Nandini Singhal created KAFKA-19967:
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Summary: Reduce GC pressure in tiered storage read path by using
direct memory buffers
Key: KAFKA-19967
URL: https://issues.apache.org/jira/browse/KAFKA-19967
Project: Kafka
Issue Type: Improvement
Components: Tiered-Storage
Reporter: Nandini Singhal
Large heap buffer allocations in the tiered storage read path cause significant
GC pressure by creating "humongous" objects that bypass young generation and go
directly to old generation. Using direct buffers for these I/O-centric
allocations would eliminate this GC overhead.
(RemoteLogManager.java:1718):
{code:java}
int updatedFetchSize = remoteStorageFetchInfo.minOneMessage() &&
firstBatchSize > maxBytes
? firstBatchSize
: maxBytes;
ByteBuffer buffer = ByteBuffer.allocate(updatedFetchSize);{code}
Where maxBytes = Math.min(fetchMaxBytes, fetchInfo.maxBytes), which can be
configured up to 55MB or
more depending on:
- replica.fetch.max.bytes (default: 1MB)
- replica.fetch.response.max.bytes (default: 10MB)
- Client-side max.partition.fetch.bytes
In the G1GC collector (Kafka's default), objects larger than half a region size
(~32MB with 64MB regions) are considered "humongous" and:
1. Skip eden and young generation entirely
2. Allocated directly in old generation
3. Can only be reclaimed during expensive full/mixed GCs
4. Trigger old GCs more frequently
Example: With a 4GB heap and InitiatingHeapOccupancyPercent=35, approximately
25 concurrent tiered storage fetch requests (25 × 55MB = 1.375GB) would trigger
an old GC. Under high read load from tiered storage, this creates continuous GC
pressure.
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