Bug: Performance regression on internode messaging
Status: Resolved
Priority: Normal
Resolution: Fixed
Created: 07/Apr/20 11:00
Resolved: 29/Jun/20 16:57

Me and [~jasonstack] have been investigating a performance regression affecting 4.0 during a 3 nodes, RF 3 write throughput test with a timeseries like workload, as shown in this plot, where blue is 3.11 and orange is 4.0:

!Oss40vsOss311.png|width=389,height=214!

 It's been a bit of a long investigation, but two clues ended up standing out:
1) An abnormal number of expired messages on 4.0 (as shown in the attached  system log), while 3.11 has almost none.
2) An abnormal GC activity (as shown in the attached gc log).

Turns out the two are related, as the [on expired callback|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/net/OutboundConnection.java#L462] creates a huge amount of strings in the {{id()}} call. The next question is what causes all those message expirations; we thoroughly reviewed the internode messaging code and the only issue we could find so far is related to the "batch pruning" calls [here|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/net/OutboundMessageQueue.java#L81] and [here|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/net/OutboundMessageQueue.java#L188]: it _seems_ too much time is spent on those, causing the event loop to fall behind in processing the rest of the messages, which will end up being expired. This is supported by the analysis of the collapsed stacks (after fixing the GC issue):
{noformat}
(tprint (top-aggregated-calls oss40nogc "EventLoopDelivery:doRun" 5))
org/apache/cassandra/net/OutboundConnection$EventLoopDelivery:doRun 3456
org/apache/cassandra/net/OutboundMessageQueue:access$600 1621
org/apache/cassandra/net/PrunableArrayQueue:prune 1621
org/apache/cassandra/net/OutboundMessageQueue$WithLock:close 1621
org/apache/cassandra/net/OutboundMessageQueue:pruneInternalQueueWithLock 1620
{noformat}

Those are the top 5 sampled calls from {{EventLoopDelivery#doRun()}} which spends half of its time pruning. But only a tiny portion of such pruning time is spent actually expiring:
{noformat}
(tprint (top-aggregated-calls oss40nogc "OutboundMessageQueue:pruneInternalQueueWithLock" 5))
org/apache/cassandra/net/OutboundMessageQueue:pruneInternalQueueWithLock 1900
org/apache/cassandra/net/PrunableArrayQueue:prune 1894
org/apache/cassandra/net/OutboundMessageQueue$1Pruner:onPruned 147
org/apache/cassandra/net/OutboundConnection$$Lambda$444/740904487:accept 147
org/apache/cassandra/net/OutboundConnection:onExpired 147
{noformat}

And indeed, the {{PrunableArrayQueue:prune()}} self time is dominant:
{noformat}
(tprint (top-self-calls oss40nogc "PrunableArrayQueue:prune" 5))
org/apache/cassandra/net/PrunableArrayQueue:prune 1718
org/apache/cassandra/net/OutboundConnection:releaseCapacity 27
java/util/concurrent/ConcurrentHashMap:replaceNode 19
java/util/concurrent/ConcurrentLinkedQueue:offer 16
java/util/concurrent/LinkedBlockingQueue:offer 15
{noformat}

That said, before proceeding with a PR to fix those issues, I'd like to understand: what's the reason to prune so often, rather than just when polling the message during delivery? If there's a reason I'm missing, let's talk about how to optimize pruning, otherwise let's get rid of that.