8356000: C1/C2-only modes use 2 compiler threads on low CPU count machines

[shipilev]

There is an unfortunate limitation with default tiered policy that we would have at least 2 threads on 1 CPU machine: 1 thread for C1, and 1 thread for C2.

But if we select C1-only or C2-only modes, we also get 2 compiler threads, for which we have no good reason. These threads would just step on each other toes. The fix changes the behavior for 1..3 CPU hosts in C1/C2-only configurations, by using 1 thread instead of 2 threads. The change for 1 CPU config is what we really need. The change in 2..3 CPU configs is an additional effect, but I think it is still good not to use 100%/66% of the CPUs in those configurations as well.

$ for I in `seq 1 8`; do build/linux-x86_64-server-release/images/jdk/bin/java \
  -XX:-TieredCompilation -XX:ActiveProcessorCount=${I} \
  -XX:+PrintFlagsFinal 2>&1 | grep "CICompilerCount "; done

# Before
     intx CICompilerCount                          = 2
     intx CICompilerCount                          = 2
     intx CICompilerCount                          = 2
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 4

# After
     intx CICompilerCount                          = 1
     intx CICompilerCount                          = 1
     intx CICompilerCount                          = 1
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 3
     intx CICompilerCount                          = 4

It is a minor bug in CompilationPolicy::initialize, but it gets in the way studying Leyden in tight CPU scenarios.

Additional testing:

• New regression test passes with the fix, fails without it
• GHA

---

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Issue

• JDK-8356000: C1/C2-only modes use 2 compiler threads on low CPU count machines (Bug - P4)

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[vnkozlov]

Should it be the general rule: don't create more compiler threads than available cpus?

[vnkozlov]

Except when specified on command line with -XX:CICompilerCount=n.
Actually your changes does not take this flag into account.