Re: [Qemu-devel] [PATCH 0/2] improve tracing

[Lluís Vilanova]

Denis V Lunev writes:

> On 07/24/2017 05:43 PM, Lluís Vilanova wrote:
>> Denis V Lunev writes:
>> 
>>> On 07/24/2017 02:34 PM, Stefan Hajnoczi wrote:
>>>> On Fri, Jul 21, 2017 at 05:31:47PM +0300, Vladimir Sementsov-Ogievskiy 
>>>> wrote:
>>>>> Current trace system have a drawback: parameters of trace functions
>>>>> are calculated even if corresponding tracepoint is disabled. Also, it
>>>>> looks like trace function are not actually inlined by compiler (at
>>>>> least for me).
>>>>> 
>>>>> Here is a fix proposal: move from function call to macros. Patch 02
>>>>> is an example, of how to reduce extra calculations with help of
>>>>> patch 01.
>>>>> 
>>>>> Vladimir Sementsov-Ogievskiy (2):
>>>>> trace: do not calculate arguments for disabled trace-points
>>>>> monitor: improve tracing in handle_qmp_command
>>>> Please use the TRACE_FOO_ENABLED macro instead of putting computation
>>>> inside the trace event arguments.  This makes the code cleaner and
>>>> easier to read.
>>> At our opinion this ENABLED is compile time check while the option
>>> could be tuned in runtime. Thus normally it would normally be
>>> enabled while the trace is silent.
>>> So, under load, we will have extra allocation, copying the command buffer,
>>> freeing memory without actual trace. In order to fix that we should
>>> do something like
>>> if (trace_event_get_state(TRACE_HANDLE_QMP_COMMAND)) {
>>> req_json = qobject_to_json(req);
>>> trace_handle_qmp_command(mon, req_json);
>>> QDECREF(req_json);
>>> }
>>> which is possible, but at our (me + Vova) opinion is ugly.
>>> That is why we are proposing to switch to macro, which
>>> will not require such tweaking.
>>> Arguments will be only evaluated when necessary and we
>>> will not have side-effects if the tracepoint is compile time
>>> enabled and run-time disabled.
>>> Though if the code above is acceptable, we can send the
>>> patch with it. No problem.
>> I completely get your point, but:
>> 
>> * I'm not sure it will have much of a performance impact.
>> * It is not obvious what's going to happen just by looking at the code of the
>> calling site.
>> 
>> I prefer to minimize the use of macros, even if that makes a few trace event
>> calls to be a bit more verbose, as in your example above. Also, I quite 
>> dislike
>> the new style you propose:
>> 
>> trace_handle_qmp_command(mon,
>> qstring_get_str(req_json = qobject_to_json(req)));
>> QDECREF(req_json);
>> 
>> 
>> Cheers,
>> Lluis
> This is a matter of overall performance. For example I can have 500 VMs.
> In order to manage them, f.e. tweak balloon I have to collect statistics.
> This happens 1 time/10 sec/VM. Libvirt issues the following

> address@hidden:handle_qmp_command mon 0x7f7fbce6bea0 cmd_name "query-balloon"
> address@hidden:handle_qmp_command mon 0x7f7fbce6bea0 cmd_name "qom-get"
> address@hidden:handle_qmp_command mon 0x7f7fbce6bea0 cmd_name 
> "query-hotpluggable-cpus"
> address@hidden:handle_qmp_command mon 0x7f7fbce6bea0 cmd_name "query-cpus"
> address@hidden:handle_qmp_command mon 0x7f7fbce6bea0 cmd_name 
> "query-blockstats"
> address@hidden:handle_qmp_command mon 0x7f7fbce6bea0 cmd_name "query-block"

> We will have 300 commands in a second in all VMs. This is not that small
> load. OK. I do think that I'll lost 2-3-5 percents of one host CPU due
> to this allocation/free/copy. There are no measurements unfortunately.
> At my opinion this matters.

Sorry for beating the point, but I just want to make sure we're on the same
page. The example above (with the state check) and the one you propose in your
patch have exactly the same performance.

The change is then only in coding style, and I think the macros you propose make
the code harder to understand:

  trace_handle_qmp_command(mon,
                           qstring_get_str(req_json = qobject_to_json(req)));
  QDECREF(req_json);

If qobject_to_json() had any side-effect, it is not obvious why it would happen
only when tracing of that event is dynaically enabled. IMO that's a recipe for
errors.


Cheers,
  Lluis