fix: reduce number of data table threads when running with future (#979)

* fix: reduce number of data table threads when running with future

* refactor: pass is sequential to worker

* test: data table threads are not changed in main session

* fix: use default

* feat: reduce blas threads to 1

* docs: namespace

* docs: namespace

* fix: roxygen

* chore: update news

DESCRIPTION

@@ -56,6 +56,7 @@ Imports:
     parallelly,
     palmerpenguins,
     paradox (>= 0.10.0),
+    RhpcBLASctl,
     uuid
 Suggests:
     Matrix,
@@ -73,7 +74,7 @@ Config/testthat/edition: 3
 Config/testthat/parallel: false
 NeedsCompilation: no
 Roxygen: list(markdown = TRUE, r6 = TRUE)
-RoxygenNote: 7.2.3.9000
+RoxygenNote: 7.2.3
 Collate:
     'mlr_reflections.R'
     'BenchmarkResult.R'

NAMESPACE

@@ -234,6 +234,8 @@ import(palmerpenguins)
 import(paradox)
 importFrom(R6,R6Class)
 importFrom(R6,is.R6)
+importFrom(RhpcBLASctl,blas_get_num_procs)
+importFrom(RhpcBLASctl,blas_set_num_threads)
 importFrom(data.table,as.data.table)
 importFrom(data.table,data.table)
 importFrom(future,nbrOfWorkers)

NEWS.md

@@ -1,5 +1,6 @@
 # mlr3 (development version)
 
+* Reduce number of threads used by `data.table` and BLAS to 1 when running `resample()` or `benchmark()` in parallel.
 * Optimize runtime of `resample()` and `benchmark()` by reducing the number of hashing operations.
 
 # mlr3 0.17.0

R/helper_exec.R

@@ -36,6 +36,8 @@ future_map = function(n, FUN, ..., MoreArgs = list()) {
     }
     stdout = if (is_sequential) NA else TRUE
 
+    MoreArgs = c(MoreArgs, list(is_sequential = is_sequential))
+
     lg$debug("Running resample() via future with %i iterations", n)
     future.apply::future_mapply(
       FUN, ..., MoreArgs = MoreArgs, SIMPLIFY = FALSE, USE.NAMES = FALSE,

R/worker.R

@@ -219,11 +219,19 @@ learner_predict = function(learner, task, row_ids = NULL) {
 }
 
 
-workhorse = function(iteration, task, learner, resampling, param_values = NULL, lgr_threshold, store_models = FALSE, pb = NULL, mode = "train") {
+workhorse = function(iteration, task, learner, resampling, param_values = NULL, lgr_threshold, store_models = FALSE, pb = NULL, mode = "train", is_sequential = TRUE) {
   if (!is.null(pb)) {
     pb(sprintf("%s|%s|i:%i", task$id, learner$id, iteration))
   }
 
+  # reduce data.table and blas threads to 1
+  if (!is_sequential) {
+    setDTthreads(1, restore_after_fork = TRUE)
+    old_blas_threads = blas_get_num_procs()
+    on.exit(blas_set_num_threads(old_blas_threads), add = TRUE)
+    blas_set_num_threads(1)
+  }
+
   # restore logger thresholds
   for (package in names(lgr_threshold)) {
     logger = lgr::get_logger(package)

R/zzz.R

@@ -10,6 +10,7 @@
 #' @importFrom uuid UUIDgenerate
 #' @importFrom parallelly availableCores
 #' @importFrom future nbrOfWorkers plan
+#' @importFrom RhpcBLASctl blas_set_num_threads blas_get_num_procs
 #'
 #' @section Learn mlr3:
 #' * Book on mlr3: \url{https://mlr3book.mlr-org.com}

tests/testthat/test_parallel.R

@@ -85,3 +85,22 @@ test_that("parallel seed", {
   })
   expect_equal(rr1$prediction()$prob, rr2$prediction()$prob)
 })
+
+test_that("data table threads are not changed in main session", {
+  old_dt_threads = getDTthreads()
+  on.exit({
+    setDTthreads(old_dt_threads)
+  }, add = TRUE)
+  setDTthreads(2L)
+
+  task = tsk("sonar")
+  learner = lrn("classif.debug", predict_type = "prob")
+  resampling = rsmp("cv", folds = 3L)
+  measure = msr("classif.auc")
+
+  rr1 = with_seed(123, with_future(future::sequential, resample(task, learner, resampling)))
+  expect_equal(getDTthreads(), 2L)
+
+  rr2 = with_seed(123, with_future(future::multisession, resample(task, learner, resampling)))
+  expect_equal(getDTthreads(), 2L)
+})