Documentation about AMD's HSA implementation?

Tue Feb 13 21:58:59 UTC 2018

That's very helpful, thanks!

On Tue, Feb 13, 2018 at 4:17 PM, Felix Kuehling <felix.kuehling at amd.com> wrote:
> On 2018-02-13 04:06 PM, Ming Yang wrote:
>> Thanks for the suggestions!  But I might ask several specific
>> questions, as I can't find the answer in those documents, to give
>> myself a quick start if that's okay. Pointing me to the
>> files/functions would be good enough.  Any explanations are
>> appreciated.   My purpose is to hack it with different scheduling
>> policy with real-time and predictability consideration.
>>
>> - Where/How is the packet scheduler implemented?  How are packets from
>> multiple queues scheduled?  What about scheduling packets from queues
>> in different address spaces?
>
> This is done mostly in firmware. The CP engine supports up to 32 queues.
> We share those between KFD and AMDGPU. KFD gets 24 queues to use.
> Usually that is 6 queues times 4 pipes. Pipes are threads in the CP
> micro engine. Within each pipe the queues are time-multiplexed.

Please correct me if I'm wrong.  CP is computing processor, like the
Execution Engine in NVIDIA GPU. Pipe is like wavefront (warp)
scheduler multiplexing queues, in order to hide memory latency.

>
> If we need more than 24 queues, or if we have more than 8 processes, the
> hardware scheduler (HWS) adds another layer scheduling, basically
> round-robin between batches of 24 queues or 8 processes. Once you get
> into such an over-subscribed scenario your performance and GPU
> utilization can suffers quite badly.

HWS is also implemented in the firmware that's closed-source?

>
>>
>> - I noticed the new support of concurrency of multi-processes in the
>> archive of this mailing list.  Could you point me to the code that
>> implements this?
>
> That's basically just a switch that tells the firmware that it is
> allowed to schedule queues from different processes at the same time.
> The upper limit is the number of VMIDs that HWS can work with. It needs
> to assign a unique VMID to each process (each VMID representing a
> separate address space, page table, etc.). If there are more processes
> than VMIDs, the HWS has to time-multiplex.

HWS dispatch packets in their order of becoming the head of the queue,
i.e., being pointed by the read_index? So in this way it's FIFO.  Or
round-robin between queues? You mentioned round-robin over batches in
the over-subscribed scenario.

This might not be a big deal for performance, but it matters for
predictability and real-time analysis.

>
>>
>> - Also another related question -- where/how is the preemption/context
>> switch between packets/queues implemented?
>
> As long as you don't oversubscribe the available VMIDs, there is no real
> context switching. Everything can run concurrently. When you start
> oversubscribing HW queues or VMIDs, the HWS firmware will start
> multiplexing. This is all handled inside the firmware and is quite
> transparent even to KFD.

I see.  So the preemption in at least AMD's implementation is not
switching out the executing kernel, but just letting new kernels to
run concurrently with the existing ones.  This means the performance
is degraded when too many workloads are submitted.  The running
kernels leave the GPU only when they are done.

Is there any reason for not preempting/switching out the existing
kernel, besides context switch overheads?  NVIDIA is not providing
this option either.  Non-preemption hurts the real-time property in
terms of priority inversion.  I understand preemption should not be
massively used but having such an option may help a lot for real-time
systems.

>
> KFD interacts with the HWS firmware through the HIQ (HSA interface
> queue). It supports packets for unmapping queues, we can send it a new
> runlist (basically a bunch of map-process and map-queue packets). The
> interesting files to look at are kfd_packet_manager.c,
> kfd_kernel_queue_<hw>.c and kfd_device_queue_manager.c.
>

So in this way, if we want to implement different scheduling policy,
we should control the submission of packets to the queues in
runtime/KFD, before getting to the firmware.  Because it's out of
access once it's submitted to the HWS in the firmware.

Best,
Mark

> Regards,
>   Felix
>
>>
>> Thanks in advance!
>>
>> Best,
>> Mark
>>
>>> On 13 Feb 2018, at 2:56 PM, Felix Kuehling <felix.kuehling at amd.com> wrote:
>>> There is also this: https://gpuopen.com/professional-compute/, which
>>> give pointer to several libraries and tools that built on top of ROCm.
>>>
>>> Another thing to keep in mind is, that ROCm is diverging from the strict
>>> HSA standard in some important ways. For example the HSA standard
>>> includes HSAIL as an intermediate representation that gets finalized on
>>> the target system, whereas ROCm compiles directly to native GPU ISA.
>>>
>>> Regards,
>>>   Felix
>>>
>>> On Tue, Feb 13, 2018 at 9:40 AM, Deucher, Alexander <Alexander.Deucher at amd.com> wrote:
>>>> The ROCm documentation is probably a good place to start:
>>>>
>>>> https://rocm.github.io/documentation.html
>>>>
>>>>
>>>> Alex
>>>>
>>>> ________________________________
>>>> From: amd-gfx <amd-gfx-bounces at lists.freedesktop.org> on behalf of Ming Yang
>>>> <minos.future at gmail.com>
>>>> Sent: Tuesday, February 13, 2018 12:00 AM
>>>> To: amd-gfx at lists.freedesktop.org
>>>> Subject: Documentation about AMD's HSA implementation?
>>>>
>>>> Hi,
>>>>
>>>> I'm interested in HSA and excited when I found AMD's fully open-stack ROCm
>>>> supporting it. Before digging into the code, I wonder if there's any
>>>> documentation available about AMD's HSA implementation, either book,
>>>> whitepaper, paper, or documentation.
>>>>
>>>> I did find helpful materials about HSA, including HSA standards on this page
>>>> (http://www.hsafoundation.com/standards/) and a nice book about HSA
>>>> (Heterogeneous System Architecture A New Compute Platform Infrastructure).
>>>> But regarding the documentation about AMD's implementation, I haven't found
>>>> anything yet.
>>>>
>>>> Please let me know if there are ones publicly accessible. If no, any
>>>> suggestions on learning the implementation of specific system components,
>>>> e.g., queue scheduling.
>>>>
>>>> Best,
>>>> Mark
>