Well, let's benchmark and see. I created a /etc/passwd file with over 6000 users each named x and some number. Here I search for the user x1, x50, x1000, x3000, and x6000. I test with getpwnam and a regex. Each test is tried 1000 times and the output follows(All tests run on a PII-450 with 384MB RAM running Redhat 9): 

#!/usr/bin/perl -w

use Benchmark;

for $x (1, 50, 1000, 3000, 6000){

timethese (1000,{
        "getpwnam${x}" => q {
        $there=getpwnam("x${x}")
        },
        "regex${x}" => q {open(PASSWD,"/etc/passwd");
        while(<PASSWD>)
        { if (/^x${x}:/){$alsothere=1};
          last if $alsothere == 1;
        }
        close PASSWD;},
});
}



Benchmark: timing 1000 iterations of getpwnam1, regex1...
 getpwnam1:  0 wallclock secs ( 0.13 usr +  0.05 sys =  0.18 CPU) @ 55
+55.56/s (n=1000)
            (warning: too few iterations for a reliable count)
    regex1:  0 wallclock secs ( 0.10 usr +  0.03 sys =  0.13 CPU) @ 76
+92.31/s (n=1000)
            (warning: too few iterations for a reliable count)
Benchmark: timing 1000 iterations of getpwnam50, regex50...
getpwnam50:  1 wallclock secs ( 0.29 usr +  0.02 sys =  0.31 CPU) @ 32
+25.81/s (n=1000)
            (warning: too few iterations for a reliable count)
   regex50:  0 wallclock secs ( 0.08 usr +  0.05 sys =  0.13 CPU) @ 76
+92.31/s (n=1000)
            (warning: too few iterations for a reliable count)
Benchmark: timing 1000 iterations of getpwnam1000, regex1000...
getpwnam1000:  2 wallclock secs ( 2.36 usr +  0.24 sys =  2.60 CPU) @ 
+384.62/s (n=1000)
 regex1000:  0 wallclock secs ( 0.10 usr +  0.04 sys =  0.14 CPU) @ 71
+42.86/s (n=1000)
            (warning: too few iterations for a reliable count)
Benchmark: timing 1000 iterations of getpwnam3000, regex3000...
getpwnam3000:  8 wallclock secs ( 7.26 usr +  0.43 sys =  7.69 CPU) @ 
+130.04/s (n=1000)
 regex3000:  1 wallclock secs ( 0.12 usr +  0.05 sys =  0.17 CPU) @ 58
+82.35/s (n=1000)
            (warning: too few iterations for a reliable count)
Benchmark: timing 1000 iterations of getpwnam6000, regex6000...
getpwnam6000: 16 wallclock secs (14.73 usr +  0.67 sys = 15.40 CPU) @ 
+64.94/s (n=1000)
 regex6000:  0 wallclock secs ( 0.13 usr +  0.06 sys =  0.19 CPU) @ 52
+63.16/s (n=1000)
            (warning: too few iterations for a reliable count)

[download]

Suprisingly the regex wins hands down. Not too shabby. I don't think you need to worry about your script being slow at all.

HTH

In reply to Re: User Existance? by pzbagel
in thread User Existance? by Anonymous Monk

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