Limiting the number of R Threads
R is single threaded, meaning that R can only execute one instruction at a time. This makes developing web applications in R which users expect to be highly performant hard. Long running computations will prevent multiple users from viewing your site concurrently.
There are a few design suggestions explored in Best Practices that can make a large impact, however, the IQSS Shiny toolchain enables basic concurrency for your Shiny applications through the use of load balancing.
The Shiny IQSS toolchain launches as many R processes as there are CPUs, divided by two. Shiny by itself takes up around 60 to 100MiB of memory without performing any work, so using a 1:1 mapping of R process to CPU would most likely result in an out of memory error in most cases.
If your application uses a lot of memory, the default of CPUs / 2 may still result in out of memory errors. You can manually set the number of R processes to launch by adding the environment variable R_NUM_PROCS to your Heroku depoyment.
Using only 1 R process.
Open the settings tab of your Shiny app on Heroku and click Reveal Config Vars.
In KEY, enter R_NUM_PROCS.
In VALUE, enter 1.
Then, click ADD
Click More in the upper-right hand corner and select Restart all dynos. This is disruptive - if you only have one dyno, user connections will be severed. When your application is restarted, your application will only launch 1 R process.
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