feat: optimizing swap on zram

Author: ryan4yin

hosts/12kingdoms-shoukei/apple-silicon.nix

@@ -1,4 +1,5 @@
 {
+  lib,
   pkgs,
   nixos-apple-silicon,
   my-asahi-firmware,
@@ -10,6 +11,8 @@
     nixos-apple-silicon.nixosModules.default
   ];
 
+  zramSwap.memoryPercent = lib.mkForce 75;
+
   # Workaround for Mesa 25.3.0 regression
   # https://github.com/nix-community/nixos-apple-silicon/issues/380
   # https://github.com/NixOS/nixpkgs/pull/461866

modules/nixos/base/zram.nix

@@ -11,10 +11,33 @@
     # Priority of the zram swap devices.
     # It should be a number higher than the priority of your disk-based swap devices
     # (so that the system will fill the zram swap devices before falling back to disk swap).
-    priority = 5;
+    priority = 100;
     # Maximum total amount of memory that can be stored in the zram swap devices (as a percentage of your total memory).
     # Defaults to 1/2 of your total RAM. Run zramctl to check how good memory is compressed.
     # This doesn’t define how much memory will be used by the zram swap devices.
     memoryPercent = 50;
   };
+
+  # Optimizing swap on zram
+  boot.kernel.sysctl = {
+    # vm.swappiness - Controls kernel preference for swapping (range: 0-200, default: 60)
+    # For in-memory swap devices like zram/zswap, values above 100 are recommended.
+    "vm.swappiness" = 180;
+
+    # vm.watermark_boost_factor - Controls aggressiveness of memory reclaim (default: 15000)
+    # Setting to 0 disables watermark boost, preventing premature memory reclamation.
+    # This allows fuller memory utilization before the kernel starts reclaiming pages.
+    "vm.watermark_boost_factor" = 0;
+
+    # vm.watermark_scale_factor - Controls kswapd wakeup frequency (range: 1-1000, default: 10)
+    # A higher value triggers background memory reclamation earlier (at 12.5% memory pressure).
+    # Value 125 means kswapd becomes active when free memory drops below 1/125 of total memory,
+    # balancing memory more proactively to prevent sudden swap storms at high swappiness values.
+    "vm.watermark_scale_factor" = 125;
+
+    # vm.page-cluster - Controls swap readahead (range: 0-6, default: 3)
+    # 0 means read only 1 page (2^0) at a time, disabling readahead.
+    # For low-latency devices like zram, readahead hurts performance by fetching unnecessary data.
+    "vm.page-cluster" = 0;
+  };
 }