Github user felixcheung commented on a diff in the pull request:
https://github.com/apache/spark/pull/18320#discussion_r123687096
--- Diff: R/pkg/inst/worker/daemon.R ---
@@ -30,8 +30,54 @@ port <- as.integer(Sys.getenv("SPARKR_WORKER_PORT"))
inputCon <- socketConnection(
port = port, open = "rb", blocking = TRUE, timeout = connectionTimeout)
+# Waits indefinitely for a socket connecion by default.
+selectTimeout <- NULL
+
+# Exit code that children send to the parent to indicate they exited.
+exitCode <- 1
+
while (TRUE) {
- ready <- socketSelect(list(inputCon))
+ ready <- socketSelect(list(inputCon), timeout = selectTimeout)
+
+ # Note that the children should be terminated in the parent. If each
child terminates
+ # itself, it appears that the resource is not released properly, that
causes an unexpected
+ # termination of this daemon due to, for example, running out of file
descriptors
+ # (see SPARK-21093). Therefore, the current implementation tries to
retrieve children
+ # that are exited (but not terminated) and then sends a kill signal to
terminate them properly
+ # in the parent.
+ #
+ # There are two paths that it attempts to send a signal to terminate the
children in the parent.
+ #
+ # 1. Every second if any socket connection is not available and if
there are child workers
+ # running.
+ # 2. Right after a socket connection is available.
+ #
+ # In other words, the parent attempts to send the signal to the children
every second if
+ # any worker is running or right before launching other worker children
from the following
+ # new socket connection.
+
+ # Only the process IDs of exited children are returned and the
termination is attempted below.
+ children <- parallel:::selectChildren(timeout = 0)
+ if (is.integer(children)) {
+ # If it is PIDs, there are workers exited but not terminated. Attempts
to terminate them
+ # by setting SIGUSR1.
+ lapply(children, function(child) {
+ # This data should be raw bytes if any data was sent from this child.
+ # Otherwise, this returns the PID.
+ data <- parallel:::readChild(child)
+ if (is.raw(data)) {
+ # This checks if the data from this child is -1 that indecides an
exited child.
--- End diff --
`indecides` -> `indicates`
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