Below I present 'submit-future' which is similar to the existing
'future' call in that it spawns a thread to execute a task, but
differs in that it will block if n submitted futures are already
running, where n is the number of available processors. I think this
could be quite handy for the producer-consumer model which lazy-seq
lends itself to, allowing one to write:
(doseq [d data-seq]
(submit-future (foo d)))
where data-seq is some feed of CPU bound tasks which you want to
process as quickly as possible. This is diverging from the OP, but
thought it might be of interest:
(let [limit (.availableProcessors (Runtime/getRuntime))
sem (java.util.concurrent.Semaphore. limit)]
(defn submit-future-call
"Takes a function of no args and yields a future object that will
invoke the function in another thread, and will cache the result and
return it on all subsequent calls to deref/@. If the computation has
not yet finished, calls to deref/@ will block.
If n futures have already been submitted, then submit-future blocks
until the completion of another future, where n is the number of
available processors."
[#^Callable task]
; take a slot (or block until a slot is free)
(.acquire sem)
(try
; create a future that will free a slot on completion
(future (try (task) (finally (.release sem))))
(catch java.util.concurrent.RejectedExecutionException e
; no task was actually submitted
(.release sem)
(throw e)))))
(defmacro submit-future
"Takes a body of expressions and yields a future object that will
invoke the body in another thread, and will cache the result and
return it on all subsequent calls to deref/@. If the computation has
not yet finished, calls to deref/@ will block.
If n futures have already been submitted, then submit-future blocks
until the completion of another future, where n is the number of
available processors."
[& body] `(submit-future-call (fn [] ~...@body)))
#_(example
user=> (submit-future (reduce + (range 100000000)))
#<core$future_call$reify__5...@6c69d02b: :pending>
user=> (submit-future (reduce + (range 100000000)))
#<core$future_call$reify__5...@38827968: :pending>
user=> (submit-future (reduce + (range 100000000)))
;; blocks at this point for a 2 processor PC until the previous
;; two futures complete
#<core$future_call$reify__5...@214c4ac9: :pending>)
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