By default, job::job() imports everything into the job by default while job::empty() imports nothing into the job by default. In some cases, you can achieve significant speed gains by setting import explicitly somewhere in between these two extremes. You can tweak the following:

  • Variables. Control this using the import argument.
  • Packages. Control this using the packages argument.
  • Options. Control this using the opts argument.

I’ll discuss a few cases where it’s meaningful to toggle these below. You can also check out the related article on using job::export() to control exports from a job.

Import specific objects

If you work in an environment with several large objects, importing will be slowed down and memory duplicated. So you want to avoid that if it’s not necessary for your job.

# Put stuff in the global environment
big_df = data.frame(x = rnorm(1e7), y = rpois(1e7, 3))  # 10 mio. rows
small_df = mtcars[1:10, ]
model = mpg ~ hp * cyl

# Import only selected variables
  print(ls())  # What was imported?
  fit = lm(model, small_df)
}, import = c(small_df, model))

In the case above, you could also be lazy and use import = "auto":

  print(ls())  # What was imported?
  fit = lm(model, small_df)
}, import = "auto")

The reason import = "auto" is not the default is that it will fail to import objects not explicitly mentioned in the code chunk. Continuing the example above, this imports the function but not mode and small_df:

stateful_function = function() {
  lm(model, small_df)

  print(ls())  # What was imported?
  fit = stateful_function()
}, import = "auto")

Fails with

# Error in stats::model.frame(formula = model, data = small_df, drop.unused.levels = TRUE) : 
#   object 'model' not found

Load specific packages

I can think of two scenarios where you want to control packages.

  1. When your job doesn’t need particular slow-to-load packages. In the example below, we use brms first but won’t use it inside the job.
  2. When you only need a package inside the job. In the example below, we use job::job() to do a slow ggplot2 render. ggplot2 won’t be loaded in the main session. (See Render plots in RStudio jobs for more details.)
# brms stuff
fit = brm(mpg ~ hp * cyl, mtcars)

# Unrelated job
big_df = data.frame(x = rnorm(1e6), y = rpois(1e6, 3))
  gg = ggplot(big_df, aes(x = x, y = y)) + 
    geom_point(alpha = 0.01, size = 0.1)
  ggsave("my_points.png", plot = gg)
}, packages = NULL)
## [1] FALSE

Set job-specific options

Say you’re doing parallel computation via the future package. You want to use mc.cores = 6 in your main session but only mc.cores = 2 in your job. First, let’s run the main session:

options(mc.cores = 6)
my_great_function = function(x) x %in% c("A", "b", "C", "d")
main_result = future.apply::future_sapply(LETTERS[1:6], my_great_function)
##     A     B     C     D     E     F 

Continuing the same session, we launch a job on two cores:

  print(options("mc.cores"))  # Verify that this option was imported
  options(mc.cores = 2)  # Overwrite existing setting
  job_result = future.apply::future_sapply(LETTERS[1:5], my_great_function)

job_result and main_result should be identical, but the former was computed on six cores while the latter was computed on two. You could also call job::job() with job::job(..., opts = NULL), just to make sure. This starts the job with default settings.

Run multiple jobs with identical settings

Say you want to launch multiple jobs in identical environments. Rather than setting options() and library() within each job, you can set the job::job() arguments programmatically. You will need to quote() the code chunk.

# Set up environment
small_df = mtcars[1:20, ]
model = mpg ~ hp * cyl
irrelevant_var = 55

# Common arguments to job::job
jobargs = list(
  import = c("small_df", "model"),
  opts = list(mc.cores = 3, warn = -1),
  packages = c("dplyr", "lubridate")

# Launch the first job
job1_code = quote({
  df = small_df %>% filter(wt < 4)
  fit = lm(model, df)
  # Check imports
  print(ls())  # "irrelevant_var" was not imported
  print(as.Date("2021-05-23") %>% round_date("month"))  # lubridate was attached
  print(options("mc.cores"))  # Option was set
  warning("You won't see me because warn = -1")
job1_args = c(jobargs, list(job1 = job1_code)), args = job1_args)

# Launch the second job
job2_code = quote({
  df = small_df %>% filter(wt > 2, cyl != 4)
  fit = lm(model, small_df)
}), args = c(jobargs, list(job2 = job2_code)))

When the jobs complete, you can inspect job1$fit and job2$fit.