Markus Weninger Markus Weninger - 1 year ago 224
Java Question

What exactly does -XX:-TieredCompilation do?

Using

java -XX:+PrintFlagsFinal
I found the
TieredCompilation
flag, and I read about it a bit online.

Yet, I still don't know exactly what happens when setting it to
false
.

I know that the compilation system supports 5 execution levels, basically splitted into interpreter, C1 and C2:


  • level 0 - interpreter

  • level 1 - C1 with full optimization (no profiling)

  • level 2 - C1 with invocation and backedge counters

  • level 3 - C1 with full profiling (level 2 + MDO)

  • level 4 - C2



Source: http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/2b2511bd3cc8/src/share/vm/runtime/advancedThresholdPolicy.hpp#l34

Two questions:

(1) By setting
-XX:-TieredCompilation
, are some of this levels just disabled? If yes, which?

(2) Is there some flag to decide whether to disable C1 or C2, or to not compile at all?

Answer Source

-XX:-TieredCompilation disables intermediate compilation tiers (1, 2, 3), so that a method is either interpreted or compiled at the maximum optimization level (C2).

As a side effect TieredCompilation flag also changes the number of compiler threads, the compilation policy and the default code cache size. Note that with TieredCompilation disabled

  • there will be less compiler threads;
  • simple compilation policy (based on method invocation and backedge counters) will be chosen instead of advanced compilation policy;
  • default reserved code cache size will be 5 times smaller.

To disable C2 compiler and to leave only C1 with no extra overhead, set -XX:TieredStopAtLevel=1.

To disable all JIT compilers and to run everything in interpreter, use -Xint.