[R] Don't cap global number of threads for serialization (#10028)

R-package/man/xgb.DMatrix.save.Rd

@@ -15,6 +15,7 @@ xgb.DMatrix.save(dmatrix, fname)
 Save xgb.DMatrix object to binary file
 }
 \examples{
+\dontshow{RhpcBLASctl::omp_set_num_threads(1)}
 data(agaricus.train, package='xgboost')
 dtrain <- with(agaricus.train, xgb.DMatrix(data, label = label, nthread = 2))
 fname <- file.path(tempdir(), "xgb.DMatrix.data")

R-package/man/xgb.dump.Rd

@@ -44,6 +44,7 @@ as a \code{character} vector. Otherwise it will return \code{TRUE}.
 Dump an xgboost model in text format.
 }
 \examples{
+\dontshow{RhpcBLASctl::omp_set_num_threads(1)}
 data(agaricus.train, package='xgboost')
 data(agaricus.test, package='xgboost')
 train <- agaricus.train

R-package/man/xgb.load.Rd

@@ -25,6 +25,7 @@ Note: a model saved as an R-object, has to be loaded using corresponding R-metho
 not \code{xgb.load}.
 }
 \examples{
+\dontshow{RhpcBLASctl::omp_set_num_threads(1)}
 data(agaricus.train, package='xgboost')
 data(agaricus.test, package='xgboost')
 

R-package/man/xgb.save.Rd

@@ -41,6 +41,7 @@ how to persist models in a future-proof way, i.e. to make the model accessible i
 releases of XGBoost.
 }
 \examples{
+\dontshow{RhpcBLASctl::omp_set_num_threads(1)}
 data(agaricus.train, package='xgboost')
 data(agaricus.test, package='xgboost')
 

R-package/man/xgb.save.raw.Rd

@@ -21,6 +21,7 @@ xgb.save.raw(model, raw_format = "ubj")
 Save xgboost model from xgboost or xgb.train
 }
 \examples{
+\dontshow{RhpcBLASctl::omp_set_num_threads(1)}
 data(agaricus.train, package='xgboost')
 data(agaricus.test, package='xgboost')
 

R-package/man/xgbConfig.Rd

@@ -25,6 +25,15 @@ values of one or more global-scope parameters. Use \code{xgb.get.config} to fetc
 values of all global-scope parameters (listed in
 \url{https://xgboost.readthedocs.io/en/stable/parameter.html}).
 }
+\details{
+Note that serialization-related functions might use a globally-configured number of threads,
+which is managed by the system's OpenMP (OMP) configuration instead. Typically, XGBoost methods
+accept an \code{nthreads} parameter, but some methods like \code{readRDS} might get executed before such
+parameter can be supplied.
+
+The number of OMP threads can in turn be configured for example through an environment variable
+\code{OMP_NUM_THREADS} (needs to be set before R is started), or through \code{RhpcBLASctl::omp_set_num_threads}.
+}
 \examples{
 # Set verbosity level to silent (0)
 xgb.set.config(verbosity = 0)