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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