R help - Dec 2024 - mclapply hanging occasionally on macos

sequoia, 15.2.  R --vanilla : 4.4.2 (2024-10-31).  I have the same
basic setup on three macs: a macbook air, a mac pro m1, and a mac mini
m4.  The following code is running into a bug on the mac pro m1 and
the mac mini, but works just fine on my macbook air.  (of course, it
doesn't do anything useful.)  it's replicable!

```
$ R --vanilla
> source("debug.R")
```

and (after putting a single word into debug.csv in order to avoid the
empty file error),

```
library(parallel)

ignore.dataset <- read.csv("debug.csv"); rm(ignore.dataset)

useless.function <- function( ) {
    y <- rnorm(3); x <- rnorm(3)
    summary( lm( y ~ x ))  ## useless
    NULL
}

run30 <- function(i) {
    message("run30=", i)
    useless.function()
}

run30( 0 )
message("many mc")
simsl <- mclapply( 1:30, run30 )
```

this replicably does not count to 30 on both desktops, but does on the air.

not fun...can someone else try this program on a macos recent desktop,
and let me know whether I am alone here?

/iaw

? Mon, 30 Dec 2024 19:16:11 -0800
Ivo Welch <ivo.welch at gmail.com> ?????:
> useless.function <- function( ) {
>     y <- rnorm(3); x <- rnorm(3)
>     summary( lm( y ~ x ))  ## useless
>     NULL
> }
> 
> run30 <- function(i) {
>     message("run30=", i)
>     useless.function()
> }
> 
> run30( 0 )
> message("many mc")
> simsl <- mclapply( 1:30, run30 )
Thank you for telling us which code you're running that eventually
hangs mclapply()! I see that your example involves running lm(), which
in turn involves BLAS and LAPACK operations. It could be that when you
first run lm() in the parent process, it starts threads that take
locks. Later, when mclapply() starts child processes, those threads
don't exist there (they stay in the parent process), but the locks
remain taken, with no-one to release them - a deadlock.

Ironically, using a parallel "socket cluster" (but not "fork
cluster")
would have avoided the problem by starting a fresh process from
scratch, without any inherited locks.

It may also be possible to disable the use of parallel threads in your
BLAS/LAPACK, but the exact procedure depends on the BLAS. What's your
sessionInfo()? If you don't get a good answer here, try
R-SIG-Mac at R-project.org.

-- 
Best regards,
Ivan