RFR: 8305770: os::Linux::available_memory() should refer MemAvailable in /proc/meminfo [v2]

[Severin Gehwolf]

On Fri, 5 May 2023 14:27:25 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> `os::Linux::available_memory()` returns available memory from cgroups or sysinfo(2). In case of the process which run on out of container, that value is based on `freeram` from sysinfo(2).
>> 
>> `freeram` is equivalent to `MemFree` in `/proc/meminfo` [1]. However it means just a free RAM. We should use `MemAvailable` when we want to know how much memory is available for the process [2]. `MemAvailable` is available in modern Linux kernel, and it has been backported some older kernels (e.g. RHEL). In `sar` from sysstat, it refers that value and shows it as `kbavail` [3].
>> 
>> AFAIK PhysicalMemory event in JFR depends on `os::Linux::available_memory()`, and it is used in automated analysis in JMC. So the JFR/JMC user could misunderstand physical memory was exhausted even if the memory was available enough.
>> 
>> [1] https://github.com/torvalds/linux/blob/c9c3395d5e3dcc6daee66c6908354d47bf98cb0c/fs/proc/meminfo.c#L59
>> [2] https://docs.kernel.org/filesystems/proc.html?highlight=memavailable
>> [3] https://github.com/sysstat/sysstat/blob/ac1df71ca252c158e8d418ded93e5ed52f5e8765/rd_stats.c#L325-L328
>
> Yasumasa Suenaga has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Introduce os::free_memory

src/hotspot/os/linux/os_linux.cpp line 278:

> 276:       return free_mem;
> 277:     }
> 278:   }

We now have this code twice in `os_linux.cpp`. Could we parameterize this and extract as a function so that we avoid the duplication from lines 226-236?

src/hotspot/share/runtime/os.hpp line 314:

> 312: 
> 313:   static julong available_memory();
> 314:   static julong free_memory();

It would probably be a good idea to add a comment describing what the actual difference is between free/available (i.e. Linux only and usually `available memory > free memory` on such systems).

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PR Review Comment: https://git.openjdk.org/jdk/pull/13398#discussion_r1187187253
PR Review Comment: https://git.openjdk.org/jdk/pull/13398#discussion_r1187189400