ThreadPriorityPolicy settings for non-root users

[David Holmes]

Hi Matthias,

This is a tricky area and it will need quite a bit of thought. The 
existing priority related code is very old, intended to address very 
specific usecases and basically AFAIK generally completely unused for 
years. We actively discourage people from messing with priority. :)

The BSD code was blindly copied from Linux (as was the bulk of the 
BSD/OS X port).

I will need to refresh myself on all the gory details so this is not 
likely to happen until the New Year as I'll be on Christmas break from 
Friday until January 7.

Cheers,
David

On 20/12/2018 3:13 am, Baesken, Matthias wrote:
> 
> Hello ,
>    currently OpenJDK supports 2 ThreadPriorityPolicy settings,  0  (normal, the default) and 1,
> the so called "aggressive" mode :
> 
> "1 : Aggressive.                                                 "\
> "    Java thread priorities map over to the entire range of      "\
> "    native thread priorities. Higher Java thread priorities map "\
> "    to higher native thread priorities. This policy should be   "\
> "    used with care, as sometimes it can cause performance       "\
> "    degradation in the application and/or the entire system. On "\
> "    Linux this policy requires root privilege.")                 \
> 
> Currently  we check directly for root in os_bsd.cpp and os_linux.cpp (the text  in globals.hpp mentions only Linux which seems to be not fully correct):
> 
> if (geteuid() != 0) { ... } in function prio_init().
> 
> (looks like the check is not done for other platforms).
> 
> However the check for root (e.g. on Linux)  hinders users to set a ****lower priority**** for a thread (== increase the "niceness" level)
> when running as a non-root user  (there might be strange ways from outside the VM with calling scripts and renice but .... ).
> 
> 
> In older JDKs (e.g. JDK8) there was a "workaround" to use for example ThreadPriorityPolicy=2 to avoid the root-check,
> but this is not possible any more in recent JDKs (10/11) after the range check (0,1) has been introduced for the ThreadPriorityPolicy flag
> (and probably the old workaround was not a good one anyway because it was undocumented).
> 
> So do you think we could introduce another XX-flag  ( AllowAggressiveThreadPriorityPolicyForAllUsers, or some better name)
> that allows using the "aggressive" mode for non-root users ? Another option would be to add another mode 2 for ThreadPriorityPolicy
> that documents the behavior (like mode 1 but without root-user check).
> 
> If I get it right, even  setting  ***higher prios*** (lower niceness) is possible for non-root users on systems configured in an appropriate way
> (using the CAP_SYS_NICE capability).
> 
> But setting  lower prio / higher niceness is even possible for normal users NOW without special config, it is just disabled by the root-check
> which is very bad.
> 
> Best regards, Matthias
>