balloon: use MADV_FREE to reclaim memory on macOS#703
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The MADV_DONTNEED flag is no-op on macOS because it preserves the data, so the OS cannot discard the pages. MADV_FREE flag actually frees the memory under pressure, which is what we need for balloon. Signed-off-by: Vadzim Dambrouski <pftbest@gmail.com>
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The current implementation of balloon on macOS is not working correctly, because it is using the wrong flag for madvise.
The MADV_DONTNEED flag is no-op on macOS because it preserves the data, so the OS cannot discard the pages. MADV_FREE flag actually frees the memory under pressure, which is what we need for balloon.
How to test this:
With MADV_DONTNEED flag nothing changes, balloon is not working:
With MADV_FREE the pressure drops right away:
Notably the per process memory usage in Activity Monitor still reports 8GB in both cases, however this numbers are not accurate, so the only way to test is to observe the pressure and total RAM usage.