[Mesa-dev] EXTERNAL: Re: Clover clEnqueue* function don't implement blocking?

[Francisco Jerez]

"Dorrington, Albert" <albert.dorrington at lmco.com> writes:

>> -----Original Message-----
>> From: Francisco Jerez [mailto:currojerez at riseup.net]
>> 
>> "Dorrington, Albert" <albert.dorrington at lmco.com> writes:
>> 
>> >> -----Original Message-----
>> >> From: Francisco Jerez [mailto:currojerez at riseup.net]
>> >  > "Dorrington, Albert" <albert.dorrington at lmco.com> writes:
>> >> >
>> >> > From reading the OpenCL spec (and perhaps I'm misinterpreting
>> >> > something
>> >> again), section 5.10 Flush and Finish says:
>> >> >
>> >> > 	Any blocking commands queued in a command-queue such as
>> >> > 	clEnqueueRead{Image|Buffer} with blocking_read set to CL_TRUE,
>> >> > 	clEnqueueWrite{Image|Buffer} with blocking_write set to CL_TRUE,
>> >> > 	clEnqueueMap{Buffer|Image} with blocking_map set to CL_TRUE or
>> >> > 	clWaitForEvents perform an implicit flush of the command-queue.
>> >> >
>> >> > From this statement, I would expect that the command-queue would
>> >> > be flushed when the blocking flag is set.
>> >>
>> >> clEnqueueRead*, clEnqueueMap* and clWaitForEvents already flush the
>> >> command queue (the first two are flushing indirectly as we try to map
>> >> a buffer referenced by the GPU).  clEnqueueWrite* doesn't flush, but
>> >> it's not clear to me that not doing it can be considered a violation
>> >> of the spec.  The guarantees given by clFlush() are rather vague (to
>> >> some extent an empty function could be a valid implementation) and it
>> >> seems to me that a compliant implementation might, for instance,
>> >> choose to batch up commands across flushes if that's the most
>> >> efficient thing to do, as long as the user has no way to tell the difference.
>> >>
>> >> I'd like to see some real-world example where clover's behavior
>> >> represents a problem before we change it to flush more frequently,
>> >> because I'm worried that changing this will actually worsen performance
>> rather than improving it.
>> >
>> > I have been working with a modified version of Mesa code, which
>> > accepts kernels compiled with AMD's compiler.  (Our project's goal is
>> > to host Mesa in an environment which does not currently support
>> > LLVM/Clang or C++11)
>> >
>> > While testing 2D image read capabilities, I have been encountering an
>> > issue where the command queue's 'queued_events' continues to be
>> > populated, with none of the events being removed until the clFinish
>> > call. At that point, I have 23,328 events in the queue and encounter a
>> > segmentation fault during the command_queue flush.
>> >
>> It would be interesting to find out what's causing the segfault exactly, the
>> more frequent flushes might just be hiding a problem of different nature.
>> Also, is it the expected behavior of your test to queue so many events
>> before trying to read back the results or doing some other sort of blocking
>> operation?  Is its source code public?
>
> The test is the Khronos conformance test for 2D image reads, so unfortunately the code is not public.
>
> In the case where I had the 23,328 events in the queue, at least two dozen kernels has been compiled,
> and each kernel had been executed 6 times, with different input parameters.
> (I would have to back the changes out to get exact numbers)
>
> From all appearances, the queued_events queue was never being flushed until the clFinish() call.
>

And how exactly does that lead to a crash?

> Previously you mentioned that the clEnqueueRead*/Map* calls are implicitly flushing the command queue;
> I've looked through the code and just don't see where the queued_events queue is being flushed.

The clover::command_queue itself isn't flushed, but the context should
be flushed transparently by the pipe driver, so commands should have
started executing even if their corresponding CL events are still
hanging from the command queue.
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