vv_seed: Parallel random number generation with reproducible results

In pbdMPI: R Interface to MPI for HPC Clusters (Programming with Big Data Project)

Parallel random number generation with reproducible results

Description

These functions control the parallel-capable L'Ecuyer-CMRG pseudo-random number generator (RNG) on clusters and in multicore parallel applications for reproducible results. Reproducibility is possible across different node and core configurations by associating the RNG streams with an application vector.

Usage

comm.set.seed(
  seed = NULL,
  diff = TRUE,
  state = NULL,
  streams = NULL,
  comm = .pbd_env$SPMD.CT$comm
)
comm.set.stream(
  name = NULL,
  reset = FALSE,
  state = NULL,
  comm = .pbd_env$SPMD.CT$comm
)
comm.get.streams(
  comm = .pbd_env$SPMD.CT$comm, 
  seed = FALSE
)

Arguments

seed	In comm.set.seed, a single value interpreted as an integer. In comm.get.streams, a logical if TRUE, return includes the current local .Random.seed.
diff	Logical indicating if the parallel instances should have different random streams.
state	In function comm.set.seed: This parameter is deprecated. In function comm.set.stream: If non-NULLit restarts a stream from a previously saved state <- comm.set.stream(). A stream state is a list with one named element, which is the 6-element L'Ecuyer-CMRG .Random.seed, probably captured earlier with state <- comm.set.stream()). The stream name, if different from a provided parameter name, has precedence, but a warning is produced. Further, the requesting rank must own the stream.
streams	An vector of sequential integers specifying the streams to be prepared on the current rank. Typically, this is used by 'comm.chunk()' to prepare correct streams for each rank, which are aligned with the vector being chunk-ed.
name	Stream number that is coercible to character, indicating to start or continue generating from that stream.
reset	If true, reset the requested stream back to its beginning.
comm	The communicator that determines MPI rank numbers.

Details

This implementation uses the function nextRNGStream in package parallel to set up streams appropriate for working on a cluster system with MPI. The main difference from parallel is that it adds a reproducibility capability with vector-based streams that works across different numbers of nodes or cores by associating streams with an application vector.

Vector-based streams are best set up with the higher level function comm.chunk instead of using comm.set.stream directly. comm.chunk will set up only the streams that each rank needs and provides the stream numbers necessary to switch between them with comm.set.stream.

The function uses parallel's nextRNGStream() and sets up the parallel stream seeds in the .pbd_env$RNG environment, which are then managed with comm.set.stream. There is only one communication broadcast in this implementation that ensures all ranks have the same seed as rank 0. Subsequently, each rank maintains only its own streams.

When rank-based streams are set up, comm.chunk with form = "number" and rng = TRUE parameters, streams are different for each rank and switching is not needed. Vector-based streams are obtained with form = "vector" and rng = TRUE parameters. In this latter case, the vector returned to each rank contains the stream numbers (and vector components) that the rank owns. Switch with comm.set.stream(v), where v is one of the stream numbers. Switching back and forth is allowed, with each stream continuing where it left off.

## RNG Notes R sessions connected by MPI begin like other R sessions as discussed in Random. On first use of random number generation, each rank computes its own seed from a combination of clock time and process id (unless it reads a previously saved workspace, which is not recommended). Because of asynchronous execution, imperfectly synchronized node clocks, and likely different process ids, this almost guarantees unique seeds and most likely results in independent streams. However, this is not reproducible and not guaranteed. Both reproducibility and guarantee are brought by the use of the L'Ecuyer-CMRG generator implementation in nextRNGStream and the use of comm.set.seed and comm.set.stream adaptation for parallel computing on cluster systems.

At a high level, the L'Ecuyer-CMRG pseudo-random number generator can take jumps (advance the seed) in its stream (about 2^191 long) so that distant substreams can be assigned. The nextRNGStream implementation takes jumps of 2^127 (about 1.7e38) to provide up to 2^64 (about 1.8e19) independent streams. See https://stat.ethz.ch/R-manual/R-devel/library/parallel/doc/parallel.pdf for more details.

In situations that require the same stream on all ranks, a simple set.seed from base R and the default RNG will suffice. comm.set.seed will also accomplish this with the diff = FALSE parameter if switching between same and different streams is needed.

Value

comm.set.seed engages the L'Ecuyer-CMRG RNG and invisibly returns the previous RNG in use (Output of RNGkind()[1]). Capturing it, enables the restoration of the previous RNG with RNGkind. See examples of use in demo/seed_rank.r and demo/seed_vec.r.

comm.set.stream invisibly returns the current stream number as character.

comm.get.streams returns the current stream name and other stream names available to the rank as a character string. Optionally, the local .Random.seed is included. This function is a debugging aid for distributed random streams.

All three functions manage and use the environment .pbd_env$RNG.

Author(s)

Wei-Chen Chen [e-mail], George Ostrouchov, Drew Schmidt, Pragneshkumar Patel, and Hao Yu.

References

Pierre L'Ecuyer, Simard, R., Chen, E.J., and Kelton, W.D. (2002) An Object-Oriented Random-Number Package with Many Long Streams and Substreams. Operations Research, 50(6), 1073-1075.

https://www.iro.umontreal.ca/~lecuyer/myftp/papers/streams00.pdf

Programming with Big Data in R Website: https://pbdr.org/

See Also

comm.chunk()

Examples

## Not run: 
### Save code in a file "demo.r" and run with 2 processors by
### SHELL> mpiexec -np 2 Rscript demo.r

spmd.code <- "
suppressMessages(library(pbdMPI, quietly = TRUE))     

comm.print(RNGkind())
comm.print(runif(5), all.rank = TRUE)

set.seed(1357)
comm.print(runif(5), all.rank = TRUE)

old.kind = comm.set.seed(1357)
comm.print(RNGkind())
comm.print(runif(5), all.rank = TRUE)

comm.set.stream(reset = TRUE)
comm.print(runif(5), all.rank = TRUE)

comm.set.seed(1357, diff = TRUE)
comm.print(runif(5), all.rank = TRUE)

state <- comm.set.stream()   ### save each rank's stream state
comm.print(runif(5), all.rank = TRUE)

comm.set.stream(state = state) ### set current RNG to state
comm.print(runif(5), all.rank = TRUE)

RNGkind(old.kind)
set.seed(1357)
comm.print(RNGkind())
comm.print(runif(5), all.rank = TRUE)

### Finish.
finalize()
"
# execmpi(spmd.code, nranks = 2L)

## End(Not run)

pbdMPI documentation built on Sept. 18, 2024, 9:07 a.m.