I'm trying to understand why a block of if...elsif statements seem to be running slow, at least to me. So I conducted a few experiments and don't know enough about the innards of perl to explain why I'm seeing what I'm seeing or how I might be able to improve things.

First the basics - here's a very simple script that executes these statements 1M times inside a subroutine: 

#!/usr/bin/perl -w

for (my $i=0; $i<1000000; $i++)
{
  test($i)
}

sub test
{
  my $a=$_[0];

#  return;
  if ($a==1) {}
  elsif ($a==2) {}
  elsif ($a==3) {}
  elsif ($a==4) {}
  elsif ($a==5) {}
  elsif ($a==6) {}
  elsif ($a==7) {}
  elsif ($a==8) {}
  elsif ($a==9) {}
}

[download]
and the difference in the times I see to execute it with and without the return statement commented out is about 3/4 of a second, so that's my approximate baselevel for executing the block of code.

The real question is I have a script that reads /proc/pid/status for all processes on my system, about 200 of them. There are 36 entries in the file and I execute the reading in a loop 1440 times (the equivalent of doing this once a minute for a day, in case anyone cares). This comes to executing that block about 1M times which is the reason for using that number in my test script above. 

#!/usr/bin/perl -w

my $data;
for (my $i=0; $i<1440; $i++)
{
  opendir PROC, '/proc';
  while (my $pid=readdir PROC)
  {
    next  if $pid!~/^\d/;

    open STAT, "</proc/$pid/status" or next;
    while ($data=<STAT>)
    {
      test($i);
    }
    close STAT;
  }
}

sub test
{
  my $a=shift;
  return;

  if ($a==1) {}
  elsif ($a==2) {}
  elsif ($a==3) {}
  elsif ($a==4) {}
  elsif ($a==5) {}
  elsif ($a==6) {}
  elsif ($a==7) {}
  elsif ($a==8) {}
  elsif ($a==9) {}
}

[download]
I would have expected the difference between running this script with the return statement commented out and not commented out to be on the order of a second or less yet it's actually over 5 seconds. This seems like a huge difference and I'd like to know if anyone can tell me why and more importantly if there's a way to improve things.

-mark

In reply to Question about curious performance of if...elsif block by markseger

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