RFR: 8334489: Add function os::used_memory

[Erik Österlund]

On Tue, 18 Jun 2024 14:11:41 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

> A function os::used_memory is needed for GC ergonomics.
> 
> A naïve implementation is:
> 
> ```c++
> julong os::used_memory() {
>   return os::physical_memory() - os::available_memory();
> }
> 
> 
> This function does not work well in a containerized environment, as the amount of physical memory may change. To understand why, we must look under the hood.
> 
> ```c++
> julong os::used_memory() {
>   return os::physical_memory() - os::available_memory();
> }
> 
> // can be expanded into
> 
> julong os::used_memory() {
>   // The os::physical_memory() call
>   julong mem_physical1 = OSContainer::memory_limit_in_bytes();
>   // The os::available_memory() call
>   julong mem_used = OSContainer::memory_usage_in_bytes();
>   julong mem_physical2 = OSContainer::memory_limit_in_bytes();
>   
>   // Uh-oh: mem_physical1 may differ from mem_physical2 at this point
>   // That means that this number is wrong
>   return mem_physical1 - (mem_physical2 - mem_used);
> }
> 
> 
> The fix is to expose OSContainer::memory_usage_in_bytes if it's available, as this call does not depend on OSContainer::memory_limit_in_bytes. The default implementation will use the naïve implementation.

Looks good to me! Thanks for fixing.

-------------

Marked as reviewed by eosterlund (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/19772#pullrequestreview-2127491819