Re: Slow down with: 'Make "info qom-tree" show children sorted'

[Daniel P . Berrangé]

On Tue, Jul 07, 2020 at 06:45:57AM +0200, Thomas Huth wrote:
> On 27/05/2020 10.47, Markus Armbruster wrote:
> > "info qom-tree" prints children in unstable order.  This is a pain
> > when diffing output for different versions to find change.  Print it
> > sorted.
> > 
> > Signed-off-by: Markus Armbruster <armbru@redhat.com>
> > ---
> >  qom/qom-hmp-cmds.c | 24 ++++++++++++++++--------
> >  1 file changed, 16 insertions(+), 8 deletions(-)
> 
>  Hi Markus,
> 
> this patch causes a slow down of the qtests which becomes quite massive
> when e.g. using the ppc64 and thourough testing. When I'm running
> 
> QTEST_QEMU_BINARY="ppc64-softmmu/qemu-system-ppc64" time \
> ./tests/qtest/device-introspect-test -m slow | tail -n 10
> 
> the test runs for ca. 6m40s here before the patch got applied, and for
> mor than 20 minutes after the patch got applied!

I think the test case itself could be optimized. Currently it does
approx

   for each device type
      info qom-tree
      device_addr type,help
      info qom-tree

it compares the before/after qom-tree to look for stray objects or
to try to trigger crashes.

The info qom-tree calls could be pushed outside the loop

   info qom-tree
   for each device type
      device_addr type,help
   info qom-tree

Taking /x86_64/device/introspect/concrete/defaults/pc-q35-5.1 as a
example, this change is the difference between 20 seconds running and
3 seconds running.

Reverting Markus' change actually didn't make much difference, only
reducing the 20 seconds to 17 seconds.

The downside is that if there is a stray object/crash, it would not
immediately associate with the device type. I'm not sure that's a
real problem though. Especially if we are running this as pre-merge
CI we'll only need to look at the patch series to find the broken
device. If this is quick enough that we can run it as standard,
instead of only with -m slow, then its a net win I think.


Regards,
Daniel
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