audit `pct_cpu` values on macOS

[simonpcouch]

Related to #1. The distributions of pct_cpu values, intended to measure CPU usage, are weird on MacOS. Here's an example that distributes a set of linear regression fits across 5 cores using forking:

library(syrup)
#> Loading required package: bench

library(future)
library(tidymodels)

# explicitly enable forking in RStudio, otherwise future will complain
rlang::local_options(parallelly.fork.enable = TRUE)

# use 5 workers via forking
plan(multicore, workers = 5)

# wrap model resampling in `syrup()` to profile CPU usage
res_mem <- syrup({
  res <-
    fit_resamples(
      linear_reg(),
      outcome ~ .,
      vfold_cv(sim_regression(1000000))
    )
})

The distribution of pct_cpu values seems spot on (except they need to be multiplied by 100, woops) when I run it on Linux. Most of the time, most R sessions are using very little CPU, though each child of the parent process briefly hits 100% while training models:

hist(res_mem$pct_cpu)

^{Created on 2024-06-24 with reprex v2.1.0}

Here's what the distribution looks like when I run it on macOS, though:

Trying to figure out if that 2.4% approximates something that's useful for debugging... this Macbook has 10 cores.

[simonpcouch]

Max was able to replicate behavior of pct_cpu maxing out around 2.4% on a 12-core machine while the "% CPU" in Activity Monitor was hitting 100%.

cc @gaborcsardi and r-lib/ps#73. syrup does a relatively straightforward transformation on the user and system CPU values outputted from ps::ps():

https://github.com/simonpcouch/syrup/blob/fe57dd21ccca34966276586863835068ce99705f/R/utils.R#L24-L42

...so the operation, per value, is something like:

interval <- .5
times_t <- ps::ps_cpu_times()
Sys.sleep(interval)
times_t_plus_1 <- ps::ps_cpu_times()

user <- times_t_plus_1[["user"]] - times_t[["user"]]
system <- times_t_plus_1[["system"]] - times_t[["system"]]

(user + system) / interval
#> [1] 0.001063258

This gives reasonable values on Linux, but not on macOS. If you have any intuition on why this might be, Gábor, it'd definitely be appreciated.

[gaborcsardi]

Seems like this is broken on new macs, Apple has changed the scale of the values they report and we need to adjust. Fixed in https://github.com/r-lib/ps/commit/b6e0b6235f03edfd87029106bffc2ab6bd1e1ca8

{
  times_t <- ps::ps_cpu_times()
  tic <- Sys.time()
  for (i in 1:10000) runif(100000)
  toc <- Sys.time()
  times_t_plus_1 <- ps::ps_cpu_times()
  interval <- as.double(toc - tic, units = "secs")

  user <- times_t_plus_1[["user"]] - times_t[["user"]]
  system <- times_t_plus_1[["system"]] - times_t[["system"]]

  (user + system) / interval
}

#> [1] 0.9935361

[gaborcsardi]

I can do a ps release this week.

[simonpcouch]

Awesome, thanks for looking into this!

[gaborcsardi]

@simonpcouch New ps is on CRAN, FYI.