Github user felixcheung commented on a diff in the pull request:
https://github.com/apache/spark/pull/18320#discussion_r123416771
--- Diff: R/pkg/inst/worker/daemon.R ---
@@ -30,8 +30,40 @@ 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
+
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
--- End diff --
If I understanding correctly, children (from `parallel:::selectChildren()`)
as integer only indicates the list of fork/child process. But it does not
indicate the child process "exited"?
When we are in a loop checking every second (ie. selectTimeout == 1), it
sounds to me like socketSelect could return in one of the following ways:
- socket available for reading (ready == TRUE)
- socket not available for reading, it's dead or exiting (ready == FALSE)
- socket not available for reading, but it's not ready to connect because
say things are running slow or the host is busy etc, and it has effectively
been just timed out since selectTimeout (ready == FALSE)
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