RFR: 8334489: Add function os::used_memory [v2]

[Johan Sjölen]

On Wed, 19 Jun 2024 10:43:50 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Ifdef instead
>
> I think in this case I'd prefer to have an ifdef LINUX in a single shared version in os.cpp rather than duplicating the function this way.

@dholmes-ora 

> I think in this case I'd prefer to have an ifdef LINUX in a single shared version in os.cpp rather than duplicating the function this way.

That's nice, let's do that.

@tstuefe 

> You subtract available from physical. Why not free from physical?

`os::free_memory()` is just `available_memory()` on all of our platforms. I thought it was weird that we have two of them which are identical, so I just picked the one that is defined.

>What is your definition of used? Used, as in "used by user processes", or used as in "used by anything on the machine, including kernel-side caches"?

The latter, but machine can be replaced by "the container". To quote:

>[memory.current] Shows the total amount of memory currently being used by the cgroup and its descendants. It includes page cache, in-kernel data structures such as inodes, and network buffers.

That should be a viable explanation for the non-container case also.

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PR Comment: https://git.openjdk.org/jdk/pull/19772#issuecomment-2179112191