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### Re: Question about curious performance of if...elsif block

by BrowserUk (Pope)
 on Jan 16, 2012 at 18:59 UTC ( #948193=note: print w/replies, xml ) Need Help??

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.

The reason it takes so long -- although in my tests of your sample code show the difference to be far less than 5 seconds -- is because for the vast majority of the cases, \$i = 10 through 1,440 (or 1,000,000), you are performing all nine comparisons before returning.

The simple solution is to avoid doing those comparisons 99% (or 99.99999^) of the time:

```sub test3 {
return if \$_[0] > 9;
my \$a = \$_[0];

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) {}
}

On my system this consistently runs even faster than your test() sub with the unconditional return (test2 below):

```C:\test>junk37
test1 took: 0.67200 seconds
test2 took: 0.30307 seconds
test3 took: 0.36421 seconds

C:\test>junk37
test1 took: 0.66500 seconds
test2 took: 0.30315 seconds
test3 took: 0.29603 seconds

C:\test>junk37
test1 took: 0.67800 seconds
test2 took: 0.30240 seconds
test3 took: 0.29488 seconds

C:\test>junk37
test1 took: 0.66600 seconds
test2 took: 0.30331 seconds
test3 took: 0.29491 seconds

C:\test>junk37
test1 took: 0.66900 seconds
test2 took: 0.30349 seconds
test3 took: 0.29475 seconds

Update: Also, if your if/else cascade is bigger than 9, then you might consider using a dispatch table:

```my @dispatch = (
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
);

sub test4 {
return if \$_[0] > 9;
\$dispatch[ \$_[0] ]->( \$_[0] );
}

This runs pretty nearly exactly the same speed as test3 above when the number of blocks is low (9 in this case), but once you get to around 20 or so, it starts to win more substantially.

And if your cases are less amenable to using an array, then you can use a hash instead.

But do construct the dispatch table outside the subroutine. Do it (every time) inside teh sub and it will slow to a crawl.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?

Replies are listed 'Best First'.
Re^2: Question about curious performance of if...elsif block
by markseger (Beadle) on Jan 16, 2012 at 19:07 UTC
If you read the base note more carefully you're see that /proc/pid/status contains 36 entries on my system.

yes I am doing the comparisons before returning and I want to so I can measure the difference between executing them and returning immediately and the difference is 5 seconds, not the 3/4 second I would have expected.

understand now?

-mark

Even with 36 cases, I still see nothing like the 5 second difference you apparently are:

```#! perl -slw
use strict;
use Time::HiRes qw[ time ];

sub test1 {
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) {}
elsif (\$a==10) {}
elsif (\$a==11) {}
elsif (\$a==12) {}
elsif (\$a==13) {}
elsif (\$a==14) {}
elsif (\$a==15) {}
elsif (\$a==16) {}
elsif (\$a==17) {}
elsif (\$a==18) {}
elsif (\$a==19) {}
elsif (\$a==20) {}
elsif (\$a==21) {}
elsif (\$a==22) {}
elsif (\$a==23) {}
elsif (\$a==24) {}
elsif (\$a==25) {}
elsif (\$a==26) {}
elsif (\$a==27) {}
elsif (\$a==28) {}
elsif (\$a==29) {}
elsif (\$a==30) {}
elsif (\$a==31) {}
elsif (\$a==32) {}
elsif (\$a==33) {}
elsif (\$a==34) {}
elsif (\$a==35) {}
elsif (\$a==36) {}
}

sub test2 {
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) {}
elsif (\$a==10) {}
elsif (\$a==11) {}
elsif (\$a==12) {}
elsif (\$a==13) {}
elsif (\$a==14) {}
elsif (\$a==15) {}
elsif (\$a==16) {}
elsif (\$a==17) {}
elsif (\$a==18) {}
elsif (\$a==19) {}
elsif (\$a==20) {}
elsif (\$a==21) {}
elsif (\$a==22) {}
elsif (\$a==23) {}
elsif (\$a==24) {}
elsif (\$a==25) {}
elsif (\$a==26) {}
elsif (\$a==27) {}
elsif (\$a==28) {}
elsif (\$a==29) {}
elsif (\$a==30) {}
elsif (\$a==31) {}
elsif (\$a==32) {}
elsif (\$a==33) {}
elsif (\$a==34) {}
elsif (\$a==35) {}
elsif (\$a==36) {}
}

sub test3 {
return if \$_[0] > 9;
my \$a = \$_[0];

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) {}
elsif (\$a==10) {}
elsif (\$a==11) {}
elsif (\$a==12) {}
elsif (\$a==13) {}
elsif (\$a==14) {}
elsif (\$a==15) {}
elsif (\$a==16) {}
elsif (\$a==17) {}
elsif (\$a==18) {}
elsif (\$a==19) {}
elsif (\$a==20) {}
elsif (\$a==21) {}
elsif (\$a==22) {}
elsif (\$a==23) {}
elsif (\$a==24) {}
elsif (\$a==25) {}
elsif (\$a==26) {}
elsif (\$a==27) {}
elsif (\$a==28) {}
elsif (\$a==29) {}
elsif (\$a==30) {}
elsif (\$a==31) {}
elsif (\$a==32) {}
elsif (\$a==33) {}
elsif (\$a==34) {}
elsif (\$a==35) {}
elsif (\$a==36) {}
}

my @dispatch = (
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
sub {},
);

sub test4 {
return if \$_[0] > 9;
\$dispatch[ \$_[0] ]->( \$_[0] );
}

my \$start = time;
test1( \$_ ) for 1 .. 1e6;
printf "test1 took: %.5f seconds\n", time - \$start;

\$start = time;
test2( \$_ ) for 1 .. 1e6;
printf "test2 took: %.5f seconds\n", time - \$start;

\$start = time;
test3( \$_ ) for 1 .. 1e6;
printf "test3 took: %.5f seconds\n", time - \$start;

\$start = time;
test4( \$_ ) for 1 .. 1e6;
printf "test4 took: %.5f seconds\n", time - \$start;

__END__
C:\test>junk37
test1 took: 1.70500 seconds
test2 took: 0.30789 seconds
test3 took: 0.29240 seconds
test4 took: 0.29207 seconds

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?

ok, one more time. try embedding my subruotine in a loop reading /proc/pid/status and measure it. THAT's my point. In a small program the timings are as expected. My question is why the unexpected performance when reading /proc?

Did you try my second script on your system? Try that without the return statement.

-mark

understand now?

Apparently not.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?

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