TestSharedSnapshotConcurrentIntegrity writes as fast as one core can
marshal for two seconds, so its transient heap scales with host speed.
On linux/386 a fast runner walks the heap into the 4GB address-space
ceiling and the run dies with out of memory. Set a 1GB memory limit for
the duration of the test so the GC paces the writer instead of the
address space; the seal-and-recycle race being tested is unaffected.
* log_buffer: share one window snapshot across all subscriber reads
Every in-memory read handed each subscriber a private pooled copy of the
window it wanted, so N subscribers reading the same data cost N copies of
up to 8MB each -- and slow consumers (grpc send backpressure) held those
copies live for their whole iteration. With hundreds of mount subscribers
that multiplied into gigabytes of live heap on the filer.
Share the bytes instead of copying per reader:
- Sealed windows get a lazily created GC-owned snapshot, made once by the
first reader and handed out zero-copy to the rest. The snapshot travels
with its window when SealBuffer shifts slots, so recycling the sealed
array never invalidates it.
- The current window keeps a shared snapshot of its append-only prefix
buf[:pos], extended on demand; each byte is copied once per window
(writer-rate-bound) instead of once per reader. At seal a fully
extended prefix becomes the sealed window's snapshot.
ReadFromBuffer now reports whether the returned buffer is a pooled copy
(flush path) or a shared view that must not be released; the read loops
only recycle pooled buffers.
With 200 subscribers consuming at grpc pace over sealed and current
windows, peak live heap drops from 5.2GB to 178MB.
* log_buffer: clear released read buffer so a panic cannot double-free it
The read loops release the previous iteration's pooled buffer and then
call ReadFromBuffer. If that call panicked before reassigning bytesBuf,
the deferred cleanup would put the same buffer into the pool a second
time, letting two future readers share one backing array. Nil the
pointer at the release site so the defer sees nothing to free.
Chris LuandGitHub