[AURORA-1840] Issue with Curator-backed discovery under heavy load

• Type: Bug
• Status: Resolved
• Priority: Blocker
• Resolution: Fixed
• Fix Version/s: 0.17.0
• Component/s: Scheduler
• Assignee: David McLaughlin
• Reporter: David McLaughlin
• Created: 01/Dec/16 07:17
• Updated: 20/Apr/17 22:26
• Resolved: 01/Dec/16 17:04

Description

We've been having some performance issues recently with our production clusters at Twitter. A side-effect of these are occassional stop-the-world GC pauses for up to 15 seconds. This has been happening at our scale for quite some time, but previous versions of the Scheduler were resilient to this and no leadership change would occur.

Since we moved to Curator, we are no longer resilient to these GC pauses. The Scheduler is now failing over any time we see a GC pause, even though these pauses are within the session timeout. Here is an example pause in the scheduler logs with the associated ZK session timeout that leads to a failover:

I1118 19:40:16.871801 51800 sched.cpp:1025] Scheduler::statusUpdate took 586236ns
I1118 19:40:16.902 [TaskGroupBatchWorker, StateMachine$Builder:389] redacted-9f565b4-067e-422f-b641-c6000f9ae2c8 state machine transition PENDING -> ASSIGNED 
I1118 19:40:16.903 [TaskGroupBatchWorker, TaskStateMachine:474] Adding work command SAVE_STATE for redacted-0-49f565b4-067e-422f-b641-c6000f9ae2c8 
I1118 19:40:16.903 [TaskGroupBatchWorker, TaskAssigner$TaskAssignerImpl:130] Offer on agent redacted (id 566ae347-c1b6-44ce-8551-b7a6cda72989-S7579) is being assigned task redacted-0-49f565b4-067e-422f-b641-c6000f9ae2c8. 
W1118 19:40:31.744 [main-SendThread(redacted:2181), ClientCnxn$SendThread:1108] Client session timed out, have not heard from server in 20743ms for sessionid 0x6584fd2b34ede86 

As you can see from the timestamps, there was a 15s GC pause (confirmed in our GC logs - a CMS promotion failure caused the pause) and this triggers a session timeout of 20s to fire. Note: we have seen GC pauses as little as 7s cause the same behavior. Removed: my ZK was rusty. 20s is 2/3 of our 30s ZK timeout, so our session timeout is being wired through fine.

We have confirmed that the Scheduler no longer fails over when deploying from HEAD with these two commits reverted and setting zk_use_curator to false:

https://github.com/apache/aurora/commit/b417be38fe1fcae6b85f7e91cea961ab272adf3f
https://github.com/apache/aurora/commit/69cba786efc2628eab566201dfea46836a1d9af5

This is a pretty big blocker for us given how expensive Scheduler failovers are (currently several minutes for us).

Issue Links

• blocks: AURORA-1669 Kill twitter/commons ZK libs when Curator replacements are vetted