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!
> 
> This causes our gitlab CI to constantly fail since the patch got merged,
> since the testing time now exceeds the 1h time limit:
> 
>  https://gitlab.com/qemu-project/qemu/-/pipelines/156767175
> 
> Sure, we can work around that problem in the CI (Alex has already a
> patch queued), but still, is there something you could do about this
> massive slowdown?

I think the answer is to stop using q_queue_insert_sorted(). The impl of
it looks like it is quadratic in complexity. Instead store the objects
in a plain array and then use qsort() at the end.

Regards,
Daniel
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