RFR (S): 8067469: G1 ignores AlwaysPreTouch

[Erik Helin]

On 2014-12-16, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have reviews for the following small change?
> 
> It adds pre-touching of allocated memory in G1, which has been forgotten
> to be implemented when changing this code in JDK-8038423.
> 
> This change adds this and refactors the pre-touch code to a single
> method in the os class.
> 
> CC'ed runtime since it changes code in os.hpp/cpp.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8067469
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8067469/webrev/

First of all, nice patch, I really like the introduction of
os::pretouch_memory. I have one question about the patch: the code you
removed in virtualspace.cpp did:

  *curr = 0;

whereas the new code you added in os::pretouch_memory does:

  char t = *p; *p = t;

Was this change intended or should os::pretouch_memory also write 0?

Thanks,
Erik

> Testing:
> jprt, manual testing with and without AlwaysPreTouch enabled
> 
> Thanks,
>   Thomas
> 
>