From: Ritwik Banerjee <ritwik.banerjee@...>
Date: 2010-10-29T22:17:39+09:00
Subject: Ruby 1.9.1 linear memory increase in a simple for loop.

The following text code shows a linear rise in memory usage. Since this
is a simple loop, I am guessing there's something fundamentally wrong
(either in my understanding of ruby's garbage collection, or in ruby's
garbage collection):

memtest.rb
----------
require 'matrix'
a = [[1,0,1,0,0,1,1,1],[0, 0, 0, 0, 0, 1, 1, 0],[1, 1, 1, 1, 0, 1, 1,
1],[0, 0, 1, 1, 1, 1, 0, 1],[0, 0, 1, 1, 1, 1, 0, 1],[0, 0, 0, 1, 1, 1,
1, 1],[0, 1, 1, 1, 1, 1, 1, 1],[0, 1, 0, 1, 1, 0, 1, 1]]

m = Matrix.rows(a)
n = Matrix.identity(8)

for i in 1..100 do
    n = n * m;
    physical_mem_usage = `ps -o rss= -p #{Process.pid}`.to_i;
    virtual_mem_usage = `ps -o vsz= -p #{Process.pid}`.to_i;
    puts "#{i}: VMU = #{virtual_mem}; PMU = #{physical_mem}."
    puts ObjectSpace.count_objects()
#   ObjectSpace.garbage_collect
    STDOUT.flush
end

---------

This code shows a linear increase in memory usage. The
ObjectSpace.count_objects call shows a steady linear increase in counts
of T_ARRAY, T_HASH, T_STRUCT, T_STRING and T_OBJECT.

Can anyone explain this? Typically memory explosions occur in complex
codes with lots of references, and in those cases the memory rise is
exponential. In the above loop, invoking garbage collection manually
(it's commented out in the snippet above) slows the memory increase, but
only by a constant factor.
For example, if the loop used up 30 kB in 100 iterations originally, it
uses up 30kB in 500 iterations after invoking garbage collection
manually.

Understanding this is critical for me since I am dealing with matrices
of enormous sizes, so even a small memory leak can jeopardize my
project.

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