JVM Flag ergonomics and constraints

[David Holmes]

On 1/02/2023 5:23 am, Ioi Lam wrote:
> CC-ing hotspot-dev for wider discussion.
> 
> On 1/31/2023 12:43 AM, Thomas Stüfe wrote:
>> Hi Ioi,
>>
>> a short question. You did the JVM flag handling revamp, right?
>>
>> I recently ran into a weird problem where FLAG_SET_ERGO would not 
>> work, turned out it ends up using the constraint function, which 
>> silently refused to set the argument if the argument violates the 
>> constraint.
>>
>> That makes perfect sense, I had an error in my code. But if we set a 
>> flag with ERGO origin, we set it in the JVM, and the JVM should adhere 
>> to constraints, not doing so is a programming error and I would expect 
>> an assert here. The same logic may be applied to other origins too, 
>> e.g. DEFAULT.
>> Do you think this makes sense?
>>
>> BTW I added debug output (setting verbose to true for ERGO), and I see 
>> other flag's ergonomics also getting ignored, which would have to be 
>> fixed too.
>>
>> Cheers, Thomas
>>
> 
> 
> This problem is also reported in 
> https://bugs.openjdk.org/browse/JDK-8224980
> 
> There are proposals for making the debug message more obvious, and/or 
> assert().
> 
> Currently we don't have documentation of what the caller of 
> FLAG_SET_ERGO is supposed to do.
> 
> There are 3 ways to set a flag. The current behavior of the following 
> two ways seems reasonable
> 
>     FLAG_SET_CMDLINE - rejects user input and exit VM
>     FLAG_SET_MGMT    - return JVMFlag::Error
> 
> With command-line, the user should inspect the limits for the current 
> environment and restart the app with an appropriate value
> 
> With FLAG_SET_MGMT, I suppose it's similar -- whoever is using the 
> management API should detect the error and adjust their parameter values 
> (by hand, or programmatically)
> 
> However, FLAG_SET_ERGO also returns JVMFlag::Error, but I don't see any 
> of our code checking for the return value. So the unwritten convention 
> is that the call must not fail.
> However, if the specified value is out of range, should we abort the VM, 
> ignore the value, or cap the value? E.g., if the range is 0~100, but you 
> call SET_ERGO with 101, should we cap the value to 100?

Generally if our code tries to sets a flags value to something invalid 
that is a programming error that should be reported right away. If the 
value is a "constant" then we should just assert that setting the value 
worked. If the value is determined by external factors that might vary 
at runtime (so no guarantee the end result will be valid) then we should 
have logic that can correct for the invalid value (or we should change 
the way we do the calculation).

Cheers,
David


> Perhaps capping would make the ergo code easier to write, especially if 
> the constraint function produces a dynamic range (depending on the 
> machine's RAM size, for example). Without capping, you will need to 
> duplicate (or share) the range detection logic between the ergo setter 
> and the constraint checker.
> 
> Thanks
> - Ioi
> 
>