[concurrency-interest] LongAdder with custom behavior?

Carl Mastrangelo notcarl at google.com
Fri Dec 1 02:18:53 EST 2017

While looking at LongAdder as a possible candidate for a concurrent
counter, I noticed three things that made it seem surprising.  I am
wondering what the rationale is for these since they don't make sense to me:

1.  LongAdder.sumThenReset seems to not be thread-safe.  If concurrent
writes happen between reading and resetting of each Cell, then the write
could get dropped.    This matches the behavior described in reset(), but
makes the class less useful.   For instance, I would like sumThenReset to
tally up all the mutations, and reset the counter back to zero without
dropping mutations.  This would make it so I call sumThenReset later, and
pick up any mutations I missed.

2.  Following up on the first point, the implementation of sumThenReset
does two volatile operations on each Cell.value.  First it reads, followed
by setting it to zero.  Why doesn't it use getAndSet on the value?  On the
surface it would appear to be fewer synchronization points.   Also, it
would nicely solve the first item.

3.   In the case that the LongAdder only ever increases in value (no
resets), it would be possible to provide a means to see if it's equal to
zero.  I believe this would be cheaper to implement than summing across all
Cells.  The list of cells could be walked until there was a single nonzero
cell, at which point it could return early.  This would be similar to the
isEmpty() method on ConcurrentLinkedQueue, which doesn't need to walk the
whole list.

The reason I bring this up is that although the LongAdder class is
non-final, there is no way to provide an isEmpty() myself.  All the access
to the Cells and value are package private.  If I did implement this, I
would have to give up the nice contention features.

Is LongAdder reusable or should I try making my own counter?  Also, were
the issues I bring up now brought up during the design?

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