ThreadPriorityPolicy settings for non-root users

David Holmes david.holmes at oracle.com
Fri Dec 21 12:48:39 UTC 2018 

On 21/12/2018 10:44 pm, Baesken, Matthias wrote:
>> I'd be interested to know what the actual usecase is. But I think a
>> native method would be the best way to handle it.
> 
> Hi David, sorry I am not sure  what native method are you talking about ?
> Do you mean  the application - coding should be rewritten from Java   into some  C/C++-JNI  +  Java  mixture  where the   setpriority - calls to adjust the threads
>    are done from the C/C++ coding part  ?

Yes - one native method to set the native priority according to the 
semantics actually desired by the application.

David

> Best regards, Matthias
> 
> 
>> -----Original Message-----
>> From: David Holmes <david.holmes at oracle.com>
>> Sent: Freitag, 21. Dezember 2018 13:33
>> To: Baesken, Matthias <matthias.baesken at sap.com>; 'hotspot-
>> dev at openjdk.java.net' <hotspot-dev at openjdk.java.net>
>> Cc: Haug, Gunter <gunter.haug at sap.com>
>> Subject: Re: ThreadPriorityPolicy settings for non-root users
>>
>> On 21/12/2018 9:09 pm, Baesken, Matthias wrote:
>>>>
>>>> Have you got real use cases for this?
>>>>
>>>
>>> Hi David, yes  we have a user  who  was setting  the "infamous"
>> ThreadPriorityPolicy=2   (+ then different prios/niceness levels  for  different
>> threads) ,  but that is now no longer working in current JDK11  .
>>
>> I'd be interested to know what the actual usecase is. But I think a
>> native method would be the best way to handle it.
>>
>> David
>>
>>> Best regards, Matthias
>>>
>>>
>>>> -----Original Message-----
>>>> From: David Holmes <david.holmes at oracle.com>
>>>> Sent: Freitag, 21. Dezember 2018 11:56
>>>> To: Baesken, Matthias <matthias.baesken at sap.com>; 'hotspot-
>>>> dev at openjdk.java.net' <hotspot-dev at openjdk.java.net>
>>>> Subject: Re: ThreadPriorityPolicy settings for non-root users
>>>>
>>>> On 21/12/2018 6:39 pm, Baesken, Matthias wrote:
>>>>> Hi  David  ,   it might be  that  the functionality  is  seen as not very helpful
>> by
>>>> some and removed  or deprecated  in  some future  Java release.
>>>>>
>>>>> However it is present in the current JDKs and should work there nicely .
>>>>
>>>> Sorry but that's a bit naive. The code is old and bit-rotted and in some
>>>> cases (Mac port) likely never used, so the idea that "it's there so it
>>>> should work" is just not realistic - sorry.
>>>>
>>>>> Currently  I have some points I do not like about the current state :
>>>>>
>>>>> - the root-check  is not consistent , it is present  on Linux /  BSD (Mac)
>> but
>>>> I don't see it on Solaris
>>>>
>>>> Wasn't needed on Solaris. User-level capabilities sufficed.
>>>>
>>>>> - It  ignores currently  the  CAP_SYS_NICE capability
>>>>
>>>> It never supported it. AFAIK the linux code doesn't really support any
>>>> capability based permissions.
>>>>
>>>>> - it ignores that  setting a  higher niceness works nicely  on most OS
>>>> (checked Linux/Solaris/BSD)  without being root  (or having special
>>>> capabilities)
>>>>
>>>> The priority control was never really about tweaking niceness levels.
>>>>
>>>>> -  the root check makes testing hard  (maybe that's why the Mac version
>>>> was a bit broken?)
>>>>
>>>> Running under sudo isn't that hard.
>>>>
>>>> Sorry I'm not very supportive here - this isn't something that needs
>>>> some minor tweaking to bring back online, it's something that may never
>>>> have worked well in the first place.
>>>>
>>>> Have you got real use cases for this?
>>>>
>>>> Cheers,
>>>> David
>>>>
>>>>>
>>>>> Best regards, Matthias
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: David Holmes <david.holmes at oracle.com>
>>>>>> Sent: Donnerstag, 20. Dezember 2018 12:06
>>>>>> To: Baesken, Matthias <matthias.baesken at sap.com>; 'hotspot-
>>>>>> dev at openjdk.java.net' <hotspot-dev at openjdk.java.net>
>>>>>> Subject: Re: ThreadPriorityPolicy settings for non-root users
>>>>>>
>>>>>> Hi Matthias,
>>>>>>
>>>>>> The more I think about this the more I see it as a huge can of worms.
>>>>>> There are very, very, limited usecases for managing the priority of
>>>>>> individual threads within a running Java application. Adjusting the
>>>>>> process priority/nice-ness is effective and much simpler.
>>>>>>
>>>>>> 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
>>>>>>>

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