Re: RFC: Memory Tiering Kernel Interfaces

Hesham Almatary
2022-05-11T09:05:40Z

On Wed, 11 May 2022 17:12:34 +1000
Alistair Popple <apopple@nvidia.com> wrote:

>
> Wei Xu <weixugc@google.com> writes:
>
> > On Tue, May 10, 2022 at 5:10 AM Aneesh Kumar K V
> > <aneesh.kumar@linux.ibm.com> wrote:
> >>
> >> On 5/10/22 3:29 PM, Hesham Almatary wrote:
> >> > Hello Yang,
> >> >
> >> > On 5/10/2022 4:24 AM, Yang Shi wrote:
> >> >> On Mon, May 9, 2022 at 7:32 AM Hesham Almatary
> >> >> <hesham.almatary@huawei.com> wrote:
> >>
> >>
> >> ...
> >>
> >> >>>
> >> >>> node 0 has a CPU and DDR memory in tier 0, node 1 has GPU and
> >> >>> DDR memory in tier 0,
> >> >>> node 2 has NVMM memory in tier 1, node 3 has some sort of
> >> >>> bigger memory (could be a bigger DDR or something) in tier 2.
> >> >>> The distances are as follows:
> >> >>>
> >> >>> -------------- --------------
> >> >>> | Node 0 | | Node 1 |
> >> >>> | ------- | | ------- |
> >> >>> | | DDR | | | | DDR | |
> >> >>> | ------- | | ------- |
> >> >>> | | | |
> >> >>> -------------- --------------
> >> >>> | 20 | 120 |
> >> >>> v v |
> >> >>> ---------------------------- |
> >> >>> | Node 2 PMEM | | 100
> >> >>> ---------------------------- |
> >> >>> | 100 |
> >> >>> v v
> >> >>> --------------------------------------
> >> >>> | Node 3 Large mem |
> >> >>> --------------------------------------
> >> >>>
> >> >>> node distances:
> >> >>> node 0 1 2 3
> >> >>> 0 10 20 20 120
> >> >>> 1 20 10 120 100
> >> >>> 2 20 120 10 100
> >> >>> 3 120 100 100 10
> >> >>>
> >> >>> /sys/devices/system/node/memory_tiers
> >> >>> 0-1
> >> >>> 2
> >> >>> 3
> >> >>>
> >> >>> N_TOPTIER_MEMORY: 0-1
> >> >>>
> >> >>>
> >> >>> In this case, we want to be able to "skip" the demotion path
> >> >>> from Node 1 to Node 2,
> >> >>>
> >> >>> and make demotion go directely to Node 3 as it is closer,
> >> >>> distance wise. How can
> >> >>>
> >> >>> we accommodate this scenario (or at least not rule it out as
> >> >>> future work) with the
> >> >>>
> >> >>> current RFC?
> >> >> If I remember correctly NUMA distance is hardcoded in SLIT by
> >> >> the firmware, it is supposed to reflect the latency. So I
> >> >> suppose it is the firmware's responsibility to have correct
> >> >> information. And the RFC assumes higher tier memory has better
> >> >> performance than lower tier memory (latency, bandwidth,
> >> >> throughput, etc), so it sounds like a buggy firmware to have
> >> >> lower tier memory with shorter distance than higher tier memory
> >> >> IMHO.
> >> >
> >> > You are correct if you're assuming the topology is all
> >> > hierarchically
> >> >
> >> > symmetric, but unfortuantely, in real hardware (e.g., my example
> >> > above)
> >> >
> >> > it is not. The distance/latency between two nodes in the same
> >> > tier
> >> >
> >> > and a third node, is different. The firmware still provides the
> >> > correct
> >> >
> >> > latency, but putting a node in a tier is up to the kernel/user,
> >> > and
> >> >
> >> > is relative: e.g., Node 3 could belong to tier 1 from Node 1's
> >> >
> >> > perspective, but to tier 2 from Node 0's.
> >> >
> >> >
> >> > A more detailed example (building on my previous one) is when
> >> > having
> >> >
> >> > the GPU connected to a switch:
> >> >
> >> > ----------------------------
> >> > | Node 2 PMEM |
> >> > ----------------------------
> >> > ^
> >> > |
> >> > -------------- --------------
> >> > | Node 0 | | Node 1 |
> >> > | ------- | | ------- |
> >> > | | DDR | | | | DDR | |
> >> > | ------- | | ------- |
> >> > | CPU | | GPU |
> >> > -------------- --------------
> >> > | |
> >> > v v
> >> > ----------------------------
> >> > | Switch |
> >> > ----------------------------
> >> > |
> >> > v
> >> > --------------------------------------
> >> > | Node 3 Large mem |
> >> > --------------------------------------
> >> >
> >> > Here, demoting from Node 1 to Node 3 directly would be faster as
> >> >
> >> > it only has to go through one hub, compared to demoting from
> >> > Node 1
> >> >
> >> > to Node 2, where it goes through two hubs. I hope that example
> >> >
> >> > clarifies things a little bit.
> >> >
> >>
> >> Alistair mentioned that we want to consider GPU memory to be
> >> expensive and want to demote from GPU to regular DRAM. In that
> >> case for the above case we should end up with
> >>
> >>
> >> tier 0 - > Node3
> >> tier 1 -> Node0, Node1
> >> tier 2 -> Node2
>
> I'm a little bit confused by the tiering here as I don't think it's
> quite what we want. As pointed out GPU memory is expensive and
> therefore we don't want anything demoting to it. That implies it
> should be in the top tier:
>
> tier 0 -> Node1
> tier 1 -> Node0, Node3
> tier 2 -> Node2
>
> Hence:
>
> node 0: allowed=2
> node 1: allowed=0,3,2
> node 2: allowed=empty
> node 3: allowed=2
>
> Alternatively Node3 could be put in tier 2 which would prevent
> demotion to PMEM via the switch/CPU:
>
> tier 0 -> Node1
> tier 1 -> Node0
> tier 2 -> Node2, Node3
>
> node 0: allowed=2,3
> node 1: allowed=0,3,2
> node 2: allowed=empty
> node 3: allowed=empty
>
Indeed. The scenario I described here is where the GPU can't/don't
demote to PMEM, but the CPU can. In this case it would work fine if we
put the GPU (Node 1) in tier 0, and rely on the fallback order.

> Both of these would be an improvement over the current situation
> upstream, which demotes everything to GPU memory and doesn't support
> demoting from the GPU (meaning reclaim on GPU memory pages everything
> to disk).
>
> >>
> >> Hence
> >>
> >> node 0: allowed=2
> >> node 1: allowed=2
> >> node 2: allowed = empty
> >> node 3: allowed = 0-1 , based on fallback order 1, 0
> >
> > If we have 3 tiers as defined above, then we'd better to have:
> >
> > node 0: allowed = 2
> > node 1: allowed = 2
> > node 2: allowed = empty
> > node 3: allowed = 0-2, based on fallback order: 1,0,2
> >
> > The firmware should provide the node distance values to reflect that
> > PMEM is slowest and should have the largest distance away from node
> > 3.
>
> Right. In my above example firmware would have to provide reasonable
> distance values to ensure optimal fallback order.
>
> >> -aneesh
> >>
> >>