RFR(xxs): 8187230: [aix] Leave OS guard page size at default for non-java threads instead of explicitly setting it

[Lindenmaier, Goetz]

Hi Thomas,

looks good except for that you missed setting the guard pages size to zero 
for compiler threads. Compiler threads are Java Threads and thus get
our guard pages. 

Best regards,
  Goetz.

> -----Original Message-----
> From: hotspot-dev [mailto:hotspot-dev-bounces at openjdk.java.net] On
> Behalf Of Thomas Stüfe
> Sent: Donnerstag, 7. September 2017 12:02
> To: ppc-aix-port-dev at openjdk.java.net
> Cc: HotSpot Open Source Developers <hotspot-dev at openjdk.java.net>
> Subject: RFR(xxs): 8187230: [aix] Leave OS guard page size at default for non-
> java threads instead of explicitly setting it
> 
> Hi all,
> 
> may I please have a review for this small change:
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8187230
> 
> Webrev:
> http://cr.openjdk.java.net/~stuefe/webrevs/8187230-aix-
> leave-os-guard-page-size-at-default-for-non-java-
> threads/webrev.00/webrev/
> 
> The change is very subtle.
> 
> Before, we would set the OS guard page size for every thread - for java
> threads disable them, for non-java threads we'd set them to 4K.
> 
> Now, we still disable them for java threads but leave them at the OS
> default size for non-java threads.
> 
> The really important part is the disabling of OS guard pages for java
> threads, where we have a VM guard pages in place and do not want to spend
> more memory on OS guards. We do not really care for the exact size of the
> OS guard pages for non-java threads, and therefore should not set it - we
> should leave the size in place the OS deems sufficient. That also spares us
> the complexity of handling the thread stack page size, which on AIX may be
> different from os::vm_page_size().
> 
> Thank you and Kind Regards, Thomas