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

[David Holmes]

On Sat, 8 Apr 2023 02:24:44 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

I'm finding this as "clear as mud". We have from the /proc description:

> MemFree
>   Total free RAM. On highmem systems, the sum of LowFree+HighFree
>
> MemAvailable
>   An estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone. The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system.

Which suggests to me that MemAvailable <= MemFree (as some memory is not actually "available") yet the example preceding the above text has:

> `MemFree:        21001236 kB`
> `MemAvailable:   27214312 kB`

so there is more available memory than there is free memory ???

And of course sysinfo(2) has:

> ` unsigned long freeram;   /* Available memory size */`

so ??? What does "available" mean?

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

PR Comment: https://git.openjdk.org/jdk/pull/13398#issuecomment-1502856794