Re: [RFC PATCH v3 0/4] Node Weights and Weighted Interleave

[Huang, Ying]

Ravi Jonnalagadda <ravis.opensrc@xxxxxxxxxx> writes:

>>> On Tue, Oct 31, 2023 at 04:56:27PM +0100, Michal Hocko wrote:
>>>> On Tue 31-10-23 11:21:42, Johannes Weiner wrote:
>>>> > On Tue, Oct 31, 2023 at 10:53:41AM +0100, Michal Hocko wrote:
>>>> > > On Mon 30-10-23 20:38:06, Gregory Price wrote:
>>
>>[snip]
>>
>>>>
>>>> > This hopefully also explains why it's a global setting. The usecase is
>>>> > different from conventional NUMA interleaving, which is used as a
>>>> > locality measure: spread shared data evenly between compute
>>>> > nodes. This one isn't about locality - the CXL tier doesn't have local
>>>> > compute. Instead, the optimal spread is based on hardware parameters,
>>>> > which is a global property rather than a per-workload one.
>>>>
>>>> Well, I am not convinced about that TBH. Sure it is probably a good fit
>>>> for this specific CXL usecase but it just doesn't fit into many others I
>>>> can think of - e.g. proportional use of those tiers based on the
>>>> workload - you get what you pay for.
>>>>
>>>> Is there any specific reason for not having a new interleave interface
>>>> which defines weights for the nodemask? Is this because the policy
>>>> itself is very dynamic or is this more driven by simplicity of use?
>>>
>>> A downside of *requiring* weights to be paired with the mempolicy is
>>> that it's then the application that would have to figure out the
>>> weights dynamically, instead of having a static host configuration. A
>>> policy of "I want to be spread for optimal bus bandwidth" translates
>>> between different hardware configurations, but optimal weights will
>>> vary depending on the type of machine a job runs on.
>>>
>>> That doesn't mean there couldn't be usecases for having weights as
>>> policy as well in other scenarios, like you allude to above. It's just
>>> so far such usecases haven't really materialized or spelled out
>>> concretely. Maybe we just want both - a global default, and the
>>> ability to override it locally.
>>
>>I think that this is a good idea.  The system-wise configuration with
>>reasonable default makes applications life much easier.  If more control
>>is needed, some kind of workload specific configuration can be added.
>
> Glad that we are in agreement here. For bandwidth expansion use cases
> that this interleave patchset is trying to cater to, most applications
> would have to follow the "reasanable defaults" for weights.
> The necessity for applications to choose different weights while
> interleaving would probably be to do capacity expansion which the
> default memory tiering implementation would anyway support and provide
> better latency.
>
>>And, instead of adding another memory policy, a cgroup-wise
>>configuration may be easier to be used.  The per-workload weight may
>>need to be adjusted when we deploying different combination of workloads
>>in the system.
>>
>>Another question is that should the weight be per-memory-tier or
>>per-node?  In this patchset, the weight is per-source-target-node
>>combination.  That is, the weight becomes a matrix instead of a vector.
>>IIUC, this is used to control cross-socket memory access in addition to
>>per-memory-type memory access.  Do you think the added complexity is
>>necessary?
>
> Pros and Cons of Node based interleave:
> Pros:
> 1. Weights can be defined for devices with different bandwidth and latency
> characteristics individually irrespective of which tier they fall into.
> 2. Defining the weight per-source-target-node would be necessary for multi
> socket systems where few devices may be closer to one socket rather than other.
> Cons:
> 1. Weights need to be programmed for all the nodes which can be tedious for
> systems with lot of NUMA nodes.

2. More complex, so need justification, for example, practical use case.

> Pros and Cons of Memory Tier based interleave:
> Pros:
> 1. Programming weight per initiator would apply for all the nodes in the tier.
> 2. Weights can be calculated considering the cumulative bandwidth of all
> the nodes in the tier and need to be programmed once for all the nodes in a
> given tier.
> 3. It may be useful in cases where numa nodes with similar latency and bandwidth
> characteristics increase, possibly with pooling use cases.

4. simpler.

> Cons:
> 1. If nodes with different bandwidth and latency characteristics are placed
> in same tier as seen in the current mainline kernel, it will be difficult to
> apply a correct interleave weight policy.
> 2. There will be a need for functionality to move nodes between different tiers
> or create new tiers to place such nodes for programming correct interleave weights.
> We are working on a patch to support it currently.

Thanks!  If we have such system, we will need this.

> 3. For systems where each numa node is having different characteristics,
> a single node might end up existing in different memory tier, which would be
> equivalent to node based interleaving.

No.  A node can only exist in one memory tier.

> On newer systems where all CXL memory from different devices under a
> port are combined to form single numa node, this scenario might be
> applicable.

You mean the different memory ranges of a NUMA node may have different
performance?  I don't think that we can deal with this.

> 4. Users may need to keep track of different memory tiers and what nodes are present
> in each tier for invoking interleave policy.

I don't think this is a con.  With node based solution, you need to know
your system too.

>>
>>> Could you elaborate on the 'get what you pay for' usecase you
>>> mentioned?
>>

--
Best Regards,
Huang, Ying