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

[Markus Armbruster]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 07/07/20 07:33, Markus Armbruster wrote:
>> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
>> 
>>> On 7/7/20 6:45 AM, 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!
>> 
>> That's surprising.
>
> It's a bit surprising indeed, but on the other hand using
> g_queue_insert_sorted results in a quadratic loop.

The surprising part is that n turns out to be large enough for n^2 to
matter *that* much.

>                                                     It should probably
> be fixed by using g_queue_sort, or switching to g_list_prepend+g_list_sort.

Yes.

Additional ideas on how to make the test less wasteful elsewhere in the
thread.