Messages in this thread | | | Subject | Re: [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology | From | Lauro Venancio <> | Date | Thu, 13 Apr 2017 18:06:57 -0300 |
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On 04/13/2017 05:21 PM, Lauro Venancio wrote: > On 04/13/2017 12:48 PM, Peter Zijlstra wrote: >> On Thu, Apr 13, 2017 at 10:56:08AM -0300, Lauro Ramos Venancio wrote: >>> Currently, on a 4 nodes NUMA machine with ring topology, two sched >>> groups are generated for the last NUMA sched domain. One group has the >>> CPUs from NUMA nodes 3, 0 and 1; the other group has the CPUs from nodes >>> 1, 2 and 3. As CPUs from nodes 1 and 3 belongs to both groups, the >>> scheduler is unable to directly move tasks between these nodes. In the >>> worst scenario, when a set of tasks are bound to nodes 1 and 3, the >>> performance is severely impacted because just one node is used while the >>> other node remains idle. >> I feel a picture would be ever so much clearer. >> >>> This patch constructs the sched groups from each CPU perspective. So, on >>> a 4 nodes machine with ring topology, while nodes 0 and 2 keep the same >>> groups as before [(3, 0, 1)(1, 2, 3)], nodes 1 and 3 have new groups >>> [(0, 1, 2)(2, 3, 0)]. This allows moving tasks between any node 2-hops >>> apart. >> So I still have no idea what specifically goes wrong and how this fixes >> it. Changelog is impenetrable. > On a 4 nodes machine with ring topology, the last sched domain level > contains groups with 3 numa nodes each. So we have four possible groups: > (0, 1, 2) (1, 2, 3) (2, 3, 0)(3, 0, 1). As we need just two groups to > fill the sched domain, currently, the groups (3, 0, 1) and (1, 2, 3) are > used for all CPUs. The problem with it is that nodes 1 and 3 belongs to > both groups, becoming impossible to move tasks between these two nodes. > > This patch uses different groups depending on the CPU they are > installed. So nodes 0 and 2 CPUs keep the same group as before: (3, 0, > 1) and (1, 2, 3). Nodes 1 and 3 CPUs use the new groups: (0, 1, 2) and > (2, 3, 0). So the first pair of groups allows movement between nodes 0 > and 2; and the second pair of groups allows movement between nodes 1 and 3. > > I will improve the changelog. > >> "From each CPU's persepective" doesn't really help, there already is a >> for_each_cpu() in. > The for_each_cpu() is used to iterate across all sched domain cpus. It > doesn't consider the CPU where the groups are being installed (parameter > cpu in build_overlap_sched_groups()). Currently, the parameter cpu is > used just for memory allocation and for ordering the groups, it doesn't > change the groups that are chosen. This patch uses the parameter cpu to > choose the first group, changing also, as consequence, the second group. >> Also, since I'm not sure what happend to the 4 node system, I cannot >> begin to imagine what would happen on the 8 node one.
Just for clarification, I am sending the nodes distance table for the two most common typologies affected by this issue.
4 nodes, ring topology node distances: node 0 1 2 3 0: 10 20 30 20 1: 20 10 20 30 2: 30 20 10 20 3: 20 30 20 10
8 node, mesh topology node distances: node 0 1 2 3 4 5 6 7 0: 10 16 16 22 16 22 16 22 1: 16 10 16 22 22 16 22 16 2: 16 16 10 16 16 16 16 22 3: 22 22 16 10 16 16 22 16 4: 16 22 16 16 10 16 16 16 5: 22 16 16 16 16 10 22 22 6: 16 22 16 22 16 22 10 16 7: 22 16 22 16 16 22 16 10
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