# Parallel Random Number Generation#

There are four main strategies implemented that can be used to produce repeatable pseudo-random numbers across multiple processes (local or distributed).

`SeedSequence`

spawning#

`SeedSequence`

implements an algorithm to process a user-provided seed,
typically as an integer of some size, and to convert it into an initial state for
a `BitGenerator`

. It uses hashing techniques to ensure that low-quality seeds
are turned into high quality initial states (at least, with very high
probability).

For example, `MT19937`

has a state consisting of 624
*uint32* integers. A naive way to take a 32-bit integer seed would be to just set
the last element of the state to the 32-bit seed and leave the rest 0s. This is
a valid state for `MT19937`

, but not a good one. The Mersenne Twister
algorithm suffers if there are too many 0s. Similarly, two adjacent 32-bit
integer seeds (i.e. `12345`

and `12346`

) would produce very similar
streams.

`SeedSequence`

avoids these problems by using successions of integer hashes
with good avalanche properties to ensure that flipping any bit in the input
has about a 50% chance of flipping any bit in the output. Two input seeds that
are very close to each other will produce initial states that are very far
from each other (with very high probability). It is also constructed in such
a way that you can provide arbitrary-sized integers or lists of integers.
`SeedSequence`

will take all of the bits that you provide and mix them
together to produce however many bits the consuming `BitGenerator`

needs to
initialize itself.

These properties together mean that we can safely mix together the usual
user-provided seed with simple incrementing counters to get `BitGenerator`

states that are (to very high probability) independent of each other. We can
wrap this together into an API that is easy to use and difficult to misuse.

```
from numpy.random import SeedSequence, default_rng
ss = SeedSequence(12345)
# Spawn off 10 child SeedSequences to pass to child processes.
child_seeds = ss.spawn(10)
streams = [default_rng(s) for s in child_seeds]
```

Child `SeedSequence`

objects can also spawn to make grandchildren, and so on.
Each `SeedSequence`

has its position in the tree of spawned `SeedSequence`

objects mixed in with the user-provided seed to generate independent (with very
high probability) streams.

```
grandchildren = child_seeds[0].spawn(4)
grand_streams = [default_rng(s) for s in grandchildren]
```

This feature lets you make local decisions about when and how to split up streams without coordination between processes. You do not have to preallocate space to avoid overlapping or request streams from a common global service. This general “tree-hashing” scheme is not unique to numpy but not yet widespread. Python has increasingly-flexible mechanisms for parallelization available, and this scheme fits in very well with that kind of use.

Using this scheme, an upper bound on the probability of a collision can be
estimated if one knows the number of streams that you derive. `SeedSequence`

hashes its inputs, both the seed and the spawn-tree-path, down to a 128-bit
pool by default. The probability that there is a collision in
that pool, pessimistically-estimated ([1]), will be about \(n^2*2^{-128}\) where
*n* is the number of streams spawned. If a program uses an aggressive million
streams, about \(2^{20}\), then the probability that at least one pair of
them are identical is about \(2^{-88}\), which is in solidly-ignorable
territory ([2]).

## Sequence of Integer Seeds#

As discussed in the previous section, `SeedSequence`

can not only take an
integer seed, it can also take an arbitrary-length sequence of (non-negative)
integers. If one exercises a little care, one can use this feature to design
*ad hoc* schemes for getting safe parallel PRNG streams with similar safety
guarantees as spawning.

For example, one common use case is that a worker process is passed one root seed integer for the whole calculation and also an integer worker ID (or something more granular like a job ID, batch ID, or something similar). If these IDs are created deterministically and uniquely, then one can derive reproducible parallel PRNG streams by combining the ID and the root seed integer in a list.

```
# default_rng() and each of the BitGenerators use SeedSequence underneath, so
# they all accept sequences of integers as seeds the same way.
from numpy.random import default_rng
def worker(root_seed, worker_id):
rng = default_rng([worker_id, root_seed])
# Do work ...
root_seed = 0x8c3c010cb4754c905776bdac5ee7501
results = [worker(root_seed, worker_id) for worker_id in range(10)]
```

This can be used to replace a number of unsafe strategies that have been used
in the past which try to combine the root seed and the ID back into a single
integer seed value. For example, it is common to see users add the worker ID to
the root seed, especially with the legacy `RandomState`

code.

```
# UNSAFE! Do not do this!
worker_seed = root_seed + worker_id
rng = np.random.RandomState(worker_seed)
```

It is true that for any one run of a parallel program constructed this way, each worker will have distinct streams. However, it is quite likely that multiple invocations of the program with different seeds will get overlapping sets of worker seeds. It is not uncommon (in the author’s self-experience) to change the root seed merely by an increment or two when doing these repeat runs. If the worker seeds are also derived by small increments of the worker ID, then subsets of the workers will return identical results, causing a bias in the overall ensemble of results.

Combining the worker ID and the root seed as a list of integers eliminates this risk. Lazy seeding practices will still be fairly safe.

This scheme does require that the extra IDs be unique and deterministically
created. This may require coordination between the worker processes. It is
recommended to place the varying IDs *before* the unvarying root seed.
`spawn`

*appends* integers after the user-provided seed, so if
you might be mixing both this *ad hoc* mechanism and spawning, or passing your
objects down to library code that might be spawning, then it is a little bit
safer to prepend your worker IDs rather than append them to avoid a collision.

```
# Good.
worker_seed = [worker_id, root_seed]
# Less good. It will *work*, but it's less flexible.
worker_seed = [root_seed, worker_id]
```

With those caveats in mind, the safety guarantees against collision are about the same as with spawning, discussed in the previous section. The algorithmic mechanisms are the same.

## Independent Streams#

`Philox`

is a counter-based RNG based which generates values by
encrypting an incrementing counter using weak cryptographic primitives. The
seed determines the key that is used for the encryption. Unique keys create
unique, independent streams. `Philox`

lets you bypass the
seeding algorithm to directly set the 128-bit key. Similar, but different, keys
will still create independent streams.

```
import secrets
from numpy.random import Philox
# 128-bit number as a seed
root_seed = secrets.getrandbits(128)
streams = [Philox(key=root_seed + stream_id) for stream_id in range(10)]
```

This scheme does require that you avoid reusing stream IDs. This may require coordination between the parallel processes.

## Jumping the BitGenerator state#

`jumped`

advances the state of the BitGenerator *as-if* a large number of
random numbers have been drawn, and returns a new instance with this state.
The specific number of draws varies by BitGenerator, and ranges from
\(2^{64}\) to \(2^{128}\). Additionally, the *as-if* draws also depend
on the size of the default random number produced by the specific BitGenerator.
The BitGenerators that support `jumped`

, along with the period of the
BitGenerator, the size of the jump and the bits in the default unsigned random
are listed below.

BitGenerator |
Period |
Jump Size |
Bits per Draw |
---|---|---|---|

MT19937 |
\(2^{19937}-1\) |
\(2^{128}\) |
32 |

PCG64 |
\(2^{128}\) |
\(~2^{127}\) ([3]) |
64 |

PCG64DXSM |
\(2^{128}\) |
\(~2^{127}\) ([3]) |
64 |

Philox |
\(2^{256}\) |
\(2^{128}\) |
64 |

The jump size is \((\phi-1)*2^{128}\) where \(\phi\) is the golden ratio. As the jumps wrap around the period, the actual distances between neighboring streams will slowly grow smaller than the jump size, but using the golden ratio this way is a classic method of constructing a low-discrepancy sequence that spreads out the states around the period optimally. You will not be able to jump enough to make those distances small enough to overlap in your lifetime.

`jumped`

can be used to produce long blocks which should be long enough to not
overlap.

```
import secrets
from numpy.random import PCG64
seed = secrets.getrandbits(128)
blocked_rng = []
rng = PCG64(seed)
for i in range(10):
blocked_rng.append(rng.jumped(i))
```

When using `jumped`

, one does have to take care not to jump to a stream that
was already used. In the above example, one could not later use
`blocked_rng[0].jumped()`

as it would overlap with `blocked_rng[1]`

. Like
with the independent streams, if the main process here wants to split off 10
more streams by jumping, then it needs to start with `range(10, 20)`

,
otherwise it would recreate the same streams. On the other hand, if you
carefully construct the streams, then you are guaranteed to have streams that
do not overlap.