koo created CASSANDRA-21535:
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Summary: Avoid extra copy for cached Mutation serialization
Key: CASSANDRA-21535
URL: https://issues.apache.org/jira/browse/CASSANDRA-21535
Project: Apache Cassandra
Issue Type: Improvement
Reporter: koo
Description
In Mutation.Serializer.serialization(), we first calculate the serialized size.
After that, current code serializes the mutation to a thread local
DataOutputBuffer.
Then unsafeToByteArray() creates a new byte array and copies all serialized
bytes.
The serialized size is already known.
So we can create a fixed size heap buffer first and serialize directly to that
buffer.
This change removes one full byte array copy when we create a cached mutation
serialization.
I also added a check that the written size is same as the calculated serialized
size.
Benchmark
I added a JMH benchmark for this case.
For every benchmark operation:
- clear cached serialization
- serialize the Mutation
- create cached serialization again
JDK 17, arm64, 1 thread:
| Value size | Before | After | Result |
|---:|---:|---:|---:|
| 128 B | 3.70 M ops/s | 3.87 M ops/s | +4% |
| 16 KiB | 608 K ops/s | 716 K ops/s | +18% |
| 256 KiB | 41.4 K ops/s | 53.1 K ops/s | +28% |
| 768 KiB | 6.83 K ops/s | 18.88 K ops/s | +177% |
The result is bigger for large mutations because current code writes the data
to a
direct buffer first, and then copies all data again to a heap byte array.
With this change, serialization writes directly to the final heap byte array.
This benchmark measures cache creation cost. It does not mean normal write
throughput will be 2.77x faster, because normal requests can reuse the cached
serialization.
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