From: Ruben <Ruben.Vandeginste@...>
Date: 2004-10-14T16:13:22+09:00
Subject: Re: GC and low file performance when large array is allocated


At Thu, 14 Oct 2004 15:54:32 +0900,
Geert Fannes wrote:
> 
> Hello,
> 
> I played some more with the test program and apparently it has nothing
> to do with the file access. The program below is a simplified version,
> which is more to the point. If I exchange the string allocation t="t"
> with t=1, there is no performance drop anymore.
> 
> #
> #begin of program
> #
> allocateBefore=true
> disableGC=false
> 
> GC.disable if disableGC
> 
> largeArray=Array.new(20000000) if allocateBefore
> 
> 100000.times{|i|t="t"}
> 
> largeArray=Array.new(20000000) if !allocateBefore
> #
> #end of program
> #
> 
> Any idea why the string allocation (and possibly deallocation) takes
> so much more time when there is a large array in memory? Can I destroy
> an object manually? This could be helpfull in combination with
> disabling the garbage collection.
> 

My guess is that in the loop 100000.times{|i|t="t"}, the garbage
collector will run out of memory, maybe once, maybe even more. And
when it runs out of memory, it will have to mark/read/follow all cells
in "largeArray", and since that array is very large, i think that's
causing the slowdown.

Try the following and you'll see it's faster:

#
#begin of program
#
allocateBefore=true
disableGC=false

GC.disable if disableGC

largeArray=Array.new(20000000) if allocateBefore
largeArray=0 # explicitly setting to 0 so that the
             # gc will not need to mark it
100000.times{|i|t="t"}

largeArray=Array.new(20000000) if !allocateBefore
#
#end of program
#

Ruben