On Tue, Jul 23, 2013 at 11:12 AM, Baishampayan Ghose <b.gh...@gmail.com> wrote:
> It's definitely got to do with the code, the right way to test it out
> will be to wrap the form in a function and then calling it twice. Like
> so -
>
> (time
> (let [x (fn [] (Thread/sleep 2000)
> (+ 1 1))]
> [(x) (x)]))
> ;=> "Elapsed time: 4002.0 msecs"
> ;=> [2 2]
>
> Hope that helps.
>
> Regards,
> BG
>
>
> On Tue, Jul 23, 2013 at 8:34 PM, Ryan Moore <niclas1...@gmail.com> wrote:
>> There is an example in the book The Joy of Clojure on p.262 that uses
>> futures that I evaluated in the REPL.
>>
>> user> (time
>> (let [x (future (do (Thread/sleep 2000)
>> (+ 1 1)))]
>> [@x @x]))
>> "Elapsed time: 2000.809 msecs"
>> [2 2]
>>
>> I figured that taking out the future would cause the execution to take twice
>> as long, however, when I try this:
>>
>> user> (time
>> (let [x (do (Thread/sleep 2000)
>> (+ 1 1))]
>> [x x]))
>> "Elapsed time: 2000.512 msecs"
>> [2 2]
>>
>> as you see it takes about the same amount of time. Does this have something
>> to do with the REPL evaluating things or maybe the newer version of Clojure
>> handles things differently from the Joy of Clojure book?
Can also show the difference using two different vars
(time (let [x (future (do (Thread/sleep 2000) (+ 1 1)))
y (future (do (Thread/sleep 3000) (+ 2 2)))]
[@x @y]))
"Elapsed time: 3003.802 msecs"
[2 4]
(time (let [x (do (Thread/sleep 2000) (+ 1 1))
y (do (Thread/sleep 3000) (+ 2 2))]
[x y]))
"Elapsed time: 5003.049 msecs"
[2 4]
Basically, [x x] doesn't do the evaluation, it happens in the let once
for x. For [@x @x] the thread is kicked off once to do the
computation, and the first @x waits for the result (if not already
available) while the second @x uses the cached value.
In my modified version, the second case the two Thread/sleep happens
in sequence, while in the first they take place in parallel thanks to
the futures.
Lars Nilsson
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