Add memoise primop #10280
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Add memoise primop #10280
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'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.
roberth
commented
Mar 21, 2024
| if ((*this)(v2->listElems()[n], v1->listElems()[n])) return false; | ||
| } | ||
|
|
||
| return n == v1->listSize() && n < v2->listSize(); |
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The ordering established by this comparator doesn't have to be exactly lexicographical.
What if we compare the size first, and only compare lexicographically if the sizes are equal?
| if ((*this)(i->value, j->value)) return true; | ||
| if ((*this)(j->value, i->value)) return false; | ||
| } | ||
| return i == v1->attrs->end() && j != v2->attrs->end(); |
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Like the list ordering, the size could be compared first.
Also, the set of names should be compared before evaluating anything extra.
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Motivation
Written by @edolstra, but PR opened by @roberth: This change comes up every now and then. I'm opening a draft PR for it, to increase visibility and so that a discussion can be had.
'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:
Context
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