random streams

[Doug Lea]

On 12/31/12 14:18, Brian Goetz wrote:
> Sharing random number generators is indeed a tricky thing.  But, this doesn't
> change the status quo, it just wraps it differently.  Doing:
>
>    rng.ints().parallel().forEach(i -> { ... i ... })
>
> is no different from submitting code like
>
>      { ... i = rng.nextInt(); i ... })
>
> to an FJP; you'll get exactly the same interleavings.


You might recall that the reason for introducing TLR in JDK7 was that
people using FJ/ParallelArray were reporting horribly contention
problems. (Initially, thread-local randoms were available only as
a utility method in FJ, then made stand-alone for JDK7.)
We'd sorta rather not let this lesson be lost :-)

>
> ThreadLocalRandom can override this to implement:
>
>       public IntStream ints() {
>           return PrimitiveStreams.repeatedly(
>               () -> TLR.current().nextInt());
>       }
>

How about ONLY adding to TLR?

-Doug



>
>
>
>
>
> On 12/31/2012 1:54 PM, Doug Lea wrote:
>> On 12/31/12 13:41, Brian Goetz wrote:
>>> On the list of requested stream sources is 'stream of random numbers'.
>>>
>>> Here's a one-line addition to Random:
>>>
>>>      public IntStream ints() {
>>>          return PrimitiveStreams.repeatedly(this::nextInt);
>>>      }
>>>
>>> Certainly the implementation is straightforward enough (modulo
>>> renaming of
>>> PrimitiveStreams and repeatedly, which are not yet nailed down.)
>>
>> This is not so straightforward under parallel operations.
>> This is a surprisingly deep topic with a lot of technical papers etc.
>> As a first pass, you'd just use ThreadLocalRandom() as sources
>> (to avoid horrible update contention), and make no promises
>> about the aggregate randomness across parallel operations.
>> However, people will come to expect that if you start off
>> computations with a common seed, then you get both replicability
>> and independence, which is not easy to deliver.
>> As a start, you'd need a better generator than the
>> one in Random (which is and must be the same algorithm
>> used in ThreadLocalRandom).
>>
>> -Doug
>>
>>
>