On Wed, May 11, 2022 at 9:40 PM Aneesh Kumar K V <aneesh.kumar@xxxxxxxxxxxxx> wrote: > > On 5/11/22 12:42 PM, Alistair Popple wrote: > > > > Wei Xu <weixugc@xxxxxxxxxx> writes: > > > >> On Tue, May 10, 2022 at 5:10 AM Aneesh Kumar K V > >> <aneesh.kumar@xxxxxxxxxxxxx> 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@xxxxxxxxxx> 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: > > > > > I didn't look closely at the topology and assumed that Node3 is the GPU > connected to the switch. Hence all the confusion. > > > > 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 > > looks good to be default and simple. > > > > > 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 > > > > and this can be configured via userspace? The per-node tier customization interface that I just mentioned should support such reconfigurations. > > 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 > >>> > >>> >
