Timothy: Each thread (call of f2) creates its own "local" atom, so I don't
think there should be any swap retries.
Gianluca: Good idea! I've only tried OpenJDK, but I will look into trying
Oracle and report back.
Andy: jvisualvm was showing pretty much all of the memory allocated in the
eden space and a little in the first survivor (no major/full GC's), and
total GC Time was very minimal.
I'm in the middle of running some more tests and will report back when I
get a chance today or tomorrow. Thanks for all the feedback on this!
On Thursday, November 19, 2015 at 12:38:55 AM UTC+9, tbc++ wrote:
>
> This sort of code is somewhat the worst case situation for atoms (or
> really for CAS). Clojure's swap! is based off the "compare-and-swap" or CAS
> operation that most x86 CPUs have as an instruction. If we expand swap! it
> looks something like this:
>
> (loop [old-val @x*]
> (let [new-val (assoc old-val :k i)]
> (if (compare-and-swap x* old-val new-val)
> new-val
> (recur @x*)))
>
> Compare-and-swap can be defined as "updates the content of the reference
> to new-val only if the current value of the reference is equal to the
> old-val).
>
> So in essence, only one core can be modifying the contents of an atom at a
> time, if the atom is modified during the execution of the swap! call, then
> swap! will continue to re-run your function until it's able to update the
> atom without it being modified during the function's execution.
>
> So let's say you have some super long task that you need to integrate into
> a ref, he's one way to do it, but probably not the best:
>
> (let [a (atom 0)]
> (dotimes [x 18]
> (future
> (swap! a long-operation-on-score some-param))))
>
>
> In this case long-operation-on-score will need to be re-run every time a
> thread modifies the atom. However if our function only needs the state of
> the ref to add to it, then we can do something like this instead:
>
> (let [a (atom 0)]
> (dotimes [x 18]
> (future
> (let [score (long-operation-on-score some-param)
> (swap! a + score)))))
>
> Now we only have a simple addition inside the swap! and we will have less
> contention between the CPUs because they will most likely be spending more
> time inside 'long-operation-on-score' instead of inside the swap.
>
> *TL;DR*: do as little work as possible inside swap! the more you have
> inside swap! the higher chance you will have of throwing away work due to
> swap! retries.
>
> Timothy
>
> On Wed, Nov 18, 2015 at 8:13 AM, gianluca torta <giat...@gmail.com
> <javascript:>> wrote:
>
>> by the way, have you tried both Oracle and Open JDK with the same results?
>> Gianluca
>>
>> On Tuesday, November 17, 2015 at 8:28:49 PM UTC+1, Andy Fingerhut wrote:
>>>
>>> David, you say "Based on jvisualvm monitoring, doesn't seem to be
>>> GC-related".
>>>
>>> What is jvisualvm showing you related to GC and/or memory allocation
>>> when you tried the 18-core version with 18 threads in the same process?
>>>
>>> Even memory allocation could become a point of contention, depending
>>> upon how the memory allocation works with many threads. e.g. Depends on
>>> whether a thread gets a large chunk of memory on a global lock, and then
>>> locally carves it up into the small pieces it needs for each individual
>>> Java 'new' allocation, or gets a global lock for every 'new'. The latter
>>> would give terrible performance as # cores increase, but I don't know how
>>> to tell whether that is the case, except by knowing more about how the
>>> memory allocator is implemented in your JVM. Maybe digging through OpenJDK
>>> source code in the right place would tell?
>>>
>>> Andy
>>>
>>> On Tue, Nov 17, 2015 at 2:00 AM, David Iba <davi...@gmail.com> wrote:
>>>
>>>> correction: that "do" should be a "doall". (My actual test code was a
>>>> bit different, but each run printed some info when it started so it
>>>> doesn't
>>>> have to do with delayed evaluation of lazy seq's or anything).
>>>>
>>>>
>>>> On Tuesday, November 17, 2015 at 6:49:16 PM UTC+9, David Iba wrote:
>>>>>
>>>>> 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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>
>
> --
> “One of the main causes of the fall of the Roman Empire was that–lacking
> zero–they had no way to indicate successful termination of their C
> programs.”
> (Robert Firth)
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