Re: [Intel-gfx] [PATCH 09/20] drm/i915/gem: Assign context id for async work

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 09/07/2020 12:07, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2020-07-09 12:01:29)
>>
>> On 08/07/2020 16:36, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2020-07-08 15:24:20)
>>>> And what is the effective behaviour you get with N contexts - emit N
>>>> concurrent operations and for N + 1 block in execbuf?
>>>
>>> Each context defines a timeline. A task is not ready to run until the
>>> task before it in its timeline is completed. So we don't block in
>>> execbuf, the scheduler waits until the request is ready before putting
>>> it into the HW queues -- i.e. the number chain of fences with everything
>>> that entails about ensuring it runs to completion [whether successfully
>>> or not, if not we then rely on the error propagation to limit the damage
>>> and report it back to the user if they kept a fence around to inspect].
>>
>> Okay but what is the benefit of N contexts in this series, before the
>> work is actually spread over ctx async width CPUs? Is there any? If not
>> I would prefer this patch is delayed until the time some actual
>> parallelism is ready to be added.
> 
> We currently submit an unbounded amount of work. This patch is added
> along with its user to restrict the amount of work allowed to run in
> parallel, and also is used to [crudely] serialise the multiple threads
> attempting to allocate space in the vm when we completely exhaust that
> address space. We need at least one fence-context id for each user, this
> took the opportunity to generalise that to N ids for each user.

Right, this is what I asked at the beginning - restricting amount of 
work run in parallel - does mean there is some "blocking"/serialisation 
during execbuf? Or it is all async but then what is restricted?

Regards,

Tvrtko


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