h5_threads: Set the Number of Threads

In h5lite: Simplified 'HDF5' Interface

Set the Number of Threads

Description

Sets how many threads h5lite uses for the work it can do in parallel. Currently that is the Blosc and Blosc2 codecs ("blosc1-..." and "blosc2-..."), which compress and decompress the blocks of each chunk in parallel. Everything uses one thread unless you ask for more.

Usage

h5_threads(n = NULL)

Arguments

n	The number of threads, from 1 to 256, or 0 to return to the default. If NULL (the default), the setting is left unchanged.

Details

More threads help most when compression is slow, as with high Zstandard levels, or when chunks are large. The setting applies to h5lite for the rest of the R session.

The default is one thread, or the value of the H5LITE_NTHREADS environment variable if it is set. Calling h5_threads() overrides it.

Threads need hdf5lib 2.2.0.1 or later. When h5lite is built with hdf5lib 2.2.0.0, Blosc always uses one thread, whatever the setting.

CRAN allows packages to use at most two threads in their examples, tests, and vignettes.

Value

With n = NULL, the number of threads h5lite will use. Otherwise, the previous setting, invisibly (0 if none had been made).

See Also

h5_compression(), vignette('compression')

Examples

h5_threads()      # 1

old <- h5_threads(2)
file <- tempfile(fileext = ".h5")
h5_write(rnorm(1e5), file, "x", compress = "blosc2-zstd-5")

h5_threads(old)   # Restore the previous setting
unlink(file)

h5lite documentation built on Oct. 31, 2026, 1:07 a.m.