I had excatly the same problem with a 32 bits mandriva 2008.1 running on 
a single core sempron with 1 Go RAM, but because I did not have any  
need to upgrade, I came back to mandriva 2007.1.

> This may not be the appropriate forum for this problem but I have
> exhausted other avenues of enquiry, including Bugzilla.  The fault
> applies to a particular release of Mandriva Linux; on 2007.0 it did
> not occur and neither in 2008.0.  Mandriva 2008 Spring (2008.1)
> however does exhibit the bug or whatever it is.  Somebody using
> Cooker, essentially 2009.0, does not see the fault.
>
> Ruby fails almost immediately for any ruby-tk script with a message
> which always has this form:
>
>   alloc: invalid block: 0x95e190: ef ef 30
>   Abort (core dumped)
>
> The address varies to some extent but the contents or whatever they
> are (registers maybe) always come up as ef ef 30.  My guess is that
> there is a failure to initialize some variable or pointer which is
> used in memory allocation.  As far as I can see the root window is
> created successfully and it may be possible to run one or two lines
> of code beyond that.  Then it breaks.  Ruby itself works without a
> problem, likewise fxruby, and Tcl/Tk scripts on their own work and so
> do combinations of Tcl/Tk and C or C++.  There are no architecture or
> version mismatches.  The test system is x86_64 dual core with Tk
> 8.5a5.  Other people have already pointed out that Mandriva should
> have moved on to 8.5b3 by now but that may be irrelevant because Tk
> does work.  I have reinstalled 2008.1 two or three times and also
> reverted to 2008.0 to prove that 2008.1 always behaves this way and
> only 2008.1.
>
> Has anybody else here seen this?  It would be helpful to me if
> someone with an installation of Mandriva Spring 2008 on a 64-bit
> machine could check this out, maybe using an example from the Pickaxe
> or Using Ruby Gems.  I could supply a simple test script if
> necessary.
>
> TIA
>
> Len