I haven't tried it, but for parallelisation it's sometimes worth starting
from more "array-oriented" code, e.g.
(defn max-diff [check-until]
(let [val (map #(Math/sqrt %) (range 1 check-until))]
(reduce max (map #(Math/abs (- %1 %2)) (map #(Math/pow % 2) val) (map
#(* % %) val)))))
On Friday, 11 April 2014 07:43:44 UTC+1, Cecil Westerhof wrote:
>
> 2014-04-11 3:10 GMT+02:00 Andy Fingerhut <[email protected]<javascript:>
> >:
>
>> Your check-concurrent2 is a much better way to use parallelism in your
>> program -- create a few threads, each of which does a lot of work.
>>
>> I haven't analyzed the values you are computing in detail, but note that
>> your program still has lots of inter-thread 'communication' because of the
>> use of a common atom in your main-function. If you can get the correct
>> final answer by having each thread calculate independent values, or
>> collections of values, and then somehow combining them together when they
>> are all finished with a bit of quick-running sequential code, that might
>> give you even better parallel speedup than check-concurrent2.
>>
>
> I display immediately found worse values and those are dependent on what
> is found in the other t
> hreads, but I will
> think about it. I am certainly impressed by the result: less as 10%
> overhead.
>
> I do not think that the inter-thread communication is a real problem,
> because when I calculate it for ten time as much values it takes 309
> seconds. That is almost exactly ten times as much time, so this
> communication is (in this case) not a bottleneck.
>
>
>
>> The use of asterisks at beginning and end of a symbol name is
>> traditionally used on Clojure (and Common Lisp, from whence this tradition
>> came) if the var is dynamically bound, meaning different threads typically
>> do things like (binding [*dynamic-var* value-to-use-in-my-thread] <code
>> here>). There is no need for that in your program, and leaving off the
>> asterisks (or "earmuffs") is common practice.
>>
>
> So my code is not to bad? :-D
>
>
>
>> For the explanation of the 1-minute delay, see the end of the last
>> example on ClojureDocs for the future function:
>>
>> http://clojuredocs.org/clojure_core/clojure.core/future
>>
>
> I added (shutdown-agents) and the delay is gone.
>
>
> On Thu, Apr 10, 2014 at 4:35 PM, Cecil Westerhof
> <[email protected]<javascript:>
>> > wrote:
>>
>>> 2014-04-10 16:45 GMT+02:00 Andy Fingerhut <[email protected]<javascript:>
>>> >:
>>>
>>> Forcing small bits of computation to be done in parallel using the tools
>>>> Clojure and the JVM have at hand, e.g. pmap, future, etc., which rely on
>>>> creating JVM Thread objects, tends to slow things down rather than speed
>>>> things up, because the extra overhead of creating threads and waiting for
>>>> them to finish is higher than the cost of sequentially doing the small
>>>> bits
>>>> of computation.
>>>>
>>>> If you want to force parallelism in your example anyway, to compare the
>>>> most-likely-slower-performance against what you have now, you can try this:
>>>>
>>>> (let [val (Math/sqrt i)
>>>> a1 (future (Math/pow val 2))
>>>> a2 (future (* val val))
>>>> diff (Math/abs (- @a1 @a2))]
>>>>
>>>
>>> The first form of concurrency was very bad indeed. I attached the script
>>> I made. (With the first form of concurrency disabled.) The sequential
>>> version takes between 1:52 and 1:56 with 18% CPU.
>>>
>>> The first form of concurrency took 2 hours and 45 minutes with 26% CPU.
>>> That is about 87 times as long, so very bad indeed. A future seems to be
>>> very expensive.
>>>
>>> The second form of concurrency takes around 31 seconds with 57% CPU. So
>>> about 3.7 times as fast. So that is a good improvement. The output is
>>> not complete the same, but that is not important in this case.
>>>
>>> By the way: is this the right way to write Clojure code? Especially: are
>>> the names max-diff and max-factor correct, or should the names be called
>>> different? For example: *max-fiff*.
>>>
>>> I call the script with:
>>> date +%T ; clojure ~/Clojure/check-power.clj ; date +%T
>>>
>>> This gives:
>>> 00:52:10
>>> 00:52:11: Start sequential
>>> 00:52:11: Different for 838 (1.136868e-13, 1.356645e-16)
>>> 00:52:11: Different for 3352 (4.547474e-13, 1.356645e-16)
>>> 00:52:11: Different for 5343 (9.094947e-13, 1.702217e-16)
>>> 00:52:11: Different for 9470 (1.818989e-12, 1.920791e-16)
>>> 00:52:11: Different for 19922 (3.637979e-12, 1.826111e-16)
>>> 00:52:11: Different for 33757 (7.275958e-12, 2.155392e-16)
>>> 00:52:11: Different for 65830 (1.455192e-11, 2.210529e-16)
>>> 00:52:11: Different for 132323 (2.910383e-11, 2.199454e-16)
>>> 00:52:11: Different for 263320 (5.820766e-11, 2.210529e-16)
>>> 00:52:12: Different for 524523 (1.164153e-10, 2.219451e-16)
>>> 00:52:12: Different for 1052879 (2.328306e-10, 2.211371e-16)
>>> 00:52:12: Different for 2097255 (4.656613e-10, 2.220337e-16)
>>> 00:52:12: Different for 4194573 (9.313226e-10, 2.220304e-16)
>>> 00:52:12: Different for 8389020 (1.862645e-09, 2.220337e-16)
>>> 00:52:13: Different for 16777781 (3.725290e-09, 2.220371e-16)
>>> 00:52:16: Different for 33554486 (7.450581e-09, 2.220442e-16)
>>> 00:52:19: Different for 67110098 (1.490116e-08, 2.220405e-16)
>>> 00:52:27: Different for 134217944 (2.980232e-08, 2.220442e-16)
>>> 00:52:42: Different for 268436271 (5.960464e-08, 2.220439e-16)
>>> 00:53:13: Different for 536871776 (1.192093e-07, 2.220442e-16)
>>> 00:54:06: Stop, max-factor: 2.220442e-16
>>>
>>>
>>> 00:54:06: Start concurrent2
>>> 00:54:06: Different for 838 (1.136868e-13, 1.356645e-16)
>>> 00:54:06: Different for 5343 (9.094947e-13, 1.702217e-16)
>>> 00:54:06: Different for 3674 (4.547474e-13, 1.237745e-16)
>>> 00:54:06: Different for 10925 (1.818989e-12, 1.664979e-16)
>>> 00:54:06: Different for 21372 (3.637979e-12, 1.702217e-16)
>>> 00:54:06: Different for 20571 (3.637979e-12, 1.768499e-16)
>>> 00:54:06: Different for 19922 (3.637979e-12, 1.826111e-16)
>>> 00:54:06: Different for 33757 (7.275958e-12, 2.155392e-16)
>>> 00:54:06: Different for 70012 (1.455192e-11, 2.078489e-16)
>>> 00:54:06: Different for 135028 (2.910383e-11, 2.155392e-16)
>>> 00:54:06: Different for 263320 (5.820766e-11, 2.210529e-16)
>>> 00:54:06: Different for 529292 (1.164153e-10, 2.199454e-16)
>>> 00:54:06: Different for 1053280 (2.328306e-10, 2.210529e-16)
>>> 00:54:06: Different for 2098092 (4.656613e-10, 2.219451e-16)
>>> 00:54:06: Different for 4211516 (9.313226e-10, 2.211371e-16)
>>> 00:54:06: Different for 4194573 (9.313226e-10, 2.220304e-16)
>>> 00:54:06: Different for 8389473 (1.862645e-09, 2.220217e-16)
>>> 00:54:07: Different for 16777781 (3.725290e-09, 2.220371e-16)
>>> 00:54:07: Different for 33555125 (7.450581e-09, 2.220400e-16)
>>> 00:54:08: Different for 67115849 (1.490116e-08, 2.220215e-16)
>>> 00:54:10: Different for 134218985 (2.980232e-08, 2.220425e-16)
>>> 00:54:14: Different for 268441753 (5.960464e-08, 2.220394e-16)
>>> 00:54:23: Different for 536872413 (1.192093e-07, 2.220440e-16)
>>> 00:54:37: Stop, max-factor: 2.220440e-16
>>> 00:55:37
>>>
>>> One thing I do not understand, between finishing (check-concurrent2) and
>>> getting back to the shell there is one minute. When calling only
>>> (check-sequential) the script returns immediately back to the shell. Why
>>> is this?
>>>
>>>
>>> On Thu, Apr 10, 2014 at 7:35 AM, Cecil Westerhof
>>> <[email protected]<javascript:>
>>>> > wrote:
>>>>
>>>>> I have the following in my code:
>>>>> (let [val (Math/sqrt i)
>>>>> diff (Math/abs (- (Math/pow val 2) (* val val)))]
>>>>>
>>>>> I expect that calculating the difference is the most expensive part of
>>>>> my code. I understood that Clojure is very good in parallellisation. How
>>>>> would I let (Math/pow val 2) and (* val val) be calculated in parallel?
>>>>>
>>>>
> --
> Cecil Westerhof
>
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