Make hackageIndexLayout faster#343
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`hackageIndexLayout` runs `fromPath` in a hot loop, for every file in the Hackage 01-index.tar. However, `fromPath` is rather wasteful when parsing `PackageIdentifier`: it concatenates together package name and version and asks `Cabal-syntax` to parse. This is very backwards, because `PackageIdentifier` is exactly a record with two fields: package name and its version. We already have the name at hand, there is no need to parse it again, and the version can be parsed more efficiently without resorting to `parsec`. This performance optimization was earlier attested in the `hackage-revdeps` package.
`hackageIndexLayout` runs `fromPath` in a hot loop, for every file in the Hackage 01-index.tar. However, `fromPath` is rather wasteful: it uses `splitFragments` (and underlying `System.FilePath.splitDirectories`) to explode `base/4.22.0.0/base.cabal` into `["base", "4.22.0.0", "base.cabal"]`. One could have expected that `splitDirectories` simply splits filepath by '/', but this is not the case. It's a general function, which does a lot of transformations such as checking for absolute paths, splitting drive letter, first retaining '/', then stripping them, etc. The patch replaces `splitDirectories` with a rather specialized operations, aiming to avoid traversing the same characters twice. It was previously attested in the `hackage-revdeps` package that this microptimization brings measurable performance benefits.
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Mikolaj
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Jul 10, 2026
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LGTM
If the speedup is confirmed in practice, this is a great improvement!
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Do we want to a) release this asap and b) make sure cabal 3.18.1 0 will allow the latest release? |
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This is not a breaking change, so presumably it can be released as |
fgaz
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Jul 11, 2026
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| case reverse basename of | ||
| -- ".cabal" reversed | ||
| 'l' : 'a' : 'b' : 'a' : 'c' : '.' : _ -> | ||
| return $ Some $ IndexPkgCabal pkgId | ||
| -- ".json" reversed | ||
| 'n' : 'o' : 's' : 'j' : '.' : _ -> | ||
| return $ Some $ IndexPkgMetadata pkgId | ||
| _ -> Nothing |
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Before these patches
hackageIndexLayouttakes up to 50% ofcabal updatetime. With the patches applied this reduces to ~10% ofcabal update, making it almost 2x faster.I originally developed this code for
hackage-revdeps, so the approach has been exercised for some time. Besides,hackageIndexLayoutis covered by tests forhackage-security.