CRITICAL: Gateway calling LookupTopicBrokers on EVERY fetch to translate
Kafka partition IDs to SeaweedFS partition ranges!
Problem (from CPU profile):
- getActualPartitionAssignment: 13.52% CPU (1.71s out of 12.65s)
- Called bc.client.LookupTopicBrokers on line 228 for EVERY fetch
- With 250 fetches/sec, this means 250 LookupTopicBrokers calls/sec!
- No caching at all - same overhead as broker had before optimization
Root Cause:
Gateway needs to translate Kafka partition IDs (0, 1, 2...) to SeaweedFS
partition ranges (0-341, 342-682, etc.) for every fetch request. This
translation requires calling LookupTopicBrokers to get partition assignments.
Without caching, every fetch request triggered:
1. gRPC call to broker (LookupTopicBrokers)
2. Broker reads from its cache (fast now after broker optimization)
3. gRPC response back to gateway
4. Gateway computes partition range mapping
The gRPC round-trip overhead was consuming 13.5% CPU even though broker
cache was fast!
Solution:
Added partitionAssignmentCache to BrokerClient:
Changes to types.go:
- Added partitionAssignmentCacheEntry struct (assignments + expiresAt)
- Added cache fields to BrokerClient:
* partitionAssignmentCache map[string]*partitionAssignmentCacheEntry
* partitionAssignmentCacheMu sync.RWMutex
* partitionAssignmentCacheTTL time.Duration
Changes to broker_client.go:
- Initialize partitionAssignmentCache in NewBrokerClientWithFilerAccessor
- Set partitionAssignmentCacheTTL to 30 seconds (same as broker)
Changes to broker_client_publish.go:
- Added "time" import
- Modified getActualPartitionAssignment() to check cache first:
* Cache HIT: Use cached assignments (fast ✅)
* Cache MISS: Call LookupTopicBrokers, cache result for 30s
- Extracted findPartitionInAssignments() helper function
* Contains range calculation and partition matching logic
* Reused for both cached and fresh lookups
Cache Behavior:
- First fetch: Cache MISS -> LookupTopicBrokers (~2ms) -> cache for 30s
- Next 7500 fetches in 30s: Cache HIT -> immediate return (~0.01ms)
- Cache automatically expires after 30s, re-validates on next fetch
Performance Impact:
With 250 fetches/sec and 5 topics:
- Before: 250 LookupTopicBrokers/sec = 500ms CPU overhead
- After: 0.17 LookupTopicBrokers/sec (5 topics / 30s TTL)
- Reduction: 99.93% fewer gRPC calls
Expected CPU Reduction:
- Before: 12.65s total, 1.71s in getActualPartitionAssignment (13.5%)
- After: ~11s total (-13.5% = 1.65s saved)
- Benefit: 13% lower CPU, more capacity for actual message processing
Cache Consistency:
- Same 30-second TTL as broker's topic config cache
- Partition assignments rarely change (only on topic reconfiguration)
- 30-second staleness is acceptable for partition mapping
- Gateway will eventually converge with broker's view
Testing:
- ✅ Compiles successfully
- Ready to deploy and measure CPU improvement
Priority: CRITICAL - Eliminates major performance bottleneck in gateway fetch path
chrislu
98ec5e03eb