RFR: 8367319: Add os interfaces to get machine and container values separately [v2]

[David Holmes]

On Mon, 6 Oct 2025 14:48:29 GMT, Casper Norrbin <cnorrbin at openjdk.org> wrote:

>> Hi everyone,
>> 
>> The current `os::` layer on Linux hides whether the JVM is running inside a container or not. When running inside a container, we replace machine values with container values where applicable, without telling the user of these methods. For most use cases, this is fine, users only care about the returned value. But for other use cases, where the value originated is important. Two examples:
>> 
>> - A user might need the physical cpu count of the machine, but `os::active_processor_count()` only returns the limited container value, which also represents something slightly different.
>> - A user might want the container memory limit and the physical RAM size, but `os::physical_memory()` only gives one number.
>> 
>> To solve this, every function that mixed container/machine values now has to explicit variants, prefixed with `machine_` and `container_`. These use the bool return + out-parameter interface, with the container functions only working on Linux. The original methods remain and continue to return the same mixed values.
>> 
>> In addition, container-specific accessors for the memory soft limit and the memory throttle limit have been added, as these values matter when running in a containerized environment.
>> 
>> `OSContainer::active_processor_count()` has also been changed to return `double` instead of `int`. The previous implementation rounded the quota/period ratio up to produce an integer for `os::active_processor_count()`. Now, when the value is requested directly from the new container API it makes more sense to preserve this fraction rather than rounding it up. We can thus keep the exact value for those that want it, then round it up to keep the same behavior in `os::active_processor_count()`.
>> 
>> Testing:
>> - Oracle tiers 1-5
>> - Container tests on cgroup v1 and v2 hosts.
>
> Casper Norrbin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixed print type

> A user might need the physical cpu count of the machine, but os::active_processor_count() only returns the limited container value, which also represents something slightly different.

That is a mis-characterization of the API. `active_processor_count()` tells you how many logical processors are available to the JVM process. That can be very different to the "physical" (**) number of processors due to partitioning at various levels (e.g. virtualization, containerization), as well as direct restrictions through API's like `taskset`.

(**) "physical" actually has no meaning these days. There is some value you can obtain through the operating system that provides the maximum number of processors that the operating system can see (and thus make available to the JVM).

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PR Comment: https://git.openjdk.org/jdk/pull/27646#issuecomment-3374848444