Re: RFC: Memory Tiering Kernel Interfaces

[Alistair Popple]

Wei Xu <weixugc@xxxxxxxxxx> writes:

> On Tue, May 10, 2022 at 4:38 AM Aneesh Kumar K.V
> <aneesh.kumar@xxxxxxxxxxxxx> wrote:
>>
>> Alistair Popple <apopple@xxxxxxxxxx> writes:
>>
>> > Wei Xu <weixugc@xxxxxxxxxx> writes:
>> >
>> >> On Thu, May 5, 2022 at 5:19 PM Alistair Popple <apopple@xxxxxxxxxx> wrote:
>> >>>
>> >>> Wei Xu <weixugc@xxxxxxxxxx> writes:
>> >>>
>> >>> [...]
>> >>>
>> >>> >> >
>> >>> >> >
>> >>> >> > Tiering Hierarchy Initialization
>> >>> >> > `=============================='
>> >>> >> >
>> >>> >> > By default, all memory nodes are in the top tier (N_TOPTIER_MEMORY).
>> >>> >> >
>> >>> >> > A device driver can remove its memory nodes from the top tier, e.g.
>> >>> >> > a dax driver can remove PMEM nodes from the top tier.
>> >>> >>
>> >>> >> With the topology built by firmware we should not need this.
>> >>>
>> >>> I agree that in an ideal world the hierarchy should be built by firmware based
>> >>> on something like the HMAT. But I also think being able to override this will be
>> >>> useful in getting there. Therefore a way of overriding the generated hierarchy
>> >>> would be good, either via sysfs or kernel boot parameter if we don't want to
>> >>> commit to a particular user interface now.
>> >>>
>> >>> However I'm less sure letting device-drivers override this is a good idea. How
>> >>> for example would a GPU driver make sure it's node is in the top tier? By moving
>> >>> every node that the driver does not know about out of N_TOPTIER_MEMORY? That
>> >>> could get messy if say there were two drivers both of which wanted their node to
>> >>> be in the top tier.
>> >>
>> >> The suggestion is to allow a device driver to opt out its memory
>> >> devices from the top-tier, not the other way around.
>> >
>> > So how would demotion work in the case of accelerators then? In that
>> > case we would want GPU memory to demote to DRAM, but that won't happen
>> > if both DRAM and GPU memory are in N_TOPTIER_MEMORY and it seems the
>> > only override available with this proposal would move GPU memory into a
>> > lower tier, which is the opposite of what's needed there.
>>
>> How about we do 3 tiers now. dax kmem devices can be registered to
>> tier 3. By default all numa nodes can be registered at tier 2 and HBM or
>> GPU can be enabled to register at tier 1. ?
>
> This makes sense.  I will send an updated RFC based on the discussions so far.

Thanks! The sense I got from LSF/MM was that we should initially try
keep things simple by limiting it to two tiers. However I don't think
there was strong opposition to adding a third tier to support
GPU+CPU+PMEM, and it does seem like it might even be simpler to just
have three tiers and assign devices as suggested.

>> -aneesh