[Rd] weird bug with parallel, RSQlite and tcltk

[Simon Urbanek]

On Dec 31, 2012, at 1:08 PM, Karl Forner wrote:

> Hello,
> 
> I spent a lot of a time on a weird bug, and I just managed to narrow it down.
> 

First, tcltk and multicore don't mix well, see the warning in the documentation (it mentions GUIs and AFAIR tcltk fires up a GUI event loop even if you don't actually create GUI elements). Second, using any kind of descriptors in parallel code is asking for trouble since those will be owned by multiple processes. If you use databases files, etc. they must be opened in the parallel code, they cannot be shared by multiple workers. The latter is ok in your code so you're probably bitten by the former.

Cheers,
Simon



> In parallel code (here with parallel::mclappy, but I got it
> doMC/multicore too), if the library(tcltk) is loaded, R hangs when
> trying to open a DB connection.
> I got the same behaviour on two different computers, one dual-core,
> and one 2 xeon quad-core.
> 
> Here's the code:
> 
> library(parallel)
> library(RSQLite)
> library(tcltk)
> #unloadNamespace("tcltk")
> 
> res <- mclapply(1:2, function(x) {
> 	db <- DBI::dbConnect("SQLite", ":memory:")
> }, mc.cores=2)
> print("Done")	
> 
> When I execute it (R --vanilla  < test_parallel_db.R), it hangs
> forever, and I have to type several times CTRL+C to interrupt it. I
> then get this message:
> 
> Warning messages:
> 1: In selectChildren(ac, 1) : error 'Interrupted system call' in select
> 2: In selectChildren(ac, 1) : error 'Interrupted system call' in select
> 
> Then, just remove library(tcltk), or uncomment
> unloadNamespace("tcltk"), and it works fine again.
> 
> I guess there's a bug somewhere, but where exactly ?
> 
> Best,
> 
> Karl Forner
> 
> Further info:
> 
> 
> R version 2.15.1 (2012-06-22) -- "Roasted Marshmallows"
> Copyright (C) 2012 The R Foundation for Statistical Computing
> ISBN 3-900051-07-0
> Platform: x86_64-unknown-linux-gnu (64-bit)
> 
> ubuntu 12.04 and 12.10
> 
> ubuntu package tk8.5
> 
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