As discovered by Todd Veldhuizen, the shell started by nix-shell has
its affinity set to a single CPU. This is because nix-shell connects
to the Nix daemon, which causes the affinity hack to be applied. So
we turn this off for Perl programs.
For instance, it's pointless to keep copy-from-other-stores running if
there are no other stores, or download-using-manifests if there are no
manifests. This also speeds things up because we don't send queries
to those substituters.
I.e.
Subroutine Nix::Store::isValidPath redefined at /nix/store/clfzsf6gi7qh5i9c0vks1ifjam47rijn-perl-5.16.2/lib/perl5/5.16.2/XSLoader.pm line 92.
and so on.
To implement binary caches efficiently, Hydra needs to be able to map
the hash part of a store path (e.g. "gbg...zr7") to the full store
path (e.g. "/nix/store/gbg...kzr7-subversion-1.7.5"). (The binary
cache mechanism uses hash parts as a key for looking up store paths to
ensure privacy.) However, doing a search in the Nix store for
/nix/store/<hash>* is expensive since it requires reading the entire
directory. queryPathFromHashPart() prevents this by doing a cheap
database lookup.
XZ compresses significantly better than bzip2. Here are the
compression ratios and execution times (using 4 cores in parallel) on
my /var/run/current-system (3.1 GiB):
bzip2: total compressed size 849.56 MiB, 30.8% [2m08]
xz -6: total compressed size 641.84 MiB, 23.4% [6m53]
xz -7: total compressed size 621.82 MiB, 22.6% [7m19]
xz -8: total compressed size 599.33 MiB, 21.8% [7m18]
xz -9: total compressed size 588.18 MiB, 21.4% [7m40]
Note that compression takes much longer. More importantly, however,
decompression is much faster:
bzip2: 1m47.274s
xz -6: 0m55.446s
xz -7: 0m54.119s
xz -8: 0m52.388s
xz -9: 0m51.842s
The only downside to using -9 is that decompression takes a fair
amount (~65 MB) of memory.
This command builds or fetches all dependencies of the given
derivation, then starts a shell with the environment variables from
the derivation. This shell also sources $stdenv/setup to initialise
the environment further.
The current directory is not changed. Thus this is a convenient way
to reproduce a build environment in an existing working tree.
Existing environment variables are left untouched (unless the
derivation overrides them). As a special hack, the original value of
$PATH is appended to the $PATH produced by $stdenv/setup.
Example session:
$ nix-build --run-env '<nixpkgs>' -A xterm
(the dependencies of xterm are built/fetched...)
$ tar xf $src
$ ./configure
$ make
$ emacs
(... hack source ...)
$ make
$ ./xterm
Without these, Nix fails on 32-bit Linux with Perl 5.14, with a
rather unhelpful error message:
Not a CODE reference at /nix/store/n6kpbacn6nn7i3i735v8j3di8aqyl07v-perl-5.14.2/lib/perl5/5.14.2/i686-linux-thread-multi/DynaLoader.pm
This is likely because the lack of -D_FILE_OFFSET_BITS=64 causes
various Perl structures to not match what the Perl interpreter
expects.
scripts.
* Include the version and architecture in the -I flag so that there is
at least a chance that a Nix binary built for one Perl version will
run on another version.
