2014-04-10 16:45 GMT+02:00 Andy Fingerhut <andy.finger...@gmail.com>:
> 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 <cldwester...@gmail.com>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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check-power.clj
Description: Binary data
