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'
...
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).
directory
/home/eelco/src/stdenv-updates
that you want to use as the directory for import such as
with (import <nixpkgs> { });
then you can say
$ nix-build -I nixpkgs=/home/eelco/src/stdenv-updates
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.
derivations added to the store by clients have "correct" output
paths (meaning that the output paths are computed by hashing the
derivation according to a certain algorithm). This means that a
malicious user could craft a special .drv file to build *any*
desired path in the store with any desired contents (so long as the
path doesn't already exist). Then the attacker just needs to wait
for a victim to come along and install the compromised path.
For instance, if Alice (the attacker) knows that the latest Firefox
derivation in Nixpkgs produces the path
/nix/store/1a5nyfd4ajxbyy97r1fslhgrv70gj8a7-firefox-5.0.1
then (provided this path doesn't already exist) she can craft a .drv
file that creates that path (i.e., has it as one of its outputs),
add it to the store using "nix-store --add", and build it with
"nix-store -r". So the fake .drv could write a Trojan to the
Firefox path. Then, if user Bob (the victim) comes along and does
$ nix-env -i firefox
$ firefox
he executes the Trojan injected by Alice.
The fix is to have the Nix daemon verify that derivation outputs are
correct (in addValidPath()). This required some refactoring to move
the hash computation code to libstore.
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).
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.
a pointer to a Value, rather than the Value directly. This improves
the effectiveness of garbage collection a lot: if the Value is
stored inside the set directly, then any live pointer to the Value
causes all other attributes in the set to be live as well.
because it defines _FILE_OFFSET_BITS. Without this, on
OpenSolaris the system headers define it to be 32, and then
the 32-bit stat() ends up being called with a 64-bit "struct
stat", or vice versa.
This also ensures that we get 64-bit file sizes everywhere.
* Remove the redundant call to stat() in parseExprFromFile().
The file cannot be a symlink because that's the exit condition
of the loop before.
values. This improves sharing and gives another speed up.
Evaluation of the NixOS system attribute is now almost 7 times
faster than the old evaluator.
use site, allowing environments to be stores as vectors of values
rather than maps. This should speed up evaluation and reduce the
number of allocations.
precedence, i.e. `with {x=1;}; with {x=2;}; x' evaluates to 2'.
This has a simpler implementation and seems more natural. There
doesn't seem to be any code in Nixpkgs or NixOS that relies on the
old behaviour.
efficiently. The symbol table ensures that there is only one copy
of each symbol, thus allowing symbols to be compared efficiently
using a pointer equality test.
that there are some places in Nixpkgs (php_configurable /
composableDerivation, it seems) that call `derivation' with
incorrect arguments (namely, the `name' attribute missing) but get
away with it because of laziness.
concatenation and string coercion. This was a big mess (see
e.g. NIX-67). Contexts are now folded into strings, so that they
don't cause evaluation errors when they're not expected. The
semantics of paths has been clarified (see nixexpr-ast.def).
toString() and coerceToString() have been merged.
Semantic change: paths are now copied to the store when they're in a
concatenation (and in most other situations - that's the
formalisation of the meaning of a path). So
"foo " + ./bla
evaluates to "foo /nix/store/hash...-bla", not "foo
/path/to/current-dir/bla". This prevents accidental impurities, and
is more consistent with the treatment of derivation outputs, e.g.,
`"foo " + bla' where `bla' is a derivation. (Here `bla' would be
replaced by the output path of `bla'.)
packages (provided that they have a `meta.description' attribute).
E.g.,
$ ./src/nix-env/nix-env -qa --description gcc
gcc-4.0.2 GNU Compiler Collection, 4.0.x (cross-compiler for sparc-linux)
gcc-4.0.2 GNU Compiler Collection, 4.0.x (cross-compiler for mips-linux)
gcc-4.0.2 GNU Compiler Collection, 4.0.x (cross-compiler for arm-linux)
gcc-4.0.2 GNU Compiler Collection, 4.0.x
instantiation, e.g. "nix-env -i" and "nix-env -qas" (but not
"nix-env -qa"). It turns out that many redundant calls to
addToStore(path) were made, which reads and hashes the entire path.
For instance, the bash bootstrap binary in Nixpkgs would be read and
hashed many times. As a result nix-env would spend around 92% of
its time in the function sha256_block (according to callgrind).
Some simple memoization fixes this.
permission to the Nix store or database. E.g., `nix-env -qa' will
work, but `nix-env -qas' won't (the latter needs DB access). The
option `--readonly-mode' forces this mode; otherwise, it's only
activated when the database cannot be opened.
