RFR: 8334489: Add function os::used_memory [v3]

[Severin Gehwolf]

On Thu, 20 Jun 2024 11:20:40 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.
>
> Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   No, not like that, like *this*

src/hotspot/share/runtime/os.cpp line 2074:

> 2072:       return mem_usage;
> 2073:     }
> 2074:   }

I'm not sure we should introduce Linux specific things in the shared os code like that. It seems the model is to use `pd_<foo>` to delegate to platform specific code? @dholmes-ora thoughts?

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PR Review Comment: https://git.openjdk.org/jdk/pull/19772#discussion_r1647437854