Github user HyukjinKwon commented on a diff in the pull request:
https://github.com/apache/spark/pull/18320#discussion_r123700234
--- Diff: R/pkg/inst/worker/daemon.R ---
@@ -30,8 +30,51 @@ 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
+ # by setting SIGUSR1.
+ lapply(children, function(child) {
--- End diff --
With the change above, it printed:
```
[1] "Wait for 4 seconds to test the last child ..."
[1] "child PID: 86866 and parent will kill given: -1"
[1] "child PID: 86865 and parent will kill given: -1"
[1] "child PID: 86864 and sent a data: arbitrary"
[1] "child PID: 86863 and sent a PID: 86863"
[1] "Wait for 7 seconds more to test the last child ..."
[1] "child PID: 86864 and sent a data: 123"
```
It looks correct.
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