5 ms·

2000x performance win

110 points by tilt 15y ago

lordlarm 15y ago
It actually was 4000x times faster by using C's inbuilt counter ("-c") instead of "wc -l". Any risks with just updating to latest version of grep instead of using the LANG=C hack?
luriel 15y ago
This is not really a 2000x performance "win", this is GNU grep(1) being so bad at dealing with UTF-8 to the point of being basically unusable. The PoSix locale system is a nightmare, but the implementation in GNU coreutils is even worse, when it is not insanely slow it is plain broken. I wasted days tracking down a bug when we moved a script from FreeBSD to a linux sytem that had an UTF-8 locale, even for plain ASCII input GNU awk could match lines erratically if the locale was not set back to C! I'm sure this bug has been fixed by now, but is by far not the only time I have found serious flaws with UTF-8 support in GNU tools, it seems this days the only way to be safe is to set the locale to C at the top of all your scripts, which is quite sad. This is also required because due to POSIX locale insanity the actual behavior of tools as basic as ls(1), and more fundamentally regexps matching changes based on the locale, making scripts that don't set their locale at the top unreliable and nonportable. Or using a toolkit with decent UTF-8 support, like the from Plan 9 that one can use on *nix systems via Plan 9 from User Space ( http://plan9.us http://plan9.us ) or 9base ( http://tools.suckless.org/9base http://tools.suckless.org/9base ), if you use those also your scripts will be much more portable, as there are all kinds of incompatibilities among for example awk implementations.
- 4ad 15y ago
  Adding to what you said, I also find Plan9 regular expressions to be significantly easier to grok in your head compared to POSIX or PCRE regular expressions that push a lot of complexity on the user's side (PCREs are not even true regexps as PCREs are not regular). Plan9 regexps are very easy to reason about, more explicit and don't look as line noise as much.
  - omaranto 15y ago
    I thought POSIX regexes allowed back references (as in "(*.)\1"), so they aren't regular either.
  - adobriyan 15y ago
    RE with backreferences is strictly more powerful then RE without them. Implying that PCRE is somehow inferior due to backreferences support is quite fuddy.
- dododo 15y ago
  here's another "fun" grep locale oddity: $ echo HI | LANG=en_US.utf8 grep '^[a-z]' HI $ echo HI | LANG=C grep '^[a-z]' $ apparently en_{GB,US}.utf8 orders a-z like aAbBcC..zZ. $ echo ZI | LANG=en_US.utf8 grep '^[a-z]' $
  - iso8859-1 15y ago
    This is what I get: $ echo HI | LANG=C grep '^[a-z]' $ echo HI | LANG=en_US.utf8 grep '^[a-z]' $ How come?
    - p9idf 15y ago
      I was able to reproduce the bug. It could be a version thing. ; grep --version GNU grep 2.6.3 ; echo A | LANG=en_US.utf8 grep '[a-z]' A
      - Ives 15y ago
        No, I have the same version but not a similar result. I also have the en_US.utf8 locale installed.
  - ot 15y ago
    I had the same problem with sort: $ sort <<EOF > Aa > aa > Ab > ab > EOF aa Aa ab Ab I was going crazy because I was getting different results in OSX and Ubuntu. Setting the LANG to POSIX fixed it.
    - Macha 15y ago
      $ sort --version sort (GNU coreutils) 8.14 For what it's worth, this gets me the same results under LANG=C and LANG=en_US.utf8
- EdiX 15y ago
  > This is not really a 2000x performance "win", this is GNU grep(1) being so bad at dealing with UTF-8 to the point of being basically unusable. > > The PoSix locale system is a nightmare, but the implementation in GNU coreutils is even worse, when it is not insanely slow it is plain broken. Actually, GNU grep as of last year was much faster than BSD grep, as long as the C locale is used: http://lists.freebsd.org/pipermail/freebsd-current/2010-August/019310.html http://lists.freebsd.org/pipermail/freebsd-current/2010-Augu... The reason why using UTF8 makes it much slower is that dealing with Unicode correctly is a very expensive business. But pretending that languages other than english do not exist is not an answer.
  - 4ad 15y ago
    The Plan9 grep deals with Unicode correctly and is very, very fast. Claiming that > dealing with Unicode correctly is a very expensive business is plain FUD. UTF-8 has zero overhead in this case (not near zero, but actual zero overhead). Grep doesn't even need to be aware of UTF-8. It doesn't need to be aware where a rune starts and ends in the byte stream. UTF-8 is designed in such a way that matching a regexp only requires byte comparisons, exactly as for ASCII.
    - EdiX 15y ago
      > The Plan9 grep deals with Unicode correctly and is very, very fast. Claiming that >> dealing with Unicode correctly is a very expensive business > is plain FUD. If you think this is true you don't know unicode enough. I don't know how plan9 grep is implemented but if it does it with near zero overhead then it's not actually supporting Unicode, but only with a small subset of Unicode that they like. I also probably agree that that's the best part of Unicode and the rest of Unicode could burn in a fire but it's still not Unicode.
      - morsch 15y ago
        > If you think this is true you don't know unicode enough. Yes, well, that is sort of a given for any statement about unicode (including this one).
    - 15y ago
- pdw 15y ago
  FWIW, more recent GNU grep releases (2.6 and later) run fine in UTF-8 mode. Actually, why is SmartOS, "the Completely Modern Operating System", shipping with a five year old grep?
  - wollw 15y ago
    When the author shows the output of "which grep" it shows the path "/opt/local/bin/grep"; "/opt" is typically for additional installations of software. I haven't used SmartOS myself but my best guess is someone installed GNU grep manually and the PATH just included "/opt/local/bin" before "/usr/bin".
    - pdw 15y ago
      I guess SmartOS reserves /usr for the Solaris base install and puts additions in /opt/local. If you look at http://wiki.joyent.com/display/jpc2/SmartOS+Smartmachine+Datasets http://wiki.joyent.com/display/jpc2/SmartOS+Smartmachine+Dat... and click "to see list of installed software", it includes grep 2.5.3.
  - rg3 15y ago
    Minor correction: the "bug" in grep was fixed in GNU grep 2.7, not 2.6. I recall that because I wrote an article about it on my blog. See the release notes for GNU grep 2.7 here: http://savannah.gnu.org/forum/forum.php?forum_id=6521 http://savannah.gnu.org/forum/forum.php?forum_id=6521
- iradik 15y ago
  Yup, the author an older version of grep 2.5.3. I have grep 2.6.3 on RHEL6 and only got a 2x boost doing a similar test.
- iradik 15y ago
  OP needs to upgrade grep to the latest version. He is using 2.5.3, and latest version is 2.10. Release notes for 2.6: "This release fixes an unexpectedly large number of flaws, from outright bugs (surprisingly many, considering this is "grep") to some occasionally debilitating performance problems." http://savannah.gnu.org/forum/forum.php?forum_id=6249 http://savannah.gnu.org/forum/forum.php?forum_id=6249
Nitramp 15y ago
Does anyone understand what the bug in grep was? UTF-8 regular expression matching shouldn't be different from ASCII at all, as far as I can tell. In UTF-8, every byte by itself can be identified as a start byte or trail byte, so if you want to match a regular expression, you don't even have to care about UTF-8 in any way. Any legal character in the regexp you want to look for can only match at a legal character start in the haystack.
- 4ad 15y ago
  GNU grep uselessly converts UTF-8 to some multibyte char internal representation. You are completely right, matching regular expressions with UTF-8 text is the same as with ASCII text, one of the reasons why UTF-8 is so good.
  - omgtehlion 15y ago
    You both are wrong. If you take arbitrary byte in ascii stream you can be sure that this is a whole char, but if you take arbitrary byte in utf-8 stream you can get a start of a char / end of char / or whole char.
    - 4ad 15y ago
      Obviously choosing an arbitrary offset in the byte stream might not align on rune boundary, but UTF-8 is great because you can determine that. You can determine the start/end boundaries in any subset of the byte stream, as opposed to most other encodings. The issue is completely unrelated to the fact that matching regexps in UTF-8 text is the same as matching regexp in ASCII text. The regular expression tool doesn't even need to care that the text is UTF-8. It's just byte comparisons, the tool doesn't even need to be aware of rune boundaries.
      - omgtehlion 15y ago
        just look into grep’s sources. it has loads of low-level optimizations to make string search faster. But all these optimizations work only with exact-witdth encodings, thats why it is related. P.S.: and sometimes there are a lot of problems with lower and upper-case letters http://www.gnu.org/s/grep/devel.html http://www.gnu.org/s/grep/devel.html
schiptsov 15y ago
what about grep on linux times with LANG=en_US.UTF-8 ?) Could it be that something is wrong with Solaris UTF-8 locales, and very old version of GNU grep from an outdated package?)
gravitronic 15y ago
For getting dtrace-like traces under Linux I strongly suggest getting oprofile (http://oprofile.sourceforge.net/news/ http://oprofile.sourceforge.net/news/) working on your target machine. I've used it on a PPC embedded board and it worked wonderfully. Do not optimize that which you have not measured.
kennystone 15y ago
I enjoyed seeing your debugging process and picked up a few tricks.
mgedmin 15y ago
Is this really a bug in grep, rather than a bug in Solaris's libc? I've never seen grep so slow, and I've been using UTF-8 locales for years. I'm not denying that grep was buggy (there's a link to grep's bug tracker to a bug that was closed more than a year ago), but I'm surprised at the magnitude of the slowdown.
- pdw 15y ago
  The official GNU grep used to be absurdly slow at UTF-8. Linux distributors very quickly noticed this and fixed it when they switched to UTF-8 by default. But GNU grep maintenance was essentially dormant for years and these patches were only integrated in 2010. For an old, unpatched GNU grep a 2000x slowdown is quite believable.
  - deleted 15y ago
    [deleted]
- iradik 15y ago
  The poster is using grep 2.5.3. Release notes for 2.6 talk about UTF-8 performance improvements: "This release fixes an unexpectedly large number of flaws, from outright bugs (surprisingly many, considering this is "grep") to some occasionally debilitating performance problems." Current version of grep is 2.10.
bjoernbu 15y ago
As far as I know, grep uses Boyer-Moore for string matching when possible. Giving a variable char size encoding such as UTF8, plain Boyer-Moore isn't possible, but it is the asymptotically faster algorithm known. Hence even perfect versions of grep will slower by arbitrarily large factors, depending on the input. So while there may be problems, expecting no difference or no significant difference between encodings is not correct either
- dexen 15y ago
  > Giving a variable char size encoding such as UTF8, plain Boyer-Moore isn't possible (...) That you get UTF-8 input and produce UTF-8 output doesn't imply you are better off using UTF-8 for processing. Translating UTF-8 to fixed-width UTF-32 and back is of linear complexity and takes small, fixed amount of memory. The only trade-off is when processing /very/ long lines -- up to four time more memory would be used for buffer. As mentioned in other posts, Unicode requires normalization of certain character combinations into other characters, so you'll be processing all input characters anyway. Just prefix an extra step to it, not even a separate loop. And so you can do Boyer-Moore with Unicode at very little extra cost :-) Some text-intensive programs of Plan 9, including grep, use internally fixed-widht format called `Rune' for unicode, exactly for reasons of performance. UTF-8 input is translated into strings of Runes for processing and translated back for output.
  - deleted 15y ago
    [deleted]
    - deleted 15y ago
      [deleted]
tantalor 15y ago
In case anybody else was wondering what LANG=C means, > The 'C' locale is defined as the "default" locale > for applications, meaning that their strings are > displayed as written in the initial code (without > passing through a translation lookup). > > There is nothing special about the C locale, except > that it will always exist and applies no string > replacements. -Malcolm Tredinnick http://mailman.linuxchix.org/pipermail/techtalk/2002-November/013691.html http://mailman.linuxchix.org/pipermail/techtalk/2002-Novembe...
- tantalor 15y ago
  > If the locale value is "C" or "POSIX", the POSIX locale is used - The Single UNIX ® Specification, Version 2 http://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html http://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html
iradik 15y ago
Anyone able to verify the author's claim? I only get a 2x improvement when switching LANG on Redhat linux on EC2: % export LANG=C % time grep done nohup.out | wc -l 152929 real 0m0.343s user 0m0.233s sys 0m0.112s % export LANG=en_US.UTF-8 % time grep done nohup.out | wc -l 152931 real 0m0.771s user 0m0.673s sys 0m0.100s % grep --version GNU grep 2.6.3 Author is using grep 2.5.3, I am using 2.6.3, so not testing the same thing.

110 points by tilt 15y ago

It actually was 4000x times faster by using C's inbuilt counter ("-c") instead of "wc -l". Any risks with just updating to latest version of grep instead of using the LANG=C hack?

luriel 15y ago

This is not really a 2000x performance "win", this is GNU grep(1) being so bad at dealing with UTF-8 to the point of being basically unusable. The PoSix locale system is a nightmare, but the implementation in GNU coreutils is even worse, when it is not insanely slow it is plain broken. I wasted days tracking down a bug when we moved a script from FreeBSD to a linux sytem that had an UTF-8 locale, even for plain ASCII input GNU awk could match lines erratically if the locale was not set back to C! I'm sure this bug has been fixed by now, but is by far not the only time I have found serious flaws with UTF-8 support in GNU tools, it seems this days the only way to be safe is to set the locale to C at the top of all your scripts, which is quite sad. This is also required because due to POSIX locale insanity the actual behavior of tools as basic as ls(1), and more fundamentally regexps matching changes based on the locale, making scripts that don't set their locale at the top unreliable and nonportable. Or using a toolkit with decent UTF-8 support, like the from Plan 9 that one can use on *nix systems via Plan 9 from User Space ( http://plan9.us http://plan9.us ) or 9base ( http://tools.suckless.org/9base http://tools.suckless.org/9base ), if you use those also your scripts will be much more portable, as there are all kinds of incompatibilities among for example awk implementations.

4ad 15y ago
Adding to what you said, I also find Plan9 regular expressions to be significantly easier to grok in your head compared to POSIX or PCRE regular expressions that push a lot of complexity on the user's side (PCREs are not even true regexps as PCREs are not regular). Plan9 regexps are very easy to reason about, more explicit and don't look as line noise as much.
- omaranto 15y ago
  I thought POSIX regexes allowed back references (as in "(*.)\1"), so they aren't regular either.
- adobriyan 15y ago
  RE with backreferences is strictly more powerful then RE without them. Implying that PCRE is somehow inferior due to backreferences support is quite fuddy.
dododo 15y ago
here's another "fun" grep locale oddity: $ echo HI | LANG=en_US.utf8 grep '^[a-z]' HI $ echo HI | LANG=C grep '^[a-z]' $ apparently en_{GB,US}.utf8 orders a-z like aAbBcC..zZ. $ echo ZI | LANG=en_US.utf8 grep '^[a-z]' $
- iso8859-1 15y ago
  This is what I get: $ echo HI | LANG=C grep '^[a-z]' $ echo HI | LANG=en_US.utf8 grep '^[a-z]' $ How come?
  - p9idf 15y ago
    I was able to reproduce the bug. It could be a version thing. ; grep --version GNU grep 2.6.3 ; echo A | LANG=en_US.utf8 grep '[a-z]' A
    - Ives 15y ago
      No, I have the same version but not a similar result. I also have the en_US.utf8 locale installed.
- ot 15y ago
  I had the same problem with sort: $ sort <<EOF > Aa > aa > Ab > ab > EOF aa Aa ab Ab I was going crazy because I was getting different results in OSX and Ubuntu. Setting the LANG to POSIX fixed it.
  - Macha 15y ago
    $ sort --version sort (GNU coreutils) 8.14 For what it's worth, this gets me the same results under LANG=C and LANG=en_US.utf8
EdiX 15y ago
> This is not really a 2000x performance "win", this is GNU grep(1) being so bad at dealing with UTF-8 to the point of being basically unusable. > > The PoSix locale system is a nightmare, but the implementation in GNU coreutils is even worse, when it is not insanely slow it is plain broken. Actually, GNU grep as of last year was much faster than BSD grep, as long as the C locale is used: http://lists.freebsd.org/pipermail/freebsd-current/2010-August/019310.html http://lists.freebsd.org/pipermail/freebsd-current/2010-Augu... The reason why using UTF8 makes it much slower is that dealing with Unicode correctly is a very expensive business. But pretending that languages other than english do not exist is not an answer.
- 4ad 15y ago
  The Plan9 grep deals with Unicode correctly and is very, very fast. Claiming that > dealing with Unicode correctly is a very expensive business is plain FUD. UTF-8 has zero overhead in this case (not near zero, but actual zero overhead). Grep doesn't even need to be aware of UTF-8. It doesn't need to be aware where a rune starts and ends in the byte stream. UTF-8 is designed in such a way that matching a regexp only requires byte comparisons, exactly as for ASCII.
  - EdiX 15y ago
    > The Plan9 grep deals with Unicode correctly and is very, very fast. Claiming that >> dealing with Unicode correctly is a very expensive business > is plain FUD. If you think this is true you don't know unicode enough. I don't know how plan9 grep is implemented but if it does it with near zero overhead then it's not actually supporting Unicode, but only with a small subset of Unicode that they like. I also probably agree that that's the best part of Unicode and the rest of Unicode could burn in a fire but it's still not Unicode.
    - morsch 15y ago
      > If you think this is true you don't know unicode enough. Yes, well, that is sort of a given for any statement about unicode (including this one).
  - 15y ago
pdw 15y ago
FWIW, more recent GNU grep releases (2.6 and later) run fine in UTF-8 mode. Actually, why is SmartOS, "the Completely Modern Operating System", shipping with a five year old grep?
- wollw 15y ago
  When the author shows the output of "which grep" it shows the path "/opt/local/bin/grep"; "/opt" is typically for additional installations of software. I haven't used SmartOS myself but my best guess is someone installed GNU grep manually and the PATH just included "/opt/local/bin" before "/usr/bin".
  - pdw 15y ago
    I guess SmartOS reserves /usr for the Solaris base install and puts additions in /opt/local. If you look at http://wiki.joyent.com/display/jpc2/SmartOS+Smartmachine+Datasets http://wiki.joyent.com/display/jpc2/SmartOS+Smartmachine+Dat... and click "to see list of installed software", it includes grep 2.5.3.
- rg3 15y ago
  Minor correction: the "bug" in grep was fixed in GNU grep 2.7, not 2.6. I recall that because I wrote an article about it on my blog. See the release notes for GNU grep 2.7 here: http://savannah.gnu.org/forum/forum.php?forum_id=6521 http://savannah.gnu.org/forum/forum.php?forum_id=6521
iradik 15y ago
Yup, the author an older version of grep 2.5.3. I have grep 2.6.3 on RHEL6 and only got a 2x boost doing a similar test.
iradik 15y ago
OP needs to upgrade grep to the latest version. He is using 2.5.3, and latest version is 2.10. Release notes for 2.6: "This release fixes an unexpectedly large number of flaws, from outright bugs (surprisingly many, considering this is "grep") to some occasionally debilitating performance problems." http://savannah.gnu.org/forum/forum.php?forum_id=6249 http://savannah.gnu.org/forum/forum.php?forum_id=6249

Nitramp 15y ago

Does anyone understand what the bug in grep was? UTF-8 regular expression matching shouldn't be different from ASCII at all, as far as I can tell. In UTF-8, every byte by itself can be identified as a start byte or trail byte, so if you want to match a regular expression, you don't even have to care about UTF-8 in any way. Any legal character in the regexp you want to look for can only match at a legal character start in the haystack.

4ad 15y ago
GNU grep uselessly converts UTF-8 to some multibyte char internal representation. You are completely right, matching regular expressions with UTF-8 text is the same as with ASCII text, one of the reasons why UTF-8 is so good.
- omgtehlion 15y ago
  You both are wrong. If you take arbitrary byte in ascii stream you can be sure that this is a whole char, but if you take arbitrary byte in utf-8 stream you can get a start of a char / end of char / or whole char.
  - 4ad 15y ago
    Obviously choosing an arbitrary offset in the byte stream might not align on rune boundary, but UTF-8 is great because you can determine that. You can determine the start/end boundaries in any subset of the byte stream, as opposed to most other encodings. The issue is completely unrelated to the fact that matching regexps in UTF-8 text is the same as matching regexp in ASCII text. The regular expression tool doesn't even need to care that the text is UTF-8. It's just byte comparisons, the tool doesn't even need to be aware of rune boundaries.
    - omgtehlion 15y ago
      just look into grep’s sources. it has loads of low-level optimizations to make string search faster. But all these optimizations work only with exact-witdth encodings, thats why it is related. P.S.: and sometimes there are a lot of problems with lower and upper-case letters http://www.gnu.org/s/grep/devel.html http://www.gnu.org/s/grep/devel.html

schiptsov 15y ago

what about grep on linux times with LANG=en_US.UTF-8 ?) Could it be that something is wrong with Solaris UTF-8 locales, and very old version of GNU grep from an outdated package?)

gravitronic 15y ago

For getting dtrace-like traces under Linux I strongly suggest getting oprofile (http://oprofile.sourceforge.net/news/ http://oprofile.sourceforge.net/news/) working on your target machine. I've used it on a PPC embedded board and it worked wonderfully. Do not optimize that which you have not measured.

kennystone 15y ago

I enjoyed seeing your debugging process and picked up a few tricks.

mgedmin 15y ago

Is this really a bug in grep, rather than a bug in Solaris's libc? I've never seen grep so slow, and I've been using UTF-8 locales for years. I'm not denying that grep was buggy (there's a link to grep's bug tracker to a bug that was closed more than a year ago), but I'm surprised at the magnitude of the slowdown.

pdw 15y ago
The official GNU grep used to be absurdly slow at UTF-8. Linux distributors very quickly noticed this and fixed it when they switched to UTF-8 by default. But GNU grep maintenance was essentially dormant for years and these patches were only integrated in 2010. For an old, unpatched GNU grep a 2000x slowdown is quite believable.
- deleted 15y ago
  [deleted]
iradik 15y ago
The poster is using grep 2.5.3. Release notes for 2.6 talk about UTF-8 performance improvements: "This release fixes an unexpectedly large number of flaws, from outright bugs (surprisingly many, considering this is "grep") to some occasionally debilitating performance problems." Current version of grep is 2.10.

bjoernbu 15y ago

As far as I know, grep uses Boyer-Moore for string matching when possible. Giving a variable char size encoding such as UTF8, plain Boyer-Moore isn't possible, but it is the asymptotically faster algorithm known. Hence even perfect versions of grep will slower by arbitrarily large factors, depending on the input. So while there may be problems, expecting no difference or no significant difference between encodings is not correct either

dexen 15y ago
> Giving a variable char size encoding such as UTF8, plain Boyer-Moore isn't possible (...) That you get UTF-8 input and produce UTF-8 output doesn't imply you are better off using UTF-8 for processing. Translating UTF-8 to fixed-width UTF-32 and back is of linear complexity and takes small, fixed amount of memory. The only trade-off is when processing /very/ long lines -- up to four time more memory would be used for buffer. As mentioned in other posts, Unicode requires normalization of certain character combinations into other characters, so you'll be processing all input characters anyway. Just prefix an extra step to it, not even a separate loop. And so you can do Boyer-Moore with Unicode at very little extra cost :-) Some text-intensive programs of Plan 9, including grep, use internally fixed-widht format called `Rune' for unicode, exactly for reasons of performance. UTF-8 input is translated into strings of Runes for processing and translated back for output.
- deleted 15y ago
  [deleted]
  - deleted 15y ago
    [deleted]

tantalor 15y ago

In case anybody else was wondering what LANG=C means, > The 'C' locale is defined as the "default" locale > for applications, meaning that their strings are > displayed as written in the initial code (without > passing through a translation lookup). > > There is nothing special about the C locale, except > that it will always exist and applies no string > replacements. -Malcolm Tredinnick http://mailman.linuxchix.org/pipermail/techtalk/2002-November/013691.html http://mailman.linuxchix.org/pipermail/techtalk/2002-Novembe...

tantalor 15y ago
> If the locale value is "C" or "POSIX", the POSIX locale is used - The Single UNIX ® Specification, Version 2 http://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html http://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html

iradik 15y ago

Anyone able to verify the author's claim? I only get a 2x improvement when switching LANG on Redhat linux on EC2: % export LANG=C % time grep done nohup.out | wc -l 152929 real 0m0.343s user 0m0.233s sys 0m0.112s % export LANG=en_US.UTF-8 % time grep done nohup.out | wc -l 152931 real 0m0.771s user 0m0.673s sys 0m0.100s % grep --version GNU grep 2.6.3 Author is using grep 2.5.3, I am using 2.6.3, so not testing the same thing.