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

["Ryan McGuire (JIRA)" <ji...@apache.org>]

Ryan McGuire created CASSANDRA-5932:
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             Summary: 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
         Attachments: compaction-makes-slow.png, eager-read-looks-promising.png, eager-read-not-consistent.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]

h5. Compactions make nodes permanently slow:

!compaction-makes-slow.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!

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!

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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