Re: Concurrency question?

Eric Sosman <esosman@comcast-dot-net.invalid>
Sat, 31 Jan 2015 14:04:46 -0500
On 1/31/2015 1:34 PM, Knute Johnson wrote:

So I have a Map<Integer,String> but it could be a List or an Array,
shouldn't make any difference. Access to the Map is controlled by a
ReadWriteLock. Several threads read values from the Map and one thread
can change values or write to or remove key/value pairs from the Map.
The thread that writes new values to the Map is the EDT, in a
DocumentListener. One of the reading threads is time consuming and if
that thread has the read lock the EDT blocks until it can obtain the
write lock. That can generate noticeable delays in data entry through
the GUI.

Since data entry is slow it doesn't matter much if the data entry
changes are available during the time period that the time consuming
thread is reading the Map. However to ensure that I have current data
at some point any read access of the Map must be controlled by a read lock.

I needed a way to make the time consuming thread not need to hold the
read lock for the entire time. My solution was to acquire the read lock
and create a List of values from the Map and return the read lock, this
is very quick. Then I iterate through the List to perform the time
consuming task.

I know that it is possible that the GUI could change the Map while the
time consuming thread is running. That isn't critical as the data would
be current the next time the time consuming thread ran. The reference
to the List goes out of scope when the time consuming thread completes.
  Any values that were in the List that were no longer in the Map should
be available for garbage collection.

Do you see any holes in my logic? Any suggestions?

     Sounds reasonable, on the assumption that out-of-date mappings won't
mess up your time-consuming operation.

     Another possibility would be to buffer on the producer side instead.
Rather than updating the Map directly, the EDT could put the updates on
a queue (quickly) and go about its business. Another thread would wait
for updates to appear on the queue, wait for and grab the write lock,
and then perform the updates. (This is a bit like the write buffers
found in modern CPU's: They don't wait for memory and cache to be
ready, but just stuff the write into a buffer and trust that it will
eventually get flushed to the hard-to-reach backing store.)

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