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

[Michal Hocko <mhocko@suse.com>]

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:
> > > This patchset implements weighted interleave and adds a new sysfs
> > > entry: /sys/devices/system/node/nodeN/accessM/il_weight.
> > > 
> > > The il_weight of a node is used by mempolicy to implement weighted
> > > interleave when `numactl --interleave=...` is invoked.  By default
> > > il_weight for a node is always 1, which preserves the default round
> > > robin interleave behavior.
> > > 
> > > Interleave weights may be set from 0-100, and denote the number of
> > > pages that should be allocated from the node when interleaving
> > > occurs.
> > > 
> > > For example, if a node's interleave weight is set to 5, 5 pages
> > > will be allocated from that node before the next node is scheduled
> > > for allocations.
> > 
> > I find this semantic rather weird TBH. First of all why do you think it
> > makes sense to have those weights global for all users? What if
> > different applications have different view on how to spred their
> > interleaved memory?
> > 
> > I do get that you might have a different tiers with largerly different
> > runtime characteristics but why would you want to interleave them into a
> > single mapping and have hard to predict runtime behavior?
> > 
> > [...]
> > > In this way it becomes possible to set an interleaving strategy
> > > that fits the available bandwidth for the devices available on
> > > the system. An example system:
> > > 
> > > Node 0 - CPU+DRAM, 400GB/s BW (200 cross socket)
> > > Node 1 - CPU+DRAM, 400GB/s BW (200 cross socket)
> > > Node 2 - CXL Memory. 64GB/s BW, on Node 0 root complex
> > > Node 3 - CXL Memory. 64GB/s BW, on Node 1 root complex
> > > 
> > > In this setup, the effective weights for nodes 0-3 for a task
> > > running on Node 0 may be [60, 20, 10, 10].
> > > 
> > > This spreads memory out across devices which all have different
> > > latency and bandwidth attributes at a way that can maximize the
> > > available resources.
> > 
> > OK, so why is this any better than not using any memory policy rely
> > on demotion to push out cold memory down the tier hierarchy?
> > 
> > What is the actual real life usecase and what kind of benefits you can
> > present?
> 
> There are two things CXL gives you: additional capacity and additional
> bus bandwidth.
> 
> The promotion/demotion mechanism is good for the capacity usecase,
> where you have a nice hot/cold gradient in the workingset and want
> placement accordingly across faster and slower memory.
> 
> The interleaving is useful when you have a flatter workingset
> distribution and poorer access locality. In that case, the CPU caches
> are less effective and the workload can be bus-bound. The workload
> might fit entirely into DRAM, but concentrating it there is
> suboptimal. Fanning it out in proportion to the relative performance
> of each memory tier gives better resuls.
> 
> We experimented with datacenter workloads on such machines last year
> and found significant performance benefits:
> 
> https://lore.kernel.org/linux-mm/YqD0%2FtzFwXvJ1gK6@cmpxchg.org/T/

Thanks, this is a useful insight.
 
> 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?

-- 
Michal Hocko
SUSE Labs