leventov commented on a change in pull request #6629: Add support parallel
combine in brokers
URL: https://github.com/apache/incubator-druid/pull/6629#discussion_r242156826
##########
File path: processing/src/main/java/org/apache/druid/query/QueryContexts.java
##########
@@ -174,6 +179,35 @@
}
}
+ private static int checkPositive(String propertyName, int val)
+ {
+ Preconditions.checkArgument(
+ val > 0,
+ "%s should be positive, but [%s]",
+ propertyName,
+ val
+ );
+ return val;
+ }
+
+ /**
+ * Return the configured number of combine threads if any. Others {@link
#NO_PARALLEL_COMBINE_THREADS}.
+ */
+ public static <T> int getNumBrokerParallelCombineThreads(Query<T> query)
+ {
+ return parseInt(query, NUM_BROKER_PARALLEL_COMBINE_THREADS,
NO_PARALLEL_COMBINE_THREADS);
Review comment:
It's essentially impossible to set `NUM_BROKER_PARALLEL_COMBINE_THREADS`
"right" even if it is done on query per query basis, that will nobody ever do.
This moment broker could be idle, the next moment a lot of queries will arrive
and setting `NUM_BROKER_PARALLEL_COMBINE_THREADS` will only make things worse.
> because automatic optimization might be wrong or not enough
My intention to employ techniques from ForkJoinPool. When you use
`parallelStream()`, it basically could not parallelize wrong or not enough. It
may not be 100% optimal because of coarse partitioning granularity (You could
only split any piece of work in two parts, not three, for example), but it is
always within 10-20% from optimum. And this actually should not be so for
combine on brokers.
The only way to really abuse `parallelStream()` is to pass it something that
could be completed in less than 10ms. But it could be safely avoided in
parallel combine too, as I shown above.
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