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Switch out the string-scanning algorithm used in the reference scanner.
The construction of aho-corasick automata made up the vast majority of
runtime when evaluating nixpkgs previously. While the actual scanning
with a constructed automaton is relatively fast, we almost never scan
for the same set of strings twice and the cost is not worth it.
An algorithm that better matches our needs is the Wu-Manber multiple
string match algorithm, which works efficiently on *long* and *random*
strings of the *same length*, which describes store paths (up to their
hash component).
This switches the refscanner crate to a Rust implementation[0][1] of
this algorithm.
This has several implications:
1. This crate does not provide a way to scan streams. I'm not sure if
this is an inherent problem with the algorithm (probably not, but
it would need buffering). Either way, related functions and
tests (which were actually unused) have been removed.
2. All strings need to be of the same length. For this reason, we
truncate the known paths after their hash part (they are still
unique, of course).
3. Passing an empty set of matches, or a match that is shorter than
the length of a store path, causes the crate to panic. We safeguard
against this by completely skipping the refscanning if there are no
known paths (i.e. when evaluating the first derivation of an eval),
and by bailing out of scanning a string that is shorter than a
store path.
On the upside, this reduces overall runtime to less 1/5 of what it was
before when evaluating `pkgs.stdenv.drvPath`.
[0]: Frankly, it's a random, research-grade MIT-licensed
crate that I found on Github:
https://github.com/jneem/wu-manber
[1]: We probably want to rewrite or at least fork the above crate, and
add things like a three-byte wide scanner. Evaluating large
portions of nixpkgs can easily lead to more than 65k derivations
being scanned for.
Change-Id: I08926778e1e5d5a87fc9ac26e0437aed8bbd9eb0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8017
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
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The README was very sparse before and we've actually had people email
us (as it says to contact us) just to ask what Tvix *is*. This should
answer some questions!
Change-Id: I0f248cb060eccfe086468afed1d648652b35dfd1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8018
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
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... not just a TODO.
Most use-cases of unsafeDiscardStringContext are for cases where a
string is processed in some ways and no longer contains a "physical"
reference, but still has its context attached in C++ Nix.
We don't need to do this. This does diverge in behaviour in use-cases
related to build scheduling, but that whole behaviour will be
different in Tvix.
Change-Id: I4056d4c09f62d44d6bd52b791db03fe5556672b5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8016
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
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... instead of a BTreeMap, as we do not need ordering guarantees here.
HashMaps are noticeably faster here (especially as we've been sorting
essentially random data!).
Change-Id: Ie92d74286df9f763c04c9b226ef1066ee8484c13
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8014
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
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This aids in debugging quite substantially.
Change-Id: Ic43232aa6165ae1c3db7ac2701938e1dfeeb418c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8013
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: tazjin <tazjin@tvl.su>
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Change-Id: Ie6bb2df16f79f7d977a7e95187a6b81e3ac0108f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8011
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
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Change-Id: I08b2dc2393819c4f0d3871fe05bdbd29e89a6d75
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8010
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
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Change-Id: I594a6652e2efe7aa6e35c7cdd84fc3097660614f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8009
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: B |
