Re: What differences and relations between SVM, HSA, HMM and Unified Memory?

[Yisheng Xie]

Hi Jean-Philippeï

On 2017/6/12 19:37, Jean-Philippe Brucker wrote:
> Hello,
> 
> On 10/06/17 05:06, Wuzongyong (Cordius Wu, Euler Dept) wrote:
>> Hi,
>>
>> Could someone explain differences and relations between the SVM(Shared
>> Virtual Memory, by Intel), HSA(Heterogeneous System Architecture, by AMD),
>> HMM(Heterogeneous Memory Management, by Glisse) and UM(Unified Memory, by
>> NVIDIA) ? Are these in the substitutional relation?
>>
>> As I understand it, these aim to solve the same thing, sharing pointers
>> between CPU and GPU(implement with ATS/PASID/PRI/IOMMU support). So far,
>> SVM and HSA can only be used by integrated gpu. And, Intel declare that
>> the root ports doesnât not have the required TLP prefix support, resulting
>>  that SVM canât be used by discrete devices. So could someone tell me the
>> required TLP prefix means what specifically?>
>> With HMM, we can use allocator like malloc to manage host and device
>> memory. Does this mean that there is no need to use SVM and HSA with HMM,
>> or HMM is the basis of SVM and HAS to implement Fine-Grained system SVM
>> defined in the opencl spec?
> 
> I can't provide an exhaustive answer, but I have done some work on SVM.
> Take it with a grain of salt though, I am not an expert.
> 
> * HSA is an architecture that provides a common programming model for CPUs
> and accelerators (GPGPUs etc). It does have SVM requirement (I/O page
> faults, PASID and compatible address spaces), though it's only a small
> part of it.
> 
> * Similarly, OpenCL provides an API for dealing with accelerators. OpenCL
> 2.0 introduced the concept of Fine-Grained System SVM, which allows to
> pass userspace pointers to devices. It is just one flavor of SVM, they
> also have coarse-grained and non-system. But they might have coined the
> name, and I believe that in the context of Linux IOMMU, when we talk about
> "SVM" it is OpenCL's fine-grained system SVM.
> [...]
> 
> While SVM is only about virtual address space,
As you mentioned, SVM is only about virtual address space, I'd like to know how to
manage the physical address especially about device's RAM, before HMM?

When OpenCL alloc a SVM pointer like:
    void* p = clSVMAlloc (
        context, // an OpenCL context where this buffer is available
        CL_MEM_READ_WRITE | CL_MEM_SVM_FINE_GRAIN_BUFFER,
        size, // amount of memory to allocate (in bytes)
        0 // alignment in bytes (0 means default)
    );

where this RAM come fromï device RAM or host RAM?

Thanks
Yisheng Xie

> HMM deals with physical
> storage. If I understand correctly, HMM allows to transparently use device
> RAM from userspace applications. So upon an I/O page fault, the mm
> subsystem will migrate data from system memory into device RAM. It would
> differ from "pure" SVM in that you would use different page directories on
> IOMMU and MMU sides, and synchronize them using MMU notifiers. But please
> don't take this at face value, I haven't had time to look into HMM yet.
> 
> Thanks,
> Jean
> 
> .
>