koo created CASSANDRA-21536:
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Summary: Profile pollution in AbstractType.writeValue makes
serialization slow for all column types
Key: CASSANDRA-21536
URL: https://issues.apache.org/jira/browse/CASSANDRA-21536
Project: Apache Cassandra
Issue Type: Improvement
Reporter: koo
*Description:*
AbstractType.writeValue() is one shared method. All column types use it.
Inside writeValue(), it calls valueLengthIfFixed(). This is a virtual call.
Many types override this method (Int32Type, LongType, UTF8Type, ...).
In a real cluster, many column types pass through writeValue(). So the profile
always sees lots types.
Because of this, the JIT cannot inline the call. It stays as a vtable call.
Also, the compiled body of writeValue() becomes big, so the JIT refuses to
inline writeValue() itself ("already compiled into a big method").
We also see the same itable/vtable stubs in async-profiler output from a real
cluster, running with default production options and a normal workload.
*How to solve:*
Add a final int field to AbstractType. Set it in the constructor. writeValue()
reads this field instead of calling valueLengthIfFixed().
A field read needs no type profile. So profile pollution has no effect on it.
The valueLengthIfFixed() method is not changed. Only the write path uses the
field.
*Result:*
Production is always the polluted state, so this is the real-world comparison
|| || before || after ||
| M ops/s | 185.0 | 210.4 |
| improvement | | +13.7% |
| valueLengthIfFixed frames in profiler | 5.9% | 0% |
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