On 5/6/20 5:24 PM, Michal Hocko wrote: >> >> Yes, if we allocate from cpu 0-3 then it should be a miss on node 0. But the >> zonelists are optimized in a way that they don't include empty zones - >> build_zonerefs_node() checks managed_zone(). As a result, node 0 zonelist has no >> node 0 zones, which confuses the stats code. We should probably document that >> numa stats are bogus on systems with memoryless nodes. This patch makes it >> somewhat more obvious by presenting nice zeroes on the memoryless node itself, >> but node 1 now include stats from node 0. > > Thanks for the clarification. So the underlying problem is that zone_statistics > operates on a preferred zone rather than node. This would be fixable but > I am not sure whether this is something worth bothering. Maybe it would > just be more convenient to document the unfortunate memory less nodes > stats situation and be done with it. Or do we have any consumers that > really do care? > >> >> NUMA_OTHER uses numa_node_id(), which would mean the node 0's cpus have node 1 >> >> in their numa_node_id() ? Is that correct? >> > >> > numa_node_id should reflect the real node the CPU is associated with. >> >> You're right, numa_node_id() is probably fine. But NUMA_OTHER is actually >> incremented at the zone where the allocation succeeds. This probably doesn't >> match Documentation/admin-guide/numastat.rst, even on a non-memoryless-node systems: >> >> other_node A process ran on this node and got memory from another node. > > Yeah, the documentation doesn't match the implementation. Maybe we > should just fix the documentation because this has been the case for > ages. > How about something like this: diff --git a/Documentation/admin-guide/numastat.rst b/Documentation/admin-guide/numastat.rst index aaf1667489f8..08ec2c2bdce3 100644 --- a/Documentation/admin-guide/numastat.rst +++ b/Documentation/admin-guide/numastat.rst @@ -6,6 +6,21 @@ Numa policy hit/miss statistics All units are pages. Hugepages have separate counters. +The numa_hit, numa_miss and numa_foreign counters reflect how well processes +are able to allocate memory from nodes they prefer. If they succeed, numa_hit +is incremented on the preferred node, otherwise numa_foreign is incremented on +the preferred node and numa_miss on the node where allocation succeeded. + +Usually preferred node is the one local to the CPU where the process executes, +but restrictions such as mempolicies can change that, so there are also two +counters based on CPU local node. local_node is similar to numa_hit and is +incremented on allocation from a node by CPU on the same node. other_node is +similar to numa_miss and is incremented on the node where allocation succeeds +from a CPU from a different node. Note there is no counter analogical to +numa_foreign. + +In more detail: + =============== ============================================================ numa_hit A process wanted to allocate memory from this node, and succeeded. @@ -14,11 +29,13 @@ numa_miss A process wanted to allocate memory from another node, but ended up with memory from this node. numa_foreign A process wanted to allocate on this node, - but ended up with memory from another one. + but ended up with memory from another node. -local_node A process ran on this node and got memory from it. +local_node A process ran on this node's CPU, + and got memory from this node. -other_node A process ran on this node and got memory from another node. +other_node A process ran on a different node's CPU + and got memory from this node. interleave_hit Interleaving wanted to allocate from this node and succeeded. @@ -28,3 +45,11 @@ For easier reading you can use the numastat utility from the numactl package (http://oss.sgi.com/projects/libnuma/). Note that it only works well right now on machines with a small number of CPUs. +Note that on systems with memoryless nodes (where a node has CPUs but no +memory) the numa_hit, numa_miss and numa_foreign statistics can be skewed +heavily. In the current kernel implementation, if a process prefers a +memoryless node (i.e. because it is running on one of its local CPU), the +implementation actually treats one of the nearest nodes with memory as the +preferred node. As a result, such allocation will not increase the numa_foreign +counter on the memoryless node, and will skew the numa_hit, numa_miss and +numa_foreign statistics of the nearest node. -- 2.26.2
