[jira] [Created] (CASSANDRA-13009) Speculative retry bugs

["Simon Zhou (JIRA)" <ji...@apache.org>]

Simon Zhou created CASSANDRA-13009:
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             Summary: Speculative retry bugs
                 Key: CASSANDRA-13009
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13009
             Project: Cassandra
          Issue Type: Bug
            Reporter: Simon Zhou
            Assignee: Simon Zhou
             Fix For: 3.0.11


There are a few issues with speculative retry:

1. Time unit bugs. These are from ColumnFamilyStore (v3.0.10):

The left hand side is in nanos, as the name suggests, while the right hand side is in millis.
{code}
sampleLatencyNanos = DatabaseDescriptor.getReadRpcTimeout() / 2;
{code}

Here coordinatorReadLatency is already in nanos and we shouldn't multiple the value by 1000. This was a regression in 8896a70 when we switch metrics library and the two libraries use different time units.

{code}
sampleLatencyNanos = (long) (metric.coordinatorReadLatency.getSnapshot().getValue(retryPolicy.threshold()) * 1000d);
{code}


2. Confusing overload protection and retry delay. As the name "sampleLatencyNanos" suggests, it should be used to keep the actually sampled delay. However, we assign it the retry threshold in the case of CUSTOM. Then we compare the retry threshold with read timeout (defaults to 5000ms). This means, if we use speculative_retry=10ms for the table, we won't be able to avoid being overloaded. We should compare the actual read latency with the read timeout for overload protection.

My proposals are:
a. We use sampled p99 delay for overload protection (do not retry if latency is already very high) and introduce retry delay for waiting time before retry.
b.  Allow users to set a overload threshold through JVM option -Dcassandra.overload.threshold. The default value remains read timeout (5000ms).




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