From: Dave Howell <groups@...>
Date: 2006-06-02T04:35:28+09:00
Subject: Re: How use Profiling, a Quick Guide.

"Boy!" I thought. "Now THIS sounds like a thread I want to read!"

The aforementioned "I" in this case being a hobbyist programmer 
using/learning Ruby 1.8.4 on OSX 10.3 (w/Developer Tools installed, of 
course)...

On May 23, 2006, at 15:10, John Carter wrote:

> On Tue, 23 May 2006, Mark Somerville wrote:
>
>> Can anyone recommend some good or tutorials on profiling and 
>> benchmarking?
>> Although I have used them both (from the standard library), I feel I 
>> lack
>> fundamental knowledge about how to go about it.
>
> I don't know of any Good one but here are some things I do...
>
>  * Always benchmark first, profile and optimize later. Otherwise you 
> don't
>    know whether it was worth it. (Hint: Often the tweaks aren't worth 
> it, or
>    make things worse)

OK, that makes sense. Not that I have any idea *how* to benchmark my 
programs, but I'll keep reading...

>  * Look at the top most items in the profile list. Usually they are
>    standard library functions. Silly things like [] or +...
>    Ask your self, "What could be calling these thing so often?
>    Can I make it do it less often? eg. Cache the result etc.)

Er, the profile list? Push "Where do I find/how do I generate a 
'profile list'?" onto the question_stack

>  * Scan down the list for the first few user written methods. Usually 
> the one
>    that you spent the most time in has the most fat to trim.

I understand that. Yay!
comprehension_counter = 1

>  * Scan the list for the routine that took the longest (as opposed
>    to the most time spent in it.) Try get that one to just do less.

I don't understand this. Took the longest != most time spent in it? But 
what if it does the most work? If I make it do less, don't I have to 
make something else do more?
question_stack << "huh? [Parse Error]"

>  * Use strace, see what the system is doing at an OS level. I once
>    improved another persons program (professional in production) by a
>    factor of 40, yes FORTY times, by noting it was doing an lseek on 
> every
>    record read and changing that to an mmap.

question_stack << "[Error: File 'strace' Not Found]"

>  * Always look out for surprises. The best moment in an optimizers 
> life is
>    when you look at something and say "WTF! Why is _that_ routine 
> taking so
>    long!?"

class Fixnum
	def increment
		self = self + 1
	end
end

comprehension_counter.increment

>  * Large number of calls to .size and .length and .count_objects 
> methods are a
>    clue some nidjit somewhere is doing something really N^2 stupid 
> like...
>     for( i=0; i < container.count_objects(); i++) {
>     }

question_stack << "What exactly is stupid about that? And what's the 
smart alternative? [Error: Unknown Language] [Error: C Allergy 
Triggered] [Trap: C Antihistamine Enabled]"

>  * Use top, vmstat, times as well.
>     Large usertime means optimize program.
>     Large system time means you really hammering the system calls (that
>     was a clue in the lseek case I mentioned above)
>     Large real time small user and system means you are waiting for
>     input from user / disk / network /...? Find out what.
>     Watch the page in page out stats. Watch the swap in swap out stats.
>     If you are paging or swapping, time to switch to memory consumption
>     optimization!

question_stack << "My 'top' doesn't have 'usertime', 'system time' or 
'realtime' values. Is this only for special kinds of top? [Error: 
Potential OS-specific or otherwise specialized tools]"

question_stack << "[Error: File 'vmstat' Not Found]"

question_stack << "[Error: File 'times' Not Found]"

>   * Use my memprofile /  newprofile tricks to try spot the
>     http://wiki.rubygarden.org/Ruby/page/show/NewProfiler
>     http://rubyforge.org/snippet/detail.php?type=snippet&id=70

question_stack << "I found them, but if I use them, what are they going 
to tell me, and what kind of actions would I want to take in response? 
[Error: Documentation Not Found]"

Fatal Error: Question Stack Overflow

Fatal Error: Clue Underrun. Initial Clue supply inadequate. Increase 
Clues and re-run.

Fatal Error: Nuby Page Fault  Fatal Error: Nuby Chapter Fault  Fatal 
Error: Nuby Volume Fault  Fatal Error: San Andreas Fault   Fatal Error: 
Nobody's Fault. Please contact your system administrator to resolve the 
problem. If you are the System Administrator, I guess you're screwed. 
Ha. Ha.