Re: [PATCH 4/4] sched/numa: Adjust imb_numa_nr to a better approximation of memory channels

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 11, 2022 at 03:30:38PM +0100, Mel Gorman wrote:
> For a single LLC per node, a NUMA imbalance is allowed up until 25%
> of CPUs sharing a node could be active. One intent of the cut-off is
> to avoid an imbalance of memory channels but there is no topological
> information based on active memory channels. Furthermore, there can
> be differences between nodes depending on the number of populated
> DIMMs.
> 
> A cut-off of 25% was arbitrary but generally worked. It does have a severe
> corner cases though when an parallel workload is using 25% of all available
> CPUs over-saturates memory channels. This can happen due to the initial
> forking of tasks that get pulled more to one node after early wakeups
> (e.g. a barrier synchronisation) that is not quickly corrected by the
> load balancer. The LB may fail to act quickly as the parallel tasks are
> considered to be poor migrate candidates due to locality or cache hotness.
> 
> On a range of modern Intel CPUs, 12.5% appears to be a better cut-off
> assuming all memory channels are populated and is used as the new cut-off
> point. A minimum of 1 is specified to allow a communicating pair to
> remain local even for CPUs with low numbers of cores. For modern AMDs,
> there are multiple LLCs and are not affected.

Can the hardware tell us about memory channels?