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Stefania commented on CASSANDRA-7392:
-------------------------------------
Ariel, thank you for your comments, which are quite valid and I plan on
tackling as soon as I have wrapped up two other tickets.
The core issue is how to monitor tasks. The existing approach should have a
cost linear to the number of _in progress_ operations, since we cancel the
future when the operation completes. This should be the same as the number of
threads. I thought about a single thread with a queue as well, but opted for
scheduling a task per operation due to the cost of keeping a shared array or
queue. It did not occur to me to consider using a thread local, which is an
excellent idea.
To make sure I understand your suggestion correctly: an entry is inserted when
the _thread local is created_, meaning when the thread local initial value is
created, so the COW array is copied once per thread. Then, when a new operation
starts, we need to atomically modify the state in the thread local. Correct?
So, this gets rid of the priority queue contention as well as the insert and
remove logarithmic costs, which is nice. The downside is the complexity of
atomically swapping in a new operation, which shouldn't be much.
bq. Just to validate my understanding, when a query is aborted it either sends
no response or sends an error response? I just want to be sure that it doesn't
silently claim there is no more data.
It should send no response, this is the same behavior for queries that are
dropped before they are started, see the top of
{{MessagingDeliveryTask.run()}}. We could also send a timeout error but I think
the coordinator times out anyway on its own, and I prefer not to change the
coordinator code.
> Abort in-progress queries that time out
> ---------------------------------------
>
> Key: CASSANDRA-7392
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7392
> Project: Cassandra
> Issue Type: New Feature
> Components: Core
> Reporter: Jonathan Ellis
> Assignee: Stefania
> Fix For: 3.x
>
>
> Currently we drop queries that time out before we get to them (because node
> is overloaded) but not queries that time out while being processed.
> (Particularly common for index queries on data that shouldn't be indexed.)
> Adding the latter and logging when we have to interrupt one gets us a poor
> man's "slow query log" for free.
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