UseNuma disables itself for no adequately explained reason

[Ben Evans]

Hi,

I've encountered an issue whereby, despite -XX:+UseNuma being
explicitly set on the command line, the final flags list that is seen
has it disabled. This is on OpenJDK 8u212.

Has anyone encountered a runtime configuration or sequence of flags
that will disable NUMA-awareness as a side-effect?

I'll see if I can pull the exact startup flags as given and share them.

Any pointers gratefully received.

Thanks,

Ben

[Michael Barker]

With JDK8 it seems to be impacted by whether the machine has multiple nodes (or not):

$ numactl -H
available: 2 nodes (0-1)
node 0 cpus: 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46
node 0 size: 63560 MB
node 0 free: 24246 MB
node 1 cpus: 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47
node 1 size: 64497 MB
node 1 free: 26020 MB
node distances:
node   0   1
  0:  10  21
  1:  21  10

$ java -XX:+PrintFlagsFinal -XX:+UseNUMA -version | grep UseNUMA
     bool UseNUMA                                  := true                                {product}
     bool UseNUMAInterleaving                      := true  

$ numactl -H
available: 1 nodes (0)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
node 0 size: 64342 MB
node 0 free: 25173 MB
node distances:
node   0
  0:  10

$ java -XX:+PrintFlagsFinal -XX:+UseNUMA -version | grep UseNUMA
     bool UseNUMA                                  := false                               {product}
     bool UseNUMAInterleaving                       = false                               {product}

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[Jean-Philippe BEMPEL]

Hi Ben, 

Parallel GC supports Numa, but I'm not sure about G1! 

Cheers 

[Ben Evans]

Thanks Jean-Phillipe.

JEP 345 - https://openjdk.java.net/jeps/345 has the answer. Currently
only Parallel supports NUMA, and looking at this app, it's using CMS.
Rather annoying that HotSpot doesn't warn about the combination, but
there we go!

Thanks,

Ben

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[Kirk Pepperdine]

I believe it does but it’s all some what flaky FWIBT.

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