RFR: 8148987: [Linux] Allow building on older systems without CPU_ALLOC support

[David Holmes]

Hi Dmitry,

On 5/02/2016 11:21 PM, Dmitry Samersoff wrote:
> David,
>
> Sorry! Wasn't clean enough.
>
> 1. I'm not sure we should handle all possible variants of old systems.
>
> i.e. it might be better to create one function for the case "everything
> present" and one other, minimal, variant for the case where we don't
> have any of required macro (either CPU_COUNT or CPU_ALLOC or both).

I'm waiting for additional feedback from others. :) Given I'm reluctant 
to even make this change in 9, I'm even more reluctant to add further 
duplication.

My main motivation to change this in 9 is to take the same basic code to 
8 so that unofficial builds on later compilers can utilize the more 
modern functionality if it is available. How to handle the lack of 
Unified Logging is still TBD.


> 2. It requires some brain efforts to catch that CPU_COUNT_S at 4851 will
> never be called if we don't have CPU_ALLOC.

Yeah I tried to make this clearer by redefining the _S versions with a 
ShouldNotReachHere() - but that is itself a macro and I couldn't get it 
to work - especially when I need to appear to "return" a number.

Thanks,
David

>    I would prefer to avoid implicit dependencies like this one, ever at
> the cost of some code duplication.
>
> -Dmitry
>
> On 2016-02-05 15:21, David Holmes wrote:
>> Hi Dmitry,
>>
>> Thanks for looking at this.
>>
>> On 5/02/2016 8:46 PM, Dmitry Samersoff wrote:
>>> David,
>>>
>>> I suspect that if the os don't have CPU_COUNT it doesn't have
>>> CPU_COUNT_S as well at 4851
>>
>> It is true that if you don't have CPU_COUNT you won't have the *_S
>> macros either but I'm handling the two separately as you can have
>> CPU_COUNT but not CPU_ALLOC. The *_S variants were added for the dynamic
>> cpu set management, so it is determined by the existence of CPU_ALLOC.
>>
>>> Is it possible to create two different version of
>>> os::active_processor_count() - simple one for old systems and more
>>> complicated one for modern linux and move define to a function level?
>>
>> There are really three variants:
>> - no CPU_COUNT or CPU_ALLOC (and *_S)
>> - no CPU_ALLOC (and *_S)
>> - everything present
>>
>> There are numerous different ways in which to represent that. I started
>> with the has-everything version and tried to minimize the ifdefs within
>> that code by isolating the CPU_ALLOC part. Then the CPU_COUNT part is
>> handled separately.
>
>
>
>>
>> Thanks,
>> David
>>
>>> -Dmitry
>>>
>>> On 2016-02-05 10:50, David Holmes wrote:
>>>> bug: https://bugs.openjdk.java.net/browse/JDK-8148987
>>>>
>>>> webrev: http://cr.openjdk.java.net/~dholmes/8148987/webrev/
>>>>
>>>> This is a tentative RFR as I'm not yet convinced we should support
>>>> building on these older platforms in JDK 9. Opinions welcome on that
>>>> point.
>>>>
>>>> The older systems do not have dynamic cpu set support (CPU_ALLOC) nor do
>>>> they have the CPU_COUNT utility macro.
>>>>
>>>> Thanks,
>>>> David
>>>
>>>
>
>