I Did Not Know: xargs -n and -P

March 16, 2014

Say you need to md5sum 46 files, all ending in “.foo” in a single directory. You might use your standard md5sum *.foo > md5sum.txt command to checksum them all in one process. Get coffee, it’s done, move on.

Oh, I should mention those 46 files are a little under 6 terabytes in total size. The standard command might take a while. I drink a lot of coffee, but whoa.

Now imagine you have a 16 core server connected via modern InfiniBand to an otherwise idle pre-production parallel filesystem with several hundred disks and multiple controllers, each with their own cache. The odds are tilting in your favor. This is especially true if you read up on this pair of options in xargs(1), which inexplicably, shamefully, I Did Not Know:

--max-args=max-args, -n max-args
       Use at most max-args  arguments  per  command  line.
       Fewer  than  max-args  arguments will be used if the
       size (see the -s option) is exceeded, unless the  -x
       option is given, in which case xargs will exit.
--max-procs=max-procs, -P max-procs
       Run up to max-procs processes at a time; the default
       is 1.  If max-procs is 0, xargs  will  run  as  many
       processes  as possible at a time.  Use the -n option
       with -P; otherwise chances are that  only  one  exec
       will be done.

Sure, I could have run this on more than one such machine connected to the same filesystem. There are a number of tools that can split up work across multiple child processes on a single machine, none of which were installed in this environment. I wanted to see what I could get this single server to do with basic commands.

46 files / 16 cores = 2.875, so let’s give this a shot:

find . -type f -name "*.foo" | xargs -P 16 -n 3 md5sum | tee md5sum.out

English: For the files ending in “.foo” in this directory, run md5sum up to 16 times in parallel with up to three files per run, show results as they happen, and save the output.

Please Note: This will absolutely not help unless you have the storage infrastructure to handle it. Your Best Buy hard drive will not handle it. It has a strong chance of making your machine unhappy.

In this case, I got something lovely from top:

  PID S %CPU COMMAND
29394 R 100.0 md5sum
29396 R 100.0 md5sum
29397 R 100.0 md5sum
29398 R 100.0 md5sum
29399 R 100.0 md5sum
29400 R 100.0 md5sum
29401 R 100.0 md5sum
29402 R 100.0 md5sum
29403 R 100.0 md5sum
29391 R 99.6 md5sum 
29392 R 99.6 md5sum 
29393 R 99.6 md5sum 
29395 R 99.6 md5sum 
29404 R 99.6 md5sum 
29405 R 99.6 md5sum 
29406 R 99.6 md5sum

Early on, there were some D states waiting for cache to warm up, and CPU dropped below 70% for one or two processes, but I’ll take it. I’ll especially take this:

real    31m33.147s

Right on, xargs. Easy parallelization on one system for single file tasks driven from a file list or search.