ps goes walkabout through /dev in search of foreign ttys
Analysis here: https://gist.github.com/1484969
Suggested fix, thus far, is to tell ps not to look for ttys in a zone that isn't the current zone. If we leave the current exhaustive search behaviour for same-zone searches then crazily-pathed ttys will still be found should they exist, and in the general case ptys found in the priority search paths listed in /etc/ttysrch will be found quickly.
Updated by Robert Mustacchi almost 9 years ago
We hit this internally at Joyent simply by having a large number of zones. What follows is from the internal analysis and attached is our commit to fix this from Bryan.
When run from the global zone, ps performance is generally terrible – and degrades with the number of zones. For example, to run "ps -e" on a system with ~190 zones takes nearly four minutes! The problem is that ps is finding processes in the non-global zones and is attempting to locate names for their TTYs via ttyname(3C) – but these device nodes do not exist in the global zone. A failing ttyname() is excruciating – it essentially reduces to a find in /dev – and when you multiply these failing ttyname() calls by the number of zones in the system, performance quickly becomes deeply pathological. The fix here is very simple: to not even bother to try to lookup a TTY when the zone of the process doesn't match that of the ps.
On a system with 192 zones, here's stock ps:
[root@00-1e-4f-42-fb-1a (us-east-1) /var/tmp]# time ps -e > /dev/null real 3m44.926s user 0m0.321s sys 3m16.811s
And running with the fix:
[root@00-1e-4f-42-fb-1a (us-east-1) /var/tmp]# time ./ps -e > /dev/null real 0m0.111s user 0m0.018s sys 0m0.093s
Yes, that's a win of 2000X...
Updated by Rich Lowe almost 9 years ago
Joshua, I'd actually (for other reasons), be very interested in which parts of the /dev walk suck the most (perhaps based on the top one or two path elements?). What I missed when looking for possibly expensive dynamic portions of /dev, is the potential for upcalls from the /dev fs.
Robert, do you guys want to get this integrated, or should I try to do it over the weekend?
If you tell me who reviewed it on your end, I could probably just submit an RTI with me as another reviewer (I looked, and am happy).