(Nio|Epoll)EventLoop.pendingTasks should not block when called outside of event loop

Expected behavior

EventLoop.pendingTasks should be (reasonably) cheap to invoke so it can be used within observability.

Actual behavior

We’ve been experimenting with overriding EventExecutorChooserFactory with more superior load balancing algorithms and found out that it might be quite non-trivial to support pending-tasks load metric (pick the least loaded event loop).

After poking around the code, it seems when querying pendingTasks of either NioEventLoop or EpollEventLoop, the method blocks if called outside of said event loop. Put it this way, it’s only cheap to query event loop’s pending tasks from within the same event loop. Perhaps I miss some (historical) context for pendingTask, but for an uneducated reader, it just defeats the purpose of a method providing visibility into an event loop.

I was hoping to pick up some context on the current behavior of pendingTasks and see if we ever want/will to change it.

[nitsanw]

@johnou I apologize if I sounded impatient 😃 I was reading your question from the GitHub email. Here’s what I got:

@nitsanw is it safe to invoke MpscQueue.size() from multiple threads?

It appears that further edits do not get sent as further e-mails.

So, yes, it’s safe to call size on any queue in jctools from any thread, no issue. Furthermore, current queue being used is either the MpscChunkedArrayQueue or MpscUnboundedArrayQueue: https://github.com/netty/netty/blob/06f3574e46e0623a45a93d36dbe7aa3d7455a995/common/src/main/java/io/netty/util/internal/PlatformDependent.java#L823

Both of which have a very fast size() method.

As a side note, I’m not sure why in the non-unsafe case you do not use MpscChunkedAtomicArrayQueue: https://github.com/netty/netty/blob/06f3574e46e0623a45a93d36dbe7aa3d7455a995/common/src/main/java/io/netty/util/internal/PlatformDependent.java#L859

Thanks

[johnou]

@nitsanw and also asked if it was unsafe and posed risks ie. livelock, but given your comment now I will assume it’s safe to invoke size() from multiple threads, thanks!