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

[Paolo Bonzini <pbonzini@redhat.com>]

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

Paolo

>> Argh, yesterday I reviewed again all the range except this patch... not
>> sure why as looking at it now it is obvious.
>>
>>> 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?
>>
>> Suggestion: add a '-u' option for unsorted mode, to use in qtests.
>>
>> Humans want the new behavior (default: sorted).
> 
> Last resort.  
> 
> I'll look into speeding up the sort first.
> 
> Work-around: drop -m slow until we get this sorted (pardon the pun).
>