Re: [PATCH RESEND 3/4] acpi, hmat: calculate abstract distance with HMAT

Huang, Ying
2023-07-25T06:50:16Z

Alistair Popple <apopple@nvidia.com> writes:

> Huang Ying <ying.huang@intel.com> writes:
>
>> A memory tiering abstract distance calculation algorithm based on ACPI
>> HMAT is implemented. The basic idea is as follows.
>>
>> The performance attributes of system default DRAM nodes are recorded
>> as the base line. Whose abstract distance is MEMTIER_ADISTANCE_DRAM.
>> Then, the ratio of the abstract distance of a memory node (target) to
>> MEMTIER_ADISTANCE_DRAM is scaled based on the ratio of the performance
>> attributes of the node to that of the default DRAM nodes.
>
> The problem I encountered here with the calculations is that HBM memory
> ended up in a lower-tiered node which isn't what I wanted (at least when
> that HBM is attached to a GPU say).

I have tested the series on a server machine with HBM (pure HBM, not
attached to a GPU). Where, HBM is placed in a higher tier than DRAM.

> I suspect this is because the calculations are based on the CPU
> point-of-view (access1) which still sees lower bandwidth to remote HBM
> than local DRAM, even though the remote GPU has higher bandwidth access
> to that memory. Perhaps we need to be considering access0 as well?
> Ie. HBM directly attached to a generic initiator should be in a higher
> tier regardless of CPU access characteristics?

What's your requirements for memory tiers on the machine? I guess you
want to put GPU attache HBM in a higher tier and put DRAM in a lower
tier. So, cold HBM pages can be demoted to DRAM when there are memory
pressure on HBM? This sounds reasonable from GPU point of view.

The above requirements may be satisfied via calculating abstract
distance based on access0 (or combined with access1). But I suspect
this will be a general solution. I guess that any memory devices that
are used mainly by the memory initiators other than CPUs want to put
themselves in a higher memory tier than DRAM, regardless of its
access0.

One solution is to put GPU HBM in the highest memory tier (with smallest
abstract distance) always in GPU device driver regardless its HMAT
performance attributes. Is it possible?

> That said I'm not entirely convinced the HMAT tables I'm testing against
> are accurate/complete.

--
Best Regards,
Huang, Ying