javanna commented on code in PR #12569:
URL: https://github.com/apache/lucene/pull/12569#discussion_r1329672558
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lucene/core/src/java/org/apache/lucene/search/TaskExecutor.java:
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@@ -22,18 +22,28 @@
import java.util.Collection;
import java.util.List;
import java.util.Objects;
+import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
-import java.util.concurrent.RunnableFuture;
+import java.util.concurrent.FutureTask;
import org.apache.lucene.util.IOUtils;
import org.apache.lucene.util.ThreadInterruptedException;
/**
* Executor wrapper responsible for the execution of concurrent tasks. Used to
parallelize search
- * across segments as well as query rewrite in some cases.
+ * across segments as well as query rewrite in some cases. Exposes a {@link
#createTask(Callable)}
+ * method to create tasks given a {@link Callable}, as well as the {@link
#invokeAll(Collection)}
+ * method to execute a set of tasks concurrently. Once all tasks are submitted
to the executor, it
+ * blocks and wait for all tasks to be completed, and then returns a list with
the obtained results.
+ * Ensures that the underlying executor is only used for top-level {@link
#invokeAll(Collection)}
+ * calls, and not for potential {@link #invokeAll(Collection)} calls made from
one of the tasks.
+ * This is to prevent deadlock with certain types of executors, as well as to
limit the level of
+ * parallelism.
Review Comment:
That's a good question. I assumed that once we are in a concurrent task,
parallelizing further may cause more harm than good, especially given the
"block and wait" for all tasks to be completed. In practice, I wonder what
other executor implementations are desirable to provide to the index searcher
that are not thread pool based: say that you have an executor that creates a
new thread for each execute request (does not seem like a good idea anyways),
would we be ok allowing for more than one level of parallelism? Is that a valid
usecase?
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