[jira] [Commented] (CASSANDRA-5932) Speculative read performance data show unexpected results

["Hubert Sugeng (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-5932?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13768448#comment-13768448 ] 

Hubert Sugeng commented on CASSANDRA-5932:
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[~enigmacurry], what was your collection interval for the metrics you've collected?

I ask because I'm observing results similar to yours for "3) Eager Reads tend to lessen the immediate performance impact of a node going down, but not consistently.". However, I've polled metrics @ a 1 second granularity to see that it's actually a multi-second stress-client outage - not just poor and inconsistent performance.

Polling metrics @ a 1 second interval, has observations that a ~20 second read operations starvation outage occurs for the stress client for all data in the cluster (even with the lowest phi_convict_threshold=6).

Analysis so far indicates that high-operations reads starve out all the C* client threads/connections, because they get stuck on awaiting for a server response whenever the key-space hits the node that is down (and by probability + high-operation reads, within 1 second each stress client thread will all hit the downed-node's key-space).



So I'm confirming that I'm also seeing this bug that Speculative reads (even with an ALWAYS setting). It isn't solving this outage for clients during high-operation reads, and based on what I understand of the feature, it should.

Thanks, guys!


                
> Speculative read performance data show unexpected results
> ---------------------------------------------------------
>
>                 Key: CASSANDRA-5932
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5932
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Ryan McGuire
>            Assignee: Aleksey Yeschenko
>             Fix For: 2.0.1
>
>         Attachments: compaction-makes-slow.png, compaction-makes-slow-stats.png, eager-read-looks-promising.png, eager-read-looks-promising-stats.png, eager-read-not-consistent.png, eager-read-not-consistent-stats.png, node-down-increase-performance.png
>
>
> I've done a series of stress tests with eager retries enabled that show undesirable behavior. I'm grouping these behaviours into one ticket as they are most likely related.
> 1) Killing off a node in a 4 node cluster actually increases performance.
> 2) Compactions make nodes slow, even after the compaction is done.
> 3) Eager Reads tend to lessen the *immediate* performance impact of a node going down, but not consistently.
> My Environment:
> 1 stress machine: node0
> 4 C* nodes: node4, node5, node6, node7
> My script:
> node0 writes some data: stress -d node4 -F 30000000 -n 30000000 -i 5 -l 2 -K 20
> node0 reads some data: stress -d node4 -n 30000000 -o read -i 5 -K 20
> h3. Examples:
> h5. A node going down increases performance:
> !node-down-increase-performance.png!
> [Data for this test here|http://ryanmcguire.info/ds/graph/graph.html?stats=stats.eager_retry.node_killed.just_20.json&metric=interval_op_rate&operation=stress-read&smoothing=1]
> At 450s, I kill -9 one of the nodes. There is a brief decrease in performance as the snitch adapts, but then it recovers... to even higher performance than before.
> h5. Compactions make nodes permanently slow:
> !compaction-makes-slow.png!
> !compaction-makes-slow-stats.png!
> The green and orange lines represent trials with eager retry enabled, they never recover their op-rate from before the compaction as the red and blue lines do.
> [Data for this test here|http://ryanmcguire.info/ds/graph/graph.html?stats=stats.eager_retry.compaction.2.json&metric=interval_op_rate&operation=stress-read&smoothing=1]
> h5. Speculative Read tends to lessen the *immediate* impact:
> !eager-read-looks-promising.png!
> !eager-read-looks-promising-stats.png!
> This graph looked the most promising to me, the two trials with eager retry, the green and orange line, at 450s showed the smallest dip in performance. 
> [Data for this test here|http://ryanmcguire.info/ds/graph/graph.html?stats=stats.eager_retry.node_killed.json&metric=interval_op_rate&operation=stress-read&smoothing=1]
> h5. But not always:
> !eager-read-not-consistent.png!
> !eager-read-not-consistent-stats.png!
> This is a retrial with the same settings as above, yet the 95percentile eager retry (red line) did poorly this time at 450s.
> [Data for this test here|http://ryanmcguire.info/ds/graph/graph.html?stats=stats.eager_retry.node_killed.just_20.rc1.try2.json&metric=interval_op_rate&operation=stress-read&smoothing=1]

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