RFR (L): 8015774: Add support for multiple code heaps

Tobias Hartmann tobi.hartmann at gmail.com
Wed Oct 2 07:06:44 PDT 2013


please review the following change.

bug: https://bugs.openjdk.java.net/browse/JDK-8015774
webrev: http://cr.openjdk.java.net/~anoll/8015774/webrev.00/

This change implements support for multiple code heaps in the code cache.
The interface of the code cache was changed accordingly and references from
other components of the VM were adapted. This includes the indirect
references from:
- the Serviceability Agent: vmStructs and the Java code cache interface
- the dtrace ustack helper script (jhelper.d)
- the pstack support library libjvm_db.c

Currently the code cache contains the following three code heaps each of
which contains CodeBlobs of a specific type:
- Non-Profiled methods: nmethods that are not profiled, i.e., those
compiled at tier 1 or 4 and native methods
- Profiled methods: nmethods that are profiled, i.e., those compiled at
tier 2 or 3
- Non-methods: Non-methods like Buffers, Adapters and Runtime Stubs

By default, 2/3 of the ReservedCodeCacheSize is used for the non-profiled
heap and 1/3 is used for the profiled heap. Experiments with a small code
cache size have shown that this configuration performs best (see [1]).
Sizes can be configured using the NonProfiledCodeHeapSize and
ProfiledCodeHeapSize parameters. By now the non-method heap has a fixed
size of 8mb. More tests have to be performed to determine reasonable
default values for different build and runtime configurations. It would be
also possible to add an additional heap for the CodeBlobs needed during VM
startup (adapters, stubs, interpreter, ...) and use the non-method heap for
non-method blobs allocated during runtime.

The main benefit of this allocation is that functions operating on nmethods
can now be optimized to only iterate over CodeBlobs on the nmethod heaps,
avoiding a full scan of the code cache including the non-methods.
Performance evaluation shows that this results in a great reduction of the
sweeping time. Further it is now possible to extend the sweeper to
selectively sweep nmethods of different compilation levels. Because it is
more expensive to recompile a highly optimized method, it makes sense to
first sweep those compiled at a lower level. Possible future optimizations
may also include multithreaded sweeping and separation of nmethod code and

The code was tested using Nashorn + Octane, DaCapo, SPECJvm2008 and jprt.

Benchmark results:
- [2] shows the score of the old and new version running Nashorn with the
Octane benchmark at different code cache sizes (10 runs each, sizes < 32mb
crash with the old version). An performance improvement of around 15% is
- [3] shows the sweep time of the NMethodSweeper during runs of the Octane
benchmark with different code cache sizes (10 runs each). The time is
greatly reduced with the new version.
- [4] shows the time ratio (TOld / TNew - 1) of the DaCapo benchmarks with
a code cache size of 64mb (30 runs with 20 warmups each). With the time
differences being smaller than the confidence intervals it is not possible
determine a difference between the two versions.
- Multiple runs of the SPECJvm2008 benchmark with different code cache
sizes did not show a difference between the two versions. This is probably
due to the fact that not much code is compiled at runtime compared to
Nashorn with the Octane benchmark.

The error bars at each data point show the 95% confidence interval.

[1] https://bugs.openjdk.java.net/secure/attachment/16372/OctaneRatio.png

Thanks and best regards,

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