oomd: calculate 'used' memory with MemAvailable instead of MemFree

The calculation for used memory in oomd_system_context_acquire is given
by MemTotal - MemFree from /proc/meminfo. This is too strict of a
calculation because it does not consider memory that is still available
for starting new applictions without swapping (MemAvailable). As a
result, systemd-oomd can start to kill processes before it is necessary.
This is more apparent on systems with low swap space.

Instead, compute 'used' memory as MemTotal - MemAvailable in
oomd_system_context_acquire and procfs_memory_get (which is used by
oomd_cgroup_context_acquire). And, rename oomd_mem_free_below to
oomd_mem_available_below for clarity.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/oom/oomd-util.h b/src/oom/oomd-util.h
index 6aada90344..de2298f3c9 100644
--- a/src/oom/oomd-util.h
+++ b/src/oom/oomd-util.h
@@ -60,8 +60,8 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(OomdCGroupContext*, oomd_cgroup_context_free);
  * Returns -ENOMEM for allocation errors. */
 int oomd_pressure_above(Hashmap *h, usec_t duration, Set **ret);
 
-/* Returns true if the amount of memory free is below the permyriad of memory specified by `threshold_permyriad`. */
-bool oomd_mem_free_below(const OomdSystemContext *ctx, int threshold_permyriad);
+/* Returns true if the amount of memory available (see proc(5)) is below the permyriad of memory specified by `threshold_permyriad`. */
+bool oomd_mem_available_below(const OomdSystemContext *ctx, int threshold_permyriad);
 
 /* Returns true if the amount of swap free is below the permyriad of swap specified by `threshold_permyriad`. */
 bool oomd_swap_free_below(const OomdSystemContext *ctx, int threshold_permyriad);