chrislu
8532acea53
CRITICAL: LookupTopicBrokers was bypassing cache, causing 26% CPU overhead!
Problem (from CPU profile):
- LookupTopicBrokers: 35.74% CPU (9s out of 25.18s)
- ReadTopicConfFromFiler: 26.41% CPU (6.65s)
- protojson.Unmarshal: 16.64% CPU (4.19s)
- LookupTopicBrokers called b.fca.ReadTopicConfFromFiler() directly on line 35
- Completely bypassed our unified topicCache!
Root Cause:
LookupTopicBrokers is called VERY frequently by clients (every fetch request
needs to know partition assignments). It was calling ReadTopicConfFromFiler
directly instead of using the cache, causing:
1. Expensive gRPC calls to filer on every lookup
2. Expensive JSON unmarshaling on every lookup
3. 26%+ CPU overhead on hot path
4. Our cache optimization was useless for this critical path
Solution:
Created getTopicConfFromCache() helper and updated all callers:
Changes to broker_topic_conf_read_write.go:
- Added getTopicConfFromCache() - public API for cached topic config reads
- Implements same caching logic: check cache -> read filer -> cache result
- Handles both positive (conf != nil) and negative (conf == nil) caching
- Refactored GetOrGenerateLocalPartition() to use new helper (code dedup)
- Now only 14 lines instead of 60 lines (removed duplication)
Changes to broker_grpc_lookup.go:
- Modified LookupTopicBrokers() to call getTopicConfFromCache()
- Changed from: b.fca.ReadTopicConfFromFiler(t) (no cache)
- Changed to: b.getTopicConfFromCache(t) (with cache)
- Added comment explaining this fixes 26% CPU overhead
Cache Strategy:
- First call: Cache MISS -> read filer + unmarshal JSON -> cache for 30s
- Next 1000+ calls in 30s: Cache HIT -> return cached config immediately
- No filer gRPC, no JSON unmarshaling, near-zero CPU
- Cache invalidated on topic create/update/delete
Expected CPU Reduction:
- Before: 26.41% on ReadTopicConfFromFiler + 16.64% on JSON unmarshal = 43% CPU
- After: <0.1% (only on cache miss every 30s)
- Expected total broker CPU: 25.18s -> ~8s (67% reduction!)
Performance Impact (with 250 lookups/sec):
- Before: 250 filer reads/sec + 250 JSON unmarshals/sec
- After: 0.17 filer reads/sec (5 topics / 30s TTL)
- Reduction: 99.93% fewer expensive operations
Code Quality:
- Eliminated code duplication (60 lines -> 14 lines in GetOrGenerateLocalPartition)
- Single source of truth for cached reads (getTopicConfFromCache)
- Clear API: "Always use getTopicConfFromCache, never ReadTopicConfFromFiler directly"
Testing:
- ✅ Compiles successfully
- Ready to deploy and measure CPU improvement
Priority: CRITICAL - Completes the cache optimization to achieve full performance fix
SeaweedMQ Message Queue on SeaweedFS (WIP, not ready)
What are the use cases it is designed for?
Message queues are like water pipes. Messages flow in the pipes to their destinations.
However, what if a flood comes? Of course, you can increase the number of partitions, add more brokers, restart, and watch the traffic level closely.
Sometimes the flood is expected. For example, backfill some old data in batch, and switch to online messages. You may want to ensure enough brokers to handle the data and reduce them later to cut cost.
SeaweedMQ is designed for use cases that need to:
- Receive and save large number of messages.
- Handle spike traffic automatically.
What is special about SeaweedMQ?
- Separate computation and storage nodes to scale independently.
- Unlimited storage space by adding volume servers.
- Unlimited message brokers to handle incoming messages.
- Offline messages can be operated as normal files.
- Scale up and down with auto split and merge message topics.
- Topics can automatically split into segments when traffic increases, and vice verse.
- Pass messages by reference instead of copying.
- Clients can optionally upload the messages first and just submit the references.
- Drastically reduce the broker load.
- Stateless brokers
- All brokers are equal. One broker is dynamically picked as the leader.
- Add brokers at any time.
- Allow rolling restart brokers or remove brokers at a pace.
Design
How it works?
Brokers are just computation nodes without storage. When a broker starts, it reports itself to masters. Among all the brokers, one of them will be selected as the leader by the masters.
A topic needs to define its partition key on its messages.
Messages for a topic are divided into segments. One segment can cover a range of partitions. A segment can be split into 2 segments, or 2 neighboring segments can be merged back to one segment.
During write time, the client will ask the broker leader for a few brokers to process the segment.
The broker leader will check whether the segment already has assigned the brokers. If not, select a few brokers based on their loads, save the selection into filer, and tell the client.
The client will write the messages for this segment to the selected brokers.
Failover
The broker leader does not contain any state. If it fails, the masters will select a different broker.
For a segment, if any one of the selected brokers is down, the remaining brokers should try to write received messages to the filer, and close the segment to the clients.
Then the clients should start a new segment. The masters should assign other healthy brokers to handle the new segment.
So any brokers can go down without losing data.
Auto Split or Merge
(The idea is learned from Pravega.)
The brokers should report its traffic load to the broker leader periodically.
If any segment has too much load, the broker leader will ask the brokers to tell the client to close current one and create two new segments.
If 2 neighboring segments have the combined load below average load per segment, the broker leader will ask the brokers to tell the client to close this 2 segments and create a new segment.