2014-07-12 11:56 GMT+02:00 James Reeves <[email protected]>:
> The JVM optimises sections of code that are frequently executed, so the
> second expression benefits from the optimisation that happens during the
> first expression. My guess is that if you were to swap the two expressions
> around, you'd find that the first expression always takes longer, no matter
> which way around it is.
>
> Benchmarking libraries like Criterium get around this by "warming up" the
> JVM with a few practice runs before recording the benchmarks. I'd advise
> using Criterium over clojure.core/time if you want to benchmark things
> accurately.
>
That may be a part of it, but there is more to it. When I use 1e9 for
range-length I get:
OutOfMemoryError GC overhead limit exceeded java.lang.Long.valueOf
(Long.java:577)
"Elapsed time: 56970.117811 msecs"
[999999988 12]
By doing first (count d) and then (count t) something is retained, which
makes the execution longer and the usage of memory higher. And I do not
understand that. Because when creating t and d everything has to be
evaluated. So why has the sequence of the statements influence on the
memory usage?
> On 12 July 2014 10:29, Cecil Westerhof <[email protected]> wrote:
>
>> I do the following (with range-length 1e8):
>> (time (let [[t d] (split-with #(< % 12) (range range-length))]
>> [(count d) (count t)]))
>>
>> (time (let [[t d] (split-with #(< % 12) (range range-length))]
>> (into [] (reverse [(count t) (count d)]))))
>>
>> This gives:
>> "Elapsed time: 68105.338314 msecs"
>> [99999988 12]
>> "Elapsed time: 11545.667394 msecs"
>> [99999988 12]
>>
>> So the first expression
>> takes as six times as long to evaluate.
>> I understood that this has to do with that in the first case there is
>> head retention. But I do not understand this. What is happening here?
>>
>>
--
Cecil Westerhof
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