> felt quite pleased with myself.  I looked for a simple test to
> demonstrate the effect.  Something with plenty of output.  Hmm, why not
> query-cpu-definitions, it produces about 32KiB.  Instrument, instrument,
> run, ... whaaaat?!?
> 
> My patches save some 7000 allocations (670 KiB total), roughly matching
> my expectations.
> 
> Turns out this is a drop in the bucket: query-cpu-definitions still
> takes some 180,000 allocations (almost 12 MiB total).  They're hiding
> behind this line in qmp_query_cpu_definitions():
> 
>     g_slist_foreach(list, x86_cpu_definition_entry, &cpu_list);
> 
> The line takes more than a quarter second for me.
> 
> Hogging the main loop for a quarter second is not good.

Wow!

> 
> Eduardo's patch reduces run time to 0.02 seconds (40,000 allocations, 9
> MiB total).  It's a smaller pig now.

Thanks for investigating this!  I'll amend the commit message with:

  Also, the hundreds of instance properties were having an impact
  on QMP commands that create temporary CPU objects.  On my
  machine, run time of qmp_query_cpu_definitions() patch changed
  from ~200ms to ~16ms after applying this patch.

Numbers were obtained with:

 $ sudo perf probe -v -x ./qemu-system-x86_64 -a 'qmp_query_cpu_definitions%return' -a 'qmp_query_cpu_definitions'
 $ echo -e '{"execute": "qmp_capabilities"}\n{"execute": "query-cpu-definitions"}\n{"execute": "quit"}' | sudo perf trace -e 'probe_qemu:*' ./qemu-system-x86_64 -S -display none -qmp stdio > /dev/null

Before:

     0.000 qemu-system-x8/3103211 probe_qemu:qmp_query_cpu_definitions(__probe_ip: 94851767056275)
   204.072 qemu-system-x8/3103211 probe_qemu:qmp_query_cpu_definitions__return(__probe_func: 94851767056275, __probe_ret_ip: 94851768499362)

After:

     0.000 qemu-system-x8/3105969 probe_qemu:qmp_query_cpu_definitions(__probe_ip: 94554723186579)
    16.445 qemu-system-x8/3105969 probe_qemu:qmp_query_cpu_definitions__return(__probe_func: 94554723186579, __probe_ret_ip: 94554724629631)


-- 
Eduardo