Ruby 2.1: Process.setproctitle()
23 Dec 2013Custom proclines are a great way to add visibility into your code’s execution. The various BSDs have long shipped a setproctitle(3) syscall for this purpose. At GitHub, we use this technique extensively in our unicorns, resques and even our git processes:
$ ps ax | grep git:
32326 git: upload-pack AOKP/kernel_samsung_smdk4412 [173.x.x.x over git] clone: create_pack_file: 75.27 MiB @ 102.00 KiB/s
32327 git: pack-objects AOKP/kernel_samsung_smdk4412 [173.x.x.x over git] writing: 9% (203294/2087544)
2671 git: upload-pack CyanogenMod/android_libcore [87.x.x.x over git] clone: create_pack_file: 31.26 MiB @ 31.00 KiB/s
2672 git: pack-objects CyanogenMod/android_libcore [87.x.x.x over git] writing: 93% (90049/96410)
$ ps ax | grep unicorn
2408 unicorn github.com[9013677] worker[01]: 1843 reqs, 2.9 req/s, 23ms avg, 6.9% util
2529 unicorn github.com[9013677] worker[02]: 1715 reqs, 2.9 req/s, 21ms avg, 6.2% util
2640 unicorn github.com[9013677] worker[03]: 1632 reqs, 3.2 req/s, 19ms avg, 6.2% util
$ pstree -s resqued
─┬─ 25012 resqued-0.7.9 master [gen 413] [1 running] config/resqued.rb
└─┬─ 21924 resqued-0.7.9 listener 413 [f48cb15] [running] config/resqued.rb
├─── 22329 resque-1.20.0: [f48cb15] Waiting for low,notifications,services
├─── 22295 resque-1.20.0: [f48cb15] Processing notifications since 1389929817: GitHub::Jobs::DeliverNotifications
Linux never gained a specialized syscall, but still supports custom /proc/pid/cmdline via one weird trick. The hack relies on the fact that execve(2) allocates a contiguous piece of memory to pass argv and envp into a new process. When the null byte separating the two values is overwritten, the kernel assumes you’ve customized your cmdline and continues to read into the envp buffer. This means your procline can grow to contain more information as long as there’s space in the envp buffer. (Environment variables are copied out of the original envp buffer, so overwriting this area of memory only affects /proc/pid/environ).
Starting with Ruby 2.1 Process.setproctitle is available as a complementary method to $0=, and can be used to customize proclines without affecting $0.