On 08/31/2018 01:18 AM, Henrik Bengtsson wrote:
Hi, I'd like to test whether a (localhost) PSOCK cluster node is still
running or not by its PID, e.g. it may have crashed / core dumped.
I'm ok with getting false-positive results due to *another* process
with the same PID has since started.
kill(sig=0) is specified by POSIX but indeed as you say there is a race
condition due to PID-reuse. In principle, detecting that a worker
process is still alive cannot be done correctly outside base R. At
user-level I would probably consider some watchdog, e.g. the parallel
tasks would be repeatedly touching a file.
In base R, one can do this correctly for forked processes via
mcparallel/mccollect, not for PSOCK cluster workers which are based on
system() (and I understand it would be a useful feature)
> j <- mcparallel(Sys.sleep(1000))
> mccollect(j, wait=FALSE)
NULL
# kill the child process
> mccollect(j, wait=FALSE)
$`1542`
NULL
More details indeed in ?mcparallel. The key part is that the job must be
started as non-detached and as soon as mccollect() collects is,
mccollect() must never be called on it again.
Tomas
I can the PID of each cluster nodes by querying them for their
Sys.getpid(), e.g.
pids <- parallel::clusterEvalQ(cl, Sys.getpid())
Is there a function in core R for testing whether a process with a
given PID exists or not? From trial'n'error, I found that on Linux:
pid_exists <- function(pid) as.logical(tools::pskill(pid, signal = 0L))
returns TRUE for existing processes and FALSE otherwise, but I'm not
sure if I can trust this. It's not a documented feature in
?tools::pskill, which also warns about 'signal' not being standardized
across OSes.
The other Linux alternative I can imagine is:
pid_exists <- function(pid) system2("ps", args = c("--pid", pid),
stdout = FALSE) == 0L
Can I expect this to work on macOS as well? What about other *nix systems?
And, finally, what can be done on Windows?
I'm sure there are packages on CRAN that provides this, but I'd like
to keep dependencies at a minimum.
I appreciate any feedback. Thxs,
Henrik
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