RFR: 8305770: os::Linux::available_memory() should refer MemAvailable in /proc/meminfo [v4]

[David Holmes]

On Mon, 8 May 2023 13:59:53 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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'upstream/master' into JDK-8305770
>  - Use nullptr in condition
>  - Fix comments
>  - Introduce os::free_memory
>  - Use JULONG_FORMAT in format string
>  - 8305770: os::Linux::available_memory() should refer MemAvailable in /proc/meminfo

Just trying to understand where this has ended up. IIUC the change:
- modifies `os::Linux::available_memory` to use `MemAvailable`
- adds `os::Linux_free_memory` to do what `os::Linux::available_memory` used to do
- changes compileBroker to use `os::free_memory` instead of `os::available_memory` so that it is unaffected by the change

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PR Review: https://git.openjdk.org/jdk/pull/13398#pullrequestreview-1417712138