From: usa@...
Date: 2014-12-01T19:46:35+00:00
Subject: [ruby-core:66630] [Backport21 - Backport #10559] Unexpected copy on write when running GC in 2.1.3

Issue #10559 has been updated by Usaku NAKAMURA.

Project changed from Backport93 to Backport21

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Backport #10559: Unexpected copy on write when running GC in 2.1.3
https://bugs.ruby-lang.org/issues/10559#change-50245

* Author: Niels Tolstrup
* Status: Open
* Priority: Normal
* Assignee: 
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Memory is duplicated when forking and calling GC in 2.1.3.

We have written a small program that reproduces the issue (see attached file).

1. The program instantiates an object and then forks into two processes.
2. The GC is called in the child process.
3. The memory allocation (as given by /proc/*pid*/smaps) changes from shared to private, thereby indicating a doubling of memory consumption.

Here is the output of the program (size is in mb):

~~~
ruby version 2.1.3
   time   pid message             shared    private
 4.011s  4723 Parent pre GC           68          0
 4.013s  4737 Child  pre GC           68          0
 8.019s  4723 Parent post GC           5         62
 8.093s  4737 Child  post GC           5         66

~~~
We have tested the program on Ubuntu 14.04 with ruby 2.1.3 and 2.1.5.

We have also tried to fork 10 children and see a 10 doubling of the memory consumption, the issue only occurs after running the GC.




---Files--------------------------------
mem.rb (1.25 KB)


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