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From b7ae3bd9672e7e1a3fc8e75d90b5bcbe75e6febf Mon Sep 17 00:00:00 2001
From: Benedikt Peetz <benedikt.peetz@b-peetz.de>
Date: Mon, 19 May 2025 21:29:54 +0200
Subject: [PATCH 3/3] memory: Avoid estimating available memory, use kernel
estimate instead
`free`, `btop` and other tools (like the `sysinfo` rust library)
use `mem_total - mem_avail` to calculate the used memory. As explained
in the already linked [kernel commit][1], `mem_avail` is an estimate of
the available memory based on the current way the kernel handles
memory.
The previous logic for calculating used memory tried to estimate
this based on `mem_free`, `buffers`, `pagecache` and
`reclaimable`. Unfortunately, as the kernel commit message predicts,
this user space estimate bitrots, as the kernel memory usage patterns change.
Thus, we now simply use the kernel estimate `mem_avail` as we know that
that is in-sync with the relevant kernel internals.
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773
---
src/blocks/memory.rs | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)
diff --git a/src/blocks/memory.rs b/src/blocks/memory.rs
index 8cf32f9ba..94d8a5fe3 100644
--- a/src/blocks/memory.rs
+++ b/src/blocks/memory.rs
@@ -145,13 +145,9 @@ pub async fn run(config: &Config, api: &CommonApi) -> Result<()> {
let buffers = mem_state.buffers as f64;
- // same logic as htop
- let used_diff = mem_free + buffers + pagecache + reclaimable;
- let mem_used = if mem_total >= used_diff {
- mem_total - used_diff
- } else {
- mem_total - mem_free
- };
+ // Userspace should use `mem_avail` for estimating the memory that is available.
+ // See: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773
+ let mem_used = mem_total - mem_avail;
// account for ZFS ARC cache
let mem_used = mem_used - zfs_shrinkable_size;
--
2.49.0
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