Marcus Olsson created CASSANDRA-14983:
-----------------------------------------
Summary: Local reads potentially blocking remote reads
Key: CASSANDRA-14983
URL: https://issues.apache.org/jira/browse/CASSANDRA-14983
Project: Cassandra
Issue Type: Bug
Components: Consistency/Coordination
Reporter: Marcus Olsson
Attachments: local_read_trace.log
Since CASSANDRA-4718 there is a fast path allowing local requests to continue
to [work in the same
thread|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/service/reads/AbstractReadExecutor.java#L157]
rather than being sent over to the read stage.
Based on the comment
{code:java}
// We delay the local (potentially blocking) read till the end to avoid
stalling remote requests.
{code}
it seems like this should be performed last in the chain to avoid blocking
remote requests but that does not seem to be the case when the local request is
a data request. The digest request(s) are sent after the data requests are sent
(and now the transient replica requests as well). When the fast path is used
for local data/transient data requests this will block the next type of request
from being sent away until the local read is finished and add additional
latency to the request.
In addition to this it seems like local requests are *always* data requests
(might not be a problem), but the log message can say either ["digest" or
"data"|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/service/reads/AbstractReadExecutor.java#L156]
as the type of request.
I have tried to run performance measurements to see the impact of this in 3.0
(by moving local requests to the end of ARE#executeAsync()) but I haven't seen
any big difference yet. I'll continue to run some more tests to see if I can
find a use case affected by this.
Attaching a trace (3.0) where this happens. Reproduction:
# Create a three node CCM cluster
# Provision data with stress (rf=3)
# In parallel:
## Start stress read run
## Run multiple manual read queries in cqlsh with tracing on and local_quorum
(as this does not always happen)
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
