Github user paul-rogers commented on the issue:
https://github.com/apache/drill/pull/928
Responses to the general comments:
> I think we need to limit the number of queries waiting for queues.
Please see the detailed notes. The ZK-based mechanism does not (AFAIK)
provide the means to limit queue size. This is why this feature is preliminary,
and may be why it has been disabled, though it has been available in the code
for several years.
> For short queries, this feature might add more overhead/latency (going to
zookeeper, extra synchronization mechanisms added etc.). This will be a problem
with high concurrency. Did we quantify that ?
This is not a feature for situations of very high concurrency (= very small
queries). It is a feature for situations in which a cluster runs enough
distributed queries that the cluster becomes overloaded. For the case with
1000s of small, single-node queries, a app could use the framework provided to
implement an in-process queue. In fact, one exists, but it is intended for unit
tests.
> Not being to cancel once enqueued is also a problem. Can we do that by
having cancel release the semaphore ?
Yes, cancelation is an issue. Since the ZK-based queues were an existing
feature, I didn't dive too deeply into how they handle query cancellation. This
would be a good area for QA to exercise.
> Also, does semaphore count give an indication how many are waiting in the
queue ?
There is no ready means to determine this, though I suspect that something
could be put together that queries the internal znode and counts the ephemeral
nodes that represent waiting processes. Doing so would be handy to display
queue size in the Drill web UI. This would be a good enhancement once we prove
that the basic mechanism works.
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