Update to resolv (0.1.2) + valgrind and R + Parallel DNS Requests with Revolution R’s ‘foreach’ and `doParallel`

By Bob Rudis (@hrbrmstr)
Fri 15 August 2014 | tags: r, rstats, valgrind, rcpp, dns, -- (permalink)

Thanks to a blog comment by @arj, I finally ran at least one of the new Rcpp-based through valgrind (resolv) and, sure enough there were a few memory leaks which are now fixed. However, I first ran valgind with a simple test R script that just did library(stats) to get a baseline (and dust off some very rusty valgrind memories). After running that through:

R --vanilla -d "valgrind --tool=memcheck --track-origins=yes" < valgrindtest.R

these are the results from an Ubuntu system:

==24555== HEAP SUMMARY:
==24555==     in use at exit: 17,713,425 bytes in 7,077 blocks
==24555==   total heap usage: 21,258 allocs, 14,181 frees, 30,580,692 bytes allocated
==24555== LEAK SUMMARY:
==24555==    definitely lost: 80 bytes in 2 blocks
==24555==    indirectly lost: 240 bytes in 20 blocks
==24555==      possibly lost: 0 bytes in 0 blocks
==24555==    still reachable: 17,713,105 bytes in 7,055 blocks
==24555==         suppressed: 0 bytes in 0 blocks
==24555== Rerun with --leak-check=full to see details of leaked memory
==24555== For counts of detected and suppressed errors, rerun with: -v
==24555== ERROR SUMMARY: 18 errors from 18 contexts (suppressed: 0 from 0)

and, this is from OS X:

==77581== HEAP SUMMARY:
==77581==     in use at exit: 30,077,961 bytes in 14,565 blocks
==77581==   total heap usage: 32,198 allocs, 17,633 frees, 49,527,117 bytes allocated
==77581== LEAK SUMMARY:
==77581==    definitely lost: 18 bytes in 1 blocks
==77581==    indirectly lost: 0 bytes in 0 blocks
==77581==      possibly lost: 3,704 bytes in 77 blocks
==77581==    still reachable: 28,814,406 bytes in 13,624 blocks
==77581==         suppressed: 1,259,833 bytes in 863 blocks
==77581== Rerun with --leak-check=full to see details of leaked memory
==77581== For counts of detected and suppressed errors, rerun with: -v
==77581== ERROR SUMMARY: 1718 errors from 213 contexts (suppressed: 906524 from 939)

(both for R 3.1.1)

I know R is a complex piece of software with many hands and some excellent (perhaps, draconian :-) review processes, so the leaks actually surprised me. I’m not conerned about the heap cleanup (the kernel will deal with that and it helps apps shut down faster)—and, these are tiny leaks and will not really be an issue—but if I hadn’t baselined this first, I would have suspected there were more errors in resolv than actually existed.

I didn’t dig into why these memory leaks are in R, but that’s definitely on the TODO list.

Speeding Up Resolution

There is no effort made by the resolv package functions to parallelize DNS requests since (for now) I just needed the base functionality. If you do want to speedup lookups when you’re doing a boatload of them, you can use the super-straightforward foreach and doParallel packages from the #spiffy Revolution R folks:


alexa <- fread("data/top-1m.csv") # http://s3.amazonaws.com/alexa-static/top-1m.csv.zip

n <- 10000 # top 'n' to resolve

registerDoParallel(cores=6) # set to what you've got
output <- foreach(i=1:n, .packages=c("Rcpp", "resolv")) %dopar% resolv_a(alexa[i,]$V2)
names(output) <- alexa[1:n,]$V2})

You can also get much fancier parallel functionality with their packages (check them out!).

I’ll post some benchmarks in a future post since I want to run valgrind on iptools and get any memory bugs squashed there next, but you could see 3-6x speedup (or significantly more) using this process. Setting up an aggressive local caching DNS server will also help speed up repeat queries (but increases chances of missing “fresh” data).

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