'builtins.memoise f x' is equal to 'f x', but uses a cache to speed up
repeated invocations of 'f' with the same 'x'. A typical application
is memoising evaluations of Nixpkgs in NixOps network
specifications. For example, with the patch below, the time to
evaluate a 10-machine NixOps network is reduced from 17.1s to 9.6s,
while memory consumption goes from 4486 MiB to 3089 MiB. (This is with
GC_INITIAL_HEAP_SIZE=16G.)
Nixpkgs patch:
diff --git a/pkgs/top-level/impure.nix b/pkgs/top-level/impure.nix
index a9f21e45aed..f641067e022 100644
--- a/pkgs/top-level/impure.nix
+++ b/pkgs/top-level/impure.nix
@@ -79,7 +79,7 @@ in
# not be passed.
assert args ? localSystem -> !(args ? system || args ? platform);
-import ./. (builtins.removeAttrs args [ "system" "platform" ] // {
+builtins.memoise or (x: x) (import ./.) (builtins.removeAttrs args [ "system" "platform" ] // {
inherit config overlays crossSystem;
# Fallback: Assume we are building packages on the current (build, in GNU
# Autotools parlance) system.
0395b9b94a