[Rd] R_CheckUserInterrupt() can be a performance bottleneck within GUIs

[Martin Becker]

tl;dr: R_CheckUserInterrupt() can be a performance bottleneck
        within GUIs. This also affects functions in the 'stats'
        package, which could be improved by changing the position
        of calls to R_CheckUserInterrupt().


Dear all,

Recently I was puzzled because some code in a package under development, 
which consisted almost entirely of a .Call() to a function written in C, 
was running much slower within RStudio compared to R in a terminal. It 
took me some time to identify the cause, so I thought I would share my 
findings; perhaps they will be helpful to others.

The performance drop was caused by R_CheckUserInterrupt(), which I call 
(perhaps too often) in my C code. While calling R_CheckUserInterrupt() 
seems to be quite cheap when running R or Rscript in a terminal, it is 
more expensive when running R within a GUI, especially within RStudio, 
as I noticed (but also, e.g., within R.app on MacOS). In fact, using a 
GUI (especially RStudio) can change the cost of (frequent) calls to 
R_CheckUserInterrupt() from negligible to critical (in real-world 
applications). Significant performance drops are also visible for 
functions in the 'stats' package, e.g., pwilcox().

The following MWE (using Rcpp) illustrates the problem. Consider the 
following code:

---

library(Rcpp)
cppFunction('double nonsense(const int n, const int m, const int check) {
   int i, j;
   double result;
   for (i=0;i<n;i++) {
     if (check) R_CheckUserInterrupt();
     result = 1.;
     for (j=1;j<=m;j++) if (j%2) result *= j; else result /=j;
   }
   return(result);
}')

tmp1 <- system.time(nonsense(1e8,10,0))[1]
tmp2 <- system.time(nonsense(1e8,10,1))[1]
cat("w/o check:",tmp1,"sec., with check:",tmp2,"sec., 
diff.:",tmp2-tmp1,"sec.\n")

tmp3 <- system.time(pwilcox(rwilcox(1e5,40,60),40,60))[1]
cat("wilcox example:",tmp3,"sec.\n")

---

Running this code when R (4.4.2) is started in a terminal window 
produces the following measurements/output (Apple M1, MacOS 15.1.1):

   w/o check: 0.525 sec., with check: 0.752 sec., diff.: 0.227 sec.
   wilcox example: 1.028 sec.

Running the same code when R is used within R.app (1.81 (8462) 
aarch64-apple-darwin20) on the same machine results in:

   w/o check: 0.525 sec., with check: 1.683 sec., diff.: 1.158 sec.
   wilcox example: 2.13 sec.

Running the same code when R is used within RStudio Desktop (2024.12.0 
Build 467) on the same machine results in:

   w/o check: 0.507 sec., with check: 22.905 sec., diff.: 22.398 sec.
   wilcox example: 29.686 sec.

So, the performance drop is already remarkable for R.app, but really 
huge for RStudio.

Presumably, checking for user interrupts within a GUI is more involved 
than within a terminal window, so there may not be much room for 
improvement in R.app or RStudio (and I know that this list is not the 
right place to suggest improvements for RStudio or to report unwanted 
behaviour). However, it might be worth considering

1. an addition to the documentation in WRE (explaining that too many 
calls to R_CheckUserInterrupt() can cause a performance bottleneck, 
especially when the code is running within a GUI),
2. check (and possibly change) the position of R_CheckUserInterrupt() in 
some base R functions. For example, moving R_CheckUserInterrupt() from 
cwilcox() to pwilcox() and qwilcox() in src/nmath/wilcox.c may lead to a 
significant improvement (while still being feasible in terms of response 
time).

Best,
Martin


-- 
apl. Prof. Dr. Martin Becker, Akad. Oberrat
Lehrstab Statistik
Quantitative Methoden
Fakultät für Empirische Humanwissenschaften und Wirtschaftswissenschaft
Universität des Saarlandes
Campus C3 1, Raum 2.17
66123 Saarbrücken
Deutschland