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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. Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com> Link: https://lore.kernel.org/r/20220520103519.1863-5-mgorman@techsingularity.net |
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| .. | ||
| autogroup.c | ||
| autogroup.h | ||
| build_policy.c | ||
| build_utility.c | ||
| clock.c | ||
| completion.c | ||
| core_sched.c | ||
| core.c | ||
| cpuacct.c | ||
| cpudeadline.c | ||
| cpudeadline.h | ||
| cpufreq_schedutil.c | ||
| cpufreq.c | ||
| cpupri.c | ||
| cpupri.h | ||
| cputime.c | ||
| deadline.c | ||
| debug.c | ||
| fair.c | ||
| features.h | ||
| idle.c | ||
| isolation.c | ||
| loadavg.c | ||
| Makefile | ||
| membarrier.c | ||
| pelt.c | ||
| pelt.h | ||
| psi.c | ||
| rt.c | ||
| sched-pelt.h | ||
| sched.h | ||
| smp.h | ||
| stats.c | ||
| stats.h | ||
| stop_task.c | ||
| swait.c | ||
| topology.c | ||
| wait_bit.c | ||
| wait.c | ||