Avoid quadratic complexity in log parser (#26568)
TL;DR: there are matching groups trying to match 1 or more occurrences of
something. We don't use the matching group. Therefore it's sufficient to test
for 1 occurrence. This reduce quadratic complexity to linear time.
---
When parsing logs of an mpich build, I'm getting a 4 minute (!!) wait
with 16 threads for regexes to run:
```
In [1]: %time p.parse("mpich.log")
Wall time: 4min 14s
```
That's really unacceptably slow...
After some digging, it seems a few regexes tend to have `O(n^2)` scaling
where `n` is the string / log line length. I don't think they *necessarily*
should scale like that, but it seems that way. The common pattern is this
```
([^:]+): error
```
which matches `: error` literally, and then one or more non-colons before that. So
for a log line like this:
```
abcdefghijklmnopqrstuvwxyz: error etc etc
```
Any of these are potential group matches when using `search` in Python:
```
abcdefghijklmnopqrstuvwxyz
bcdefghijklmnopqrstuvwxyz
cdefghijklmnopqrstuvwxyz
⋮
yz
z
```
but clearly the capture group should return the longest match.
My hypothesis is that Python has a very bad implementation of `search`
that somehow considers all of these, even though it can be implemented
in linear time by scanning for `: error` first, and then greedily expanding
the longest possible `[^:]+` match to the left. If Python indeed considers
all possible matches, then with `n` matches of length `1 .. n` you
see the `O(n^2)` slowness (i verified this by replacing + with {1,k}
and doubling `k`, it doubles the execution time indeed).
This PR fixes this by removing the `+`, so effectively changing the
O(n^2) into a O(n) worst case.
The reason we are fine with dropping `+` is that we don't use the
capture group anywhere, so, we just ensure `:: error` is not a match
but `x: error` is.
After going from O(n^2) to O(n), the 15MB mpich build log is parsed
in `1.288s`, so about 200x faster.
Just to be sure I've also updated `^CMake Error.*:` to `^CMake Error`,
so that it does not match with all the possible `:`'s in the line.
Another option is to use `.*?` there to make it quit scanning as soon as
possible, but what line that starts with `CMake Error` that does not have
a colon is really a false positive...
This commit is contained in:
Harmen Stoppels
83
lib/spack/external/ctest_log_parser.py
vendored
83
lib/spack/external/ctest_log_parser.py
vendored
@@ -77,47 +77,7 @@
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from six import StringIO
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from six import string_types
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class prefilter(object):
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"""Make regular expressions faster with a simple prefiltering predicate.
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Some regular expressions seem to be much more costly than others. In
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most cases, we can evaluate a simple precondition, e.g.::
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lambda x: "error" in x
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to avoid evaluating expensive regexes on all lines in a file. This
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can reduce parse time for large files by orders of magnitude when
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evaluating lots of expressions.
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A ``prefilter`` object is designed to act like a regex,, but
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``search`` and ``match`` check the precondition before bothering to
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evaluate the regular expression.
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Note that ``match`` and ``search`` just return ``True`` and ``False``
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at the moment. Make them return a ``MatchObject`` or ``None`` if it
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becomes necessary.
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"""
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def __init__(self, precondition, *patterns):
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self.patterns = [re.compile(p) for p in patterns]
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self.pre = precondition
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self.pattern = "\n ".join(
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('MERGED:',) + patterns)
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def search(self, text):
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return self.pre(text) and any(p.search(text) for p in self.patterns)
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def match(self, text):
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return self.pre(text) and any(p.match(text) for p in self.patterns)
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_error_matches = [
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prefilter(
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lambda x: any(s in x for s in (
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'Error:', 'error', 'undefined reference', 'multiply defined')),
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"([^:]+): error[ \\t]*[0-9]+[ \\t]*:",
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"([^:]+): (Error:|error|undefined reference|multiply defined)",
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"([^ :]+) ?: (error|fatal error|catastrophic error)",
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"([^:]+)\\(([^\\)]+)\\) ?: (error|fatal error|catastrophic error)"),
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"^FAIL: ",
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"^FATAL: ",
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"^failed ",
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@@ -127,6 +87,8 @@ def match(self, text):
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"^[Ss]egmentation [Vv]iolation",
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"^[Ss]egmentation [Ff]ault",
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":.*[Pp]ermission [Dd]enied",
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"[^ :]:[0-9]+: [^ \\t]",
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"[^:]: error[ \\t]*[0-9]+[ \\t]*:",
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"^Error ([0-9]+):",
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"^Fatal",
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"^[Ee]rror: ",
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@@ -136,6 +98,9 @@ def match(self, text):
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"^cc[^C]*CC: ERROR File = ([^,]+), Line = ([0-9]+)",
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"^ld([^:])*:([ \\t])*ERROR([^:])*:",
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"^ild:([ \\t])*\\(undefined symbol\\)",
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"[^ :] : (error|fatal error|catastrophic error)",
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"[^:]: (Error:|error|undefined reference|multiply defined)",
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"[^:]\\([^\\)]+\\) ?: (error|fatal error|catastrophic error)",
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"^fatal error C[0-9]+:",
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": syntax error ",
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"^collect2: ld returned 1 exit status",
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@@ -144,7 +109,7 @@ def match(self, text):
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"^Unresolved:",
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"Undefined symbol",
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"^Undefined[ \\t]+first referenced",
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"^CMake Error.*:",
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"^CMake Error",
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":[ \\t]cannot find",
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":[ \\t]can't find",
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": \\*\\*\\* No rule to make target [`'].*\\'. Stop",
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@@ -180,6 +145,7 @@ def match(self, text):
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"instantiated from ",
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"candidates are:",
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": warning",
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": WARNING",
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": \\(Warning\\)",
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": note",
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" ok",
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@@ -187,38 +153,32 @@ def match(self, text):
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"makefile:",
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"Makefile:",
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":[ \\t]+Where:",
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"([^ :]+):([0-9]+): Warning",
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"[^ :]:[0-9]+: Warning",
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"------ Build started: .* ------",
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]
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#: Regexes to match file/line numbers in error/warning messages
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_warning_matches = [
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prefilter(
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lambda x: 'warning' in x,
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"([^ :]+):([0-9]+): warning:",
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"([^:]+): warning ([0-9]+):",
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"([^:]+): warning[ \\t]*[0-9]+[ \\t]*:",
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"([^ :]+) : warning",
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"([^:]+): warning"),
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prefilter(
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lambda x: 'note:' in x,
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"^([^ :]+):([0-9]+): note:"),
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prefilter(
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lambda x: any(s in x for s in ('Warning', 'Warnung')),
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"^(Warning|Warnung) ([0-9]+):",
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"^(Warning|Warnung)[ :]",
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"^cxx: Warning:",
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"([^ :]+):([0-9]+): (Warning|Warnung)",
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"^CMake Warning.*:"),
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"file: .* has no symbols",
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"[^ :]:[0-9]+: warning:",
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"[^ :]:[0-9]+: note:",
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"^cc[^C]*CC: WARNING File = ([^,]+), Line = ([0-9]+)",
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"^ld([^:])*:([ \\t])*WARNING([^:])*:",
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"[^:]: warning [0-9]+:",
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"^\"[^\"]+\", line [0-9]+: [Ww](arning|arnung)",
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"[^:]: warning[ \\t]*[0-9]+[ \\t]*:",
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"^(Warning|Warnung) ([0-9]+):",
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"^(Warning|Warnung)[ :]",
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"WARNING: ",
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"[^ :] : warning",
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"[^:]: warning",
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"\", line [0-9]+\\.[0-9]+: [0-9]+-[0-9]+ \\([WI]\\)",
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"^cxx: Warning:",
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"file: .* has no symbols",
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"[^ :]:[0-9]+: (Warning|Warnung)",
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"\\([0-9]*\\): remark #[0-9]*",
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"\".*\", line [0-9]+: remark\\([0-9]*\\):",
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"cc-[0-9]* CC: REMARK File = .*, Line = [0-9]*",
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"^CMake Warning",
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"^\\[WARNING\\]",
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]
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@@ -349,8 +309,7 @@ def _profile_match(matches, exceptions, line, match_times, exc_times):
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def _parse(lines, offset, profile):
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def compile(regex_array):
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return [regex if isinstance(regex, prefilter) else re.compile(regex)
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for regex in regex_array]
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return [re.compile(regex) for regex in regex_array]
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error_matches = compile(_error_matches)
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error_exceptions = compile(_error_exceptions)
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