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<p style="margin: 0px 0px 1.2em ! important;">I was writing a
simple utility and I decided to use regexps to parse filenames.
(I know, now I have two problems :-) )</p>
<p style="margin: 0px 0px 1.2em ! important;">I was surprised at
how slow it ran, so I did a profiling build. The profiled code
runs reasonably quickly, and is 7x faster, which makes it a bit
hard to figure out where the slowdown is happening in the
non-profiled code. I’m wondering if I’m doing something wrong,
or if there’s a bug in <code style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">regex-tdfa</code>
or in ghc.</p>
<p style="margin: 0px 0px 1.2em ! important;">I’ve pared my code
down to just the following:</p>
<pre style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code class="hljs language-Haskell" style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block ! important;display: block; overflow-x: auto; padding: 0.5em; color: rgb(51, 51, 51); background: rgb(248, 248, 248) none repeat scroll 0% 0%; -moz-text-size-adjust: none;"><span class="hljs-import"><span class="hljs-keyword" style="color: rgb(51, 51, 51); font-weight: bold;">import</span> Text.Regex.TDFA <span class="hljs-container">((=~)</span>)</span>
<span class="hljs-title" style="color: rgb(153, 0, 0); font-weight: bold;">main</span> :: <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">IO</span> ()
<span class="hljs-title" style="color: rgb(153, 0, 0); font-weight: bold;">main</span> = <span class="hljs-keyword" style="color: rgb(51, 51, 51); font-weight: bold;">do</span>
entries <- map parseFilename . lines <$> getContents
<span class="hljs-keyword" style="color: rgb(51, 51, 51); font-weight: bold;">let</span> check (<span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">Right</span> (_, t)) = last t == '<span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">Z'</span>
check _ = <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">False</span>
print $ all check entries
<span class="hljs-title" style="color: rgb(153, 0, 0); font-weight: bold;">parseFilename</span> :: <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">String</span> -> <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">Either</span> <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">String</span> (<span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">String</span>, <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">String</span>)
<span class="hljs-title" style="color: rgb(153, 0, 0); font-weight: bold;">parseFilename</span> fn = <span class="hljs-keyword" style="color: rgb(51, 51, 51); font-weight: bold;">case</span> (fn =~ pattern :: [[<span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">String</span>]]) <span class="hljs-keyword" style="color: rgb(51, 51, 51); font-weight: bold;">of</span>
[[_, full, _, time]] -> <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">Right</span> $ (full, time)
_ -> <span class="hljs-type" style="color: rgb(68, 85, 136); font-weight: bold;">Left</span> fn
<span class="hljs-keyword" style="color: rgb(51, 51, 51); font-weight: bold;">where</span>
pattern =
<span class="hljs-string" style="color: rgb(221, 17, 68);">"^\\./duplicity-(full|inc|new)(-signatures)?\\.\
\([0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]T[0-9][0-9][0-9][0-9][0-9][0-9]Z)\\."</span>
</code></pre>
<p style="margin: 0px 0px 1.2em ! important;">The relevant part of
my <code style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">.cabal</code>
file looks like this:</p>
<pre style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code class="hljs language-Cabal" style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block ! important;display: block; overflow-x: auto; padding: 0.5em; color: rgb(51, 51, 51); background: rgb(248, 248, 248) none repeat scroll 0% 0%; -moz-text-size-adjust: none;">executable DuplicityAnalyzer
main-is: DuplicityAnalyzer.hs
build-depends:
base >=4.6 && <4.11,
regex-tdfa >= 1.0 && <1.3
default-language: Haskell2010
ghc-options: -Wall -rtsopts
</code></pre>
<p style="margin: 0px 0px 1.2em ! important;">To run the
profiling, I do:</p>
<pre style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;font-size: 1em; line-height: 1.2em;margin: 1.2em 0px;"><code class="hljs language-Bash" style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;white-space: pre; overflow: auto; border-radius: 3px; border: 1px solid rgb(204, 204, 204); padding: 0.5em 0.7em; display: block ! important;display: block; overflow-x: auto; padding: 0.5em; color: rgb(51, 51, 51); background: rgb(248, 248, 248) none repeat scroll 0% 0%; -moz-text-size-adjust: none;">cabal clean
cabal configure --enable-profiling
cabal build
dist/build/DuplicityAnalyzer/DuplicityAnalyzer <names.in +RTS -sprofiling-summary.out -p
</code></pre>
<p style="margin: 0px 0px 1.2em ! important;">The <code style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">MUT</code>
time in the non-profiling build is 7x bigger, and the <code style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">%GC</code>
time goes from 8% to 21%. I’ve put the actual output in a <a
href="https://gist.github.com/neilmayhew/247a30738c0e294902e7c2830ca2c6f5">gist</a>.
I’ve also put my test input file there, in case anyone wants to
try this themselves.</p>
<p style="margin: 0px 0px 1.2em ! important;">I’ve done my testing
with NixOS (ghc 8.0.2) and Debian with the Haskell Platform (ghc
8.2.1) and the results are basically the same. I even tried
using Docker containers with Debian Jessie and Debian Stretch,
just to eliminate any OS influence, and the results are still
the same. I’ve tried it on an i5-2500K, i5-3317U and Xeon
E5-1620.</p>
<p style="margin: 0px 0px 1.2em ! important;">I also wrote a dummy
implementation of <code style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">=~</code>
that ignores the regex pattern and does a hard-coded manual
parse, and that produces times just slightly better than the
profiled ones. So I don’t think there’s a problem in my outer
code that uses <code style="font-size: 0.95em; font-family: Consolas,Inconsolata,Deja Vu Sans Mono,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">=~</code>.</p>
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