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Wacky string coercion semantics caused expressions like
exec = "${./my-script} params...";
to evaluate to a path (‘/path/my-script params’), because
anti-quotations are desuged to string concatenation:
exec = ./my-script + " params...";
By constrast, adding a space at the start would yield a string as
expected:
exec = " ${./my-script} params...";
Now the first example also evaluates to a string.
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We now print all output paths of a package, e.g.
openssl-1.0.0i bin=/nix/store/gq2mvh0wb9l90djvsagln3aqywqmr6vl-openssl-1.0.0i-bin;man=/nix/store/7zwf5r5hsdarl3n86dasvb4chm2xzw9n-openssl-1.0.0i-man;/nix/store/cj7xvk7fjp9q887359j75pw3pzjfmqf1-openssl-1.0.0i
or (in XML mode)
<item attrPath="openssl" name="openssl-1.0.0i" system="x86_64-linux">
<output name="bin" path="/nix/store/gq2mvh0wb9l90djvsagln3aqywqmr6vl-openssl-1.0.0i-bin" />
<output name="man" path="/nix/store/7zwf5r5hsdarl3n86dasvb4chm2xzw9n-openssl-1.0.0i-man" />
<output name="out" path="/nix/store/cj7xvk7fjp9q887359j75pw3pzjfmqf1-openssl-1.0.0i" />
</item>
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This allows adding attributes like
attr = if stdenv.system == "bla" then something else null;
without changing the resulting derivation on non-<bla> platforms.
We once considered adding a special "ignore" value for this purpose,
but using null seems more elegant.
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Since the called function can return its argument attribute set
(e.g. "a"), the latter should not be allocated on the stack.
Reported by Shea.
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This allows repairing corrupted derivations and other source files.
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This saves about 4 MB when evaluating a NixOS system configuration.
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More precisely, in concatLists, if all lists except one are empty,
then just return the non-empty list. This reduces the number of list
element allocations by 32% when evaluating a NixOS system
configuration.
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This can serve as a generic efficient list builder. For instance, the
function ‘catAttrs’ in Nixpkgs can be rewritten from
attr: l: fold (s: l: if hasAttr attr s then [(getAttr attr s)] ++ l else l) [] l
to
attr: l: builtins.concatLists (map (s: if hasAttr attr s then [(getAttr attr s)] else []) l)
Statistics before:
time elapsed: 1.08683
size of a value: 24
environments allocated: 1384376 (35809568 bytes)
list elements: 6946783 (55574264 bytes)
list concatenations: 37434
values allocated: 1760440 (42250560 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18273
number of thunks: 1297673
number of thunks avoided: 1380759
number of attr lookups: 430802
number of primop calls: 628912
number of function calls: 1333544
Statistics after (including new catAttrs):
time elapsed: 0.959854
size of a value: 24
environments allocated: 1010198 (26829296 bytes)
list elements: 1984878 (15879024 bytes)
list concatenations: 30488
values allocated: 1589760 (38154240 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18274
number of thunks: 1040925
number of thunks avoided: 1038428
number of attr lookups: 438419
number of primop calls: 474844
number of function calls: 959366
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Evaluation of a NixOS configuration spends quite a lot of time in the
"filter" function in Nixpkgs. As implemented in Nixpkgs, this is a
O(n^2) operation, so it's a good candidate for providing a more
efficient (i.e. primop) implementation. Using it gives a ~10% speed
increase and a significant reduction in the number of evaluations.
Statistics before (on a NixOS system config):
time elapsed: 1.3258
size of a value: 24
environments allocated: 1980939 (50127080 bytes)
list elements: 14679308 (117434464 bytes)
list concatenations: 50828
values allocated: 2098938 (50374512 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18271
number of thunks: 1645752
number of thunks avoided: 1921196
number of attr lookups: 430798
number of primop calls: 838807
number of function calls: 1930107
Statistics after:
time elapsed: 1.17982
size of a value: 24
environments allocated: 1543334 (39624560 bytes)
list elements: 9612638 (76901104 bytes)
list concatenations: 37434
values allocated: 1854933 (44518392 bytes)
attribute sets allocated: 392040
right-biased unions: 186334
values copied in right-biased unions: 591137
symbols in symbol table: 18272
number of thunks: 1392467
number of thunks avoided: 1507311
number of attr lookups: 430801
number of primop calls: 691600
number of function calls: 1492502
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Setting the environment variable NIX_COUNT_CALLS to 1 enables some
basic profiling in the evaluator. It will count calls to functions
and primops as well as evaluations of attributes.
For example, to see where evaluation of a NixOS configuration spends
its time:
$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 ./src/nix-instantiate/nix-instantiate '<nixos>' -A system --readonly-mode
...
calls to 39 primops:
239532 head
233962 tail
191252 hasAttr
...
calls to 1595 functions:
224157 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:19'
221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:14'
221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:10'
...
evaluations of 7088 attributes:
167377 undefined position
132459 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:119:41'
47322 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:13:21'
...
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Put all Nix configuration flags in a Settings object.
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EvalState::eval(). This gives a 12% speedup on ‘nix-instantiate
/etc/nixos/nixos/ -A system --readonly-mode’ (from 1.01s to 0.89s).
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attributes.
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and allocate them only once.
* Move Value and related functions into value.hh.
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/etc/nixos/nixos -A system" spent about 10% of its time in
dynamic_cast.
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file.
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a string. This happens in the NixOS option system.
* Remove a bogus comparison of a unsigned integer with -1.
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prevents files from being evaluated and stored as values multiple
times. For instance, evaluation of the ‘system’ attribute in NixOS
causes ‘nixpkgs/pkgs/lib/lists.nix’ to be evaluated 2019 times.
Caching gives a modest speedup and a decent memory footprint
reduction (e.g., from 1.44s to 1.28s, and from 81 MiB to 59 MiB with
GC_INITIAL_HEAP_SIZE=100000 on my system).
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possible for other Nix expressions to use corepkgs (mostly useful
for the buildenv function).
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brackets, e.g.
import <nixpkgs/pkgs/lib>
are resolved by looking them up relative to the elements listed in
the search path. This allows us to get rid of hacks like
import "${builtins.getEnv "NIXPKGS_ALL"}/pkgs/lib"
The search path can be specified through the ‘-I’ command-line flag
and through the colon-separated ‘NIX_PATH’ environment variable,
e.g.,
$ nix-build -I /etc/nixos ...
If a file is not found in the search path, an error message is
lazily thrown.
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the EvalState class.
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x.y.z or default
(as originally proposed in
https://mail.cs.uu.nl/pipermail/nix-dev/2009-September/002989.html).
For instance, an expression like
stdenv.lib.attrByPath ["features" "ckSched"] false args
can now be written as
args.features.ckSched or false
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an attribute path. This is a refactoring to support default values.
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write ‘attrs ? a.b’ to test whether ‘attrs’ has an attribute ‘a’
containing an attribute ‘b’. This is more convenient than ‘attrs ?
a && attrs.a ? b’.
Slight change in the semantics: it's no longer an error if the
left-hand side of ‘?’ is not an attribute set. In that case it just
returns false. So, ‘null ? foo’ no longer throws an error.
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little RAM. Even if the memory isn't actually used, it can cause
problems with the overcommit heuristics in the kernel. So use a VM
space of 25% of RAM, up to 384 MB.
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collector.
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latter.
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elements. This prevents the vector from having to resize itself.
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* Simplify the representation of attributes in the AST.
* Change the behaviour of listToAttrs() in case of duplicate names.
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significantly reduces the number of values allocated (e.g. from 8.7m
to 4.9m for the Bittorrent test).
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tree). This saves a lot of memory. The vector should be sorted so
that names can be looked up using binary search, but this is not the
case yet. (Surprisingly, looking up attributes using linear search
doesn't have a big impact on performance.)
Memory consumption for
$ nix-instantiate /etc/nixos/nixos/tests -A bittorrent.test --readonly-mode
on x86_64-linux with GC enabled is now 185 MiB (compared to 946
MiB on the trunk).
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* Fix the stats.
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* Clear pointers in Values after overwriting them to make sure that no
objects are kept alive unnecessarily.
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improves GC effectiveness a bit more (because a live value doesn't
keep other values in the environment plus the parent environments
alive), and removes the need for copy nodes.
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