Could you also show how you are running these functions in parallel and
time them ? The way you start the functions can have as much impact as the
functions themselves.
Regards,
Niels
On Tuesday, November 17, 2015 at 6:38:39 AM UTC+1, David Iba wrote:
>
> I have functions f1 and f2 below, and let's say they run in T1 and T2
> amount of time when running a single instance/thread. The issue I'm facing
> is that parallelizing f2 across 18 cores takes anywhere from 2-5X T2, and
> for more complex funcs takes absurdly long.
>
>
> 1. (defn f1 []
> 2. (apply + (range 2e9)))
> 3.
> 4. ;; Note: each call to (f2) makes its own x* atom, so the 'swap!'
> should never retry.
> 5. (defn f2 []
> 6. (let [x* (atom {})]
> 7. (loop [i 1e9]
> 8. (when-not (zero? i)
> 9. (swap! x* assoc :k i)
> 10. (recur (dec i))))))
>
>
> Of note:
> - On a 4-core machine, both f1 and f2 parallelize well (roungly T1 and T2
> for 4 runs in parallel)
> - running 18 f1's in parallel on the 18-core machine also parallelizes
> well.
> - Disabling hyperthreading doesn't help.
> - Based on jvisualvm monitoring, doesn't seem to be GC-related
> - also tried on dedicated 18-core ec2 instance with same issues, so not
> shared-tenancy-related
> - if I make a jar that runs a single f2 and launch 18 in parallel, it
> parallelizes well (so I don't think it's machine/aws-related)
>
> Could it be that the 18 f2's in parallel on a single JVM instance is
> overworking the STM with all the swap's? Any other theories?
>
> Thanks!
>
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