Re: dash performance regression with [ in latest github code

[Jan Pechanec <Jan.Pechanec@xxxxxxxxxx>]

My apology, by "github code" I really meant the official git repo as
specified on http://gondor.apana.org.au/~herbert/dash/:

  https://git.kernel.org/pub/scm/utils/dash/dash.git

Cheers,
Jan

On Tue, Feb 25, 2025 at 04:04:47PM +0100, Jan Pechanec wrote:
> Hi,
> 
> thank you for working on dash.  I was testing it recently and it worked
> really well.
> 
> However, I noticed the dash code from github does filename pattern
> matching even for code like "[ x = x ] && echo ok".  I believe the
> unquoted space after '[' should not trigger pattern matching but rather
> only to invoke the test/[ utility, as before.  It seems it works fine
> though and only doing some extra unneeded work which may not be
> immediatelly noticeable.
> 
> dash installed on my Oracle Linux 9:
> 
> janp:len49:~/_INST/dash$ strings /usr/bin/dash | grep dash
> dash-0.5.11.5-4.el9.x86_64.debug
> janp:len49:~/_INST/dash$ time dash -c 'i=0; while :; do : $((i=i+1)); [ $i -eq 500000 ] && break; done'
> 
> real    0m0.752s
> user    0m0.748s
> sys     0m0.002s
> 
> dash from github (commit b3e38adf6718801e7f06267b438c45caec9523bb) take
> way more time to do the same thing:
> 
> janp:len49:~/_INST/dash$ time ./src/dash -c 'i=0; while :; do : $((i=i+1)); [ $i -eq 500000 ] && break; done'
> 
> real    0m4.202s
> user    0m1.361s
> sys     0m2.804s
> 
> For the latter, strace shows open, fstat, getdents*, and close system
> calls for each iteration and it depends on number of files in the
> current directory.  With more files, it takes more time:
> 
> janp:len49:/etc$ time ~/_INST/dash/src/dash -c 'i=0; while :; do : $((i=i+1)); [ $i -eq 500000 ] && break; done'
> real    0m15.591s
> user    0m5.704s
> sys     0m9.828s
> 
> If I change [ to test, the dash github version behaves as before, and
> possibly even faster:
> 
> janp:len49:~/_INST/dash$ time ~/_INST/dash/src/dash -c 'i=0; while :; do : $((i=i+1)); test $i -eq 500000 && break; done'
> 
> real    0m0.662s
> user    0m0.659s
> sys     0m0.002s
> 
> Even bash would be faster than the current github version of dash:
> 
> janp:len49:~/_INST/dash$ time bash -c 'i=0; while :; do : $((i=i+1)); [ $i -eq 500000 ] && break; done'
> real    0m1.943s
> user    0m1.939s
> sys     0m0.002s
> 
> Unfortunately, I do not have time to work on a patch.
> 
> Best regards,
> Jan
> 
> -- 
> Jan Pechanec <jan.pechanec@xxxxxxxxxx>

-- 
Jan Pechanec <jan.pechanec@xxxxxxxxxx>