* filer.sync: pipelined subscription with adaptive batching for faster catch-up
The SubscribeMetadata pipeline was fully serial: reading a log entry from a
volume server, unmarshaling, filtering, and calling stream.Send() all happened
one-at-a-time. stream.Send() blocked the entire pipeline until the client
acknowledged each event, limiting throughput to ~80 events/sec regardless of
the -concurrency setting.
Three server-side optimizations that stack:
1. Pipelined sender: decouple stream.Send() from the read loop via a buffered
channel (1024 messages). A dedicated goroutine handles gRPC delivery while
the reader continues processing the next events.
2. Adaptive batching: when event timestamps are >2min behind wall clock
(backlog catch-up), drain multiple events from the channel and pack them
into a single stream.Send() using a new `repeated events` field on
SubscribeMetadataResponse. When events are recent (real-time), send
one-by-one for low latency. Old clients ignore the new field (backward
compatible).
3. Persisted log readahead: run the OrderedLogVisitor in a background
goroutine so volume server I/O for the next log file overlaps with event
processing and gRPC delivery.
4. Event-driven aggregated subscription: replace time.Sleep(1127ms) polling
in SubscribeMetadata with notification-driven wake-up using the
MetaLogBuffer subscriber mechanism, reducing real-time latency from
~1127ms to sub-millisecond.
Combined, these create a 3-stage pipeline:
[Volume I/O → readahead buffer] → [Filter → send buffer] → [gRPC Send]
Test results (simulated backlog with 50µs gRPC latency per Send):
direct (old): 2100 events 2100 sends 168ms 12,512 events/sec
pipelined+batched: 2100 events 14 sends 40ms 52,856 events/sec
Speedup: 4.2x single-stream throughput
Ref: #8771
* filer.sync: require client opt-in for batch event delivery
Add ClientSupportsBatching field to SubscribeMetadataRequest. The server
only packs events into the Events batch field when the client explicitly
sets this flag to true. Old clients (Java SDK, third-party) that don't
set the flag get one-event-per-Send, preserving backward compatibility.
All Go callers (FollowMetadata, MetaAggregator) set the flag to true
since their recv loops already unpack batched events.
* filer.sync: clear batch Events field after Send to release references
Prevents the envelope message from holding references to the rest of the
batch after gRPC serialization, allowing the GC to collect them sooner.
* filer.sync: fix Send deadlock, add error propagation test, event-driven local subscribe
- pipelinedSender.Send: add case <-s.done to unblock when sender goroutine
exits (fixes deadlock when errCh was already consumed by a prior Send).
- pipelinedSender.reportErr: remove for-range drain on sendCh that could
block indefinitely. Send() now detects exit via s.done instead.
- SubscribeLocalMetadata: replace remaining time.Sleep(1127ms) in the
gap-detected-no-memory-data path with event-driven listenersCond.Wait(),
consistent with the rest of the subscription paths.
- Add TestPipelinedSenderErrorPropagation: verifies error surfaces via
Send and Close when the underlying stream fails.
- Replace goto with labeled break in test simulatePipeline.
* filer.sync: check error returns in test code
- direct_send: check slowStream.Send error return
- pipelined_batched_send: check sender.Close error return
- simulatePipeline: return error from sender.Close, propagate to callers
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Co-authored-by: Copilot <copilot@github.com>
Chris Lu