Andy: Interesting. Thanks for educating me on the fact that atom swap's
don't use the STM. Your theory seems plausible... I will try those tests
next time I launch the 18-core instance, but yeah, not sure how
illuminating the results will be.
Niels: along the lines of this (so that each thread prints its time as well
as printing the overall time):
1. (time
2. (let [f f1
3. n-runs 18
4. futs (do (for [i (range n-runs)]
5. (future (time (f)))))]
6. (doseq [fut futs]
7. @fut)))
On Tuesday, November 17, 2015 at 5:33:01 PM UTC+9, Niels van Klaveren wrote:
>
> 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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