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MEDIUM: cpu-topo: change "performance" to consider per-core capacity

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Running the "performance" policy on highly heterogenous systems yields
bad choices when there are sufficiently more small than big cores,
and/or when there are multiple cluster types, because on such setups,
the higher the frequency, the lower the number of cores, despite small
differences in frequencies. In such cases, we quickly end up with
"performance" only choosing the small or the medium cores, which is
contrary to the original intent, which was to select performance cores.
This is what happens on boards like the Orion O6 for example where only
the 4 medium cores and 2 big cores are choosen, evicting the 2 biggest
cores and the 4 smallest ones.

Here we're changing the sorting method to sort CPU clusters by average
per-CPU capacity, and we evict clusters whose per-CPU capacity falls
below 80% of the previous one. Per-core capacity allows to detect
discrepancies between CPU cores, and to continue to focus on high
performance ones as a priority.
This commit is contained in:
Willy Tarreau 2025-05-13 16:12:52 +02:00
parent 5ab2c815f1
commit 6c88e27cf4
2 changed files with 21 additions and 16 deletions
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19
doc/configuration.txt
View file

@ -2098,15 +2098,16 @@ cpu-policy <policy>
admins to validate setups.
- performance exactly like group-by-cluster above, except that CPU
clusters whose performance is less than half of the
next more performant one are evicted. These are
typically "little" or "efficient" cores, whose addition
generally doesn't bring significant gains and can
easily be counter-productive (e.g. TLS handshakes).
Often, keeping such cores for other tasks such as
network handling is much more effective. On development
systems, these can also be used to run auxiliary tools
such as load generators and monitoring tools.
clusters composed of cores whose performance is less
than 80% of those of the next more performant one are
evicted. These are typically "little" or "efficient"
cores, whose addition generally doesn't bring significant
gains and can easily be counter-productive (e.g. TLS
handshakes). Often, keeping such cores for other tasks
such as network handling is much more effective. On
development systems, these can also be used to run
auxiliary tools such as load generators and monitoring
tools.
- resource this is like group-by-cluster above, except that only
the smallest and most efficient CPU cluster will be

18
src/cpu_topo.c
View file

@ -1316,7 +1316,7 @@ static int cpu_policy_group_by_ccx(int policy, int tmin, int tmax, int gmin, int
/* the "performance" cpu-policy:
* - does nothing if nbthread or thread-groups are set
* - eliminates clusters whose total capacity is below half of others
* - eliminates clusters whose average capacity is less than 80% that of others
* - tries to create one thread-group per cluster, with as many
* threads as CPUs in the cluster, and bind all the threads of
* this group to all the CPUs of the cluster.
@ -1329,22 +1329,26 @@ static int cpu_policy_performance(int policy, int tmin, int tmax, int gmin, int
if (global.nbthread || global.nbtgroups)
return 0;
/* sort clusters by reverse capacity */
cpu_cluster_reorder_by_capa(ha_cpu_clusters, cpu_topo_maxcpus);
/* sort clusters by average reverse capacity */
cpu_cluster_reorder_by_avg_capa(ha_cpu_clusters, cpu_topo_maxcpus);
capa = 0;
for (cluster = 0; cluster < cpu_topo_maxcpus; cluster++) {
if (capa && ha_cpu_clusters[cluster].capa < capa / 2) {
/* This cluster is more than twice as slow as the
* previous one, we're not interested in using it.
if (capa && ha_cpu_clusters[cluster].capa * 10 < ha_cpu_clusters[cluster].nb_cpu * capa * 8) {
/* This cluster is made of cores delivering less than
* 80% of the performance of those of the previous
* cluster, previous one, we're not interested in
* using it.
*/
for (cpu = 0; cpu <= cpu_topo_lastcpu; cpu++) {
if (ha_cpu_topo[cpu].cl_gid == ha_cpu_clusters[cluster].idx)
ha_cpu_topo[cpu].st |= HA_CPU_F_IGNORED;
}
}
else if (ha_cpu_clusters[cluster].nb_cpu)
capa = ha_cpu_clusters[cluster].capa / ha_cpu_clusters[cluster].nb_cpu;
else
capa = ha_cpu_clusters[cluster].capa;
capa = 0;
}
cpu_cluster_reorder_by_index(ha_cpu_clusters, cpu_topo_maxcpus);