RFR: 8334489: Add function os::used_memory

[Johan Sjölen]

On Wed, 19 Jun 2024 12:30:30 GMT, Severin Gehwolf <sgehwolf 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.
>
> src/hotspot/os/linux/os_linux.cpp line 275:
> 
>> 273: 
>> 274: julong os::used_memory() {
>> 275:   if (OSContainer::is_containerized()) {
> 
> I'm curious. What's the intended use case of this? https://bugs.openjdk.org/browse/JDK-8329758 perhaps? If this is getting used more broadly, https://bugs.openjdk.org/browse/JDK-8261242, would seem to get bumped in priority as that still affects physical Linux systems.

I think you hit the nail on the head regarding intended use case. It's probably important that that issue gets resolved, yes.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19772#discussion_r1646504917