[Mesa-dev] [PATCH] gallium/util: don't let children of fork & exec inherit our thread affinity

[Michel Dänzer]

On 2018-10-28 11:27 a.m., Gustaw Smolarczyk wrote:
> pon., 17 wrz 2018 o 18:24 Michel Dänzer <michel at daenzer.net> napisał(a):
>>
>> On 2018-09-15 3:04 a.m., Marek Olšák wrote:
>>> On Fri, Sep 14, 2018 at 4:53 AM, Michel Dänzer <michel at daenzer.net> wrote:
>>>>
>>>> Last but not least, this doesn't solve the issue of apps such as
>>>> blender, which spawn their own worker threads after initializing OpenGL
>>>> (possibly not themselves directly, but via the toolkit or another
>>>> library; e.g. GTK+4 uses OpenGL by default), inheriting the thread affinity.
>>>>
>>>>
>>>> Due to these issues, setting the thread affinity needs to be disabled by
>>>> default, and only white-listed for applications where it's known safe
>>>> and beneficial. This sucks, but I'm afraid that's the reality until
>>>> there's better API available which allows solving these issues.
>>>
>>> We don't have the bandwidth to maintain whitelists. This will either
>>> have to be always on or always off.
>>>
>>> On the positive side, only Ryzens with multiple CCXs get all the
>>> benefits and disadvantages.
>>
>> In other words, only people who spent relatively large amounts of money
>> for relatively high-end CPUs will be affected (I'm sure they'll be glad
>> to know that "common people" aren't affected. ;). Affected applications
>> will see their performance decreased by a factor of 2-8 (the number of
>> CCXs in the CPU).
>>
>> OTOH, only a relatively small number of games will get a significant
>> benefit from the thread affinity, and the benefit will be smaller than a
>> factor of 2. This cannot justify risking a performance drop of up to a
>> factor of 8, no matter how small the risk.
>>
>> Therefore, the appropriate mechanism is a whitelist.
> 
> Hi,
> 
> What was the conclusion of this discussion? I don't see any
> whitelist/blacklist for this feature.
> 
> I have just tested blender and it still renders on only a single CCX
> on mesa from git master. Also, there is a bug report that suggests
> this regressed performance in at least one game [1].

I hooked up that bug report to the 18.3 blocker bug.


> If you think enabling it by default is the way to go, we should also
> implement a blacklist so that it can be turned off in such cases.

I stand by my opinion that a white-list is appropriate, not a
black-list. It's pretty much the same as mesa_glthread.


> [1] https://bugs.freedesktop.org/show_bug.cgi?id=108330


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer