[erlang-questions] Optimization for the ++ operator

Gustav Simonsson <>
Fri Jun 1 15:09:09 CEST 2012

On 2012-06-01 12:07, Björn-Egil Dahlberg wrote:
> 1 jun 2012 kl. 10:46 skrev "José Valim" < 
> <mailto:>>:
>>     The '++' operator does not verify that the second argument is a list.
>>     Therefore, the compiler will rewrite the first expression to simply:
>>     [$F,$o,$o|Bar]
>> Yes, thanks. For some weird reason I thought Erlang restricted the 
>> right side to be a list in such cases.
>>     Have you made sure that you run each test in a newly spawned
>>     process? 
>>     Do you run the test for long enough time?
>> Running in a newly spawned process did the trick, the results are now 
>> consistent.
>> I was already running it 1000 times, here is the code (without using 
>> spawn):
>> https://www.refheap.com/paste/2954
>> I assume having a "clean" heap (reducing the chance of garbage 
>> collection) is the reason why it is a good idea to run benchmarks in 
>> a newly spawned process?
> If you want to vary one parameter in a test, make sure all other 
> parameters stays the same otherwise you have no idea what you are 
> measuring!
> Code loading, heaps, position of mars .. Everything has to be taken 
> into account.

The effect of Mars magnetic field on that of Earth, even if Earth's 
aphelion and the perihelion of Mars were to coincide and the 
interplanetary distance were to be at its theoretical minimum, is 
negligible compared to the normal variations in Earth's magnetic field.

Therefore it's very unlikely that the relative position of Mars would 
affect the probability of a memory bitflip occurring due to cosmic 
radiation and thus affecting the benchmark.

Gustav Simonsson
Erlang/OTP team

>> Thanks Björn!
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