RFR: 8027915: TestParallelHeapSizeFlags fails with unexpected heap size on sparcv9

[Erik Helin]

Stefan, Jon,

I forgot to remove the "@ignore" line from the actual test,
TestParallelHeapSizeFlags.java. Please see new webrev at:
http://cr.openjdk.java.net/~ehelin/8027915/webrev.02/

and the very small (one line) incremental webrev at:
http://cr.openjdk.java.net/~ehelin/8027915/webrev.01-02/

Thanks,
Erik

On Thursday 08 May 2014 17:34:56 PM Erik Helin wrote:
> Hi Stefan,
> 
> thanks for your thorough review! Please see new webrev at:
> http://cr.openjdk.java.net/~ehelin/8027915/webrev.01/
> 
> and incremental webrev at:
> http://cr.openjdk.java.net/~ehelin/8027915/webrev.00-01/
> 
> and comments inline!
> 
> On 2014-05-06 13:12, Stefan Johansson wrote:
> > Hi Erik,
> > 
> > Thanks for making page_size_for_region more strict. A few comments:
> > 
> > src/share/vm/runtime/os.cpp
> > 
> > With this fix page_size_for_region doesn't care about alignment at all
> > (old version wasn't much better) and to me that feels wrong. I would
> > expect to get a page size that the given region sizes is aligned to, but
> > I might be missing some use case where that isn't needed.
> 
> Agree, page_size_for_region should check alignment for large pages.
> Added fix and regression tests.
> 
> On 2014-05-06 13:12, Stefan Johansson wrote:
> > src/share/vm/gc_implementation/parallelScavenge/generationSizer.cpp
> > 
> > For the heap we want both min and max to be a multiple of the page size
> > so we shoud call page_size_for_region for both _min_heap_byte_size and
> > _max_heap_byte_size and use the smallest of the two returned page sizes.
> 
> Agree, fixed.
> 
> On 2014-05-06 13:12, Stefan Johansson wrote:
> > src/share/vm/memory/heap.cpp
> > 
> > The current patch will not change the default behavior so I'm fine with
> > just calling page_size_for_region once using reserved_size, but someone
> > with more insight might know if we should handle it the same way as the
> > generationSizer and use both committed and reserved.
> 
> Since I'm not sure how the compiler guys want to handle this, I opted
> for the safe alternative and updated the patch to be backwards
> compatible with the old code (except slightly more strict).
> 
> Thanks,
> Erik
> 
> > Thanks,
> > Stefan
> > 
> > On 2014-05-06 10:44, Erik Helin wrote:
> >> Hi all,
> >> 
> >> this patch changes
> >> os::page_size_for_region(min_size, max_size, min_pages) to always
> >> guarantee that the returned page size <= max_size / min_pages. Prior
> >> to this patch, page_size_for_region could return a page size larger
> >> than max_size / min_pages. This can cause some unexpected behavior
> >> for code using page_size_for_region, such as the following regression
> >> tests:
> >> 
> >> http://cr.openjdk.java.net/~ehelin/8027915/regression-test/
> >> 
> >> As the test shows, if you have a 2 MB region and you must use at least
> >> 2 pages, you would still get back 2 MB as the page size on a Linux
> >> machine that supports 2 MB large pages. The reason for this is
> >> explained in a comment above the function in os.hpp:
> >> 
> >> // The current implementation ignores min_pages if a larger page
> >> // size is an exact multiple of both region_min_size and
> >> // region_max_size.  This allows larger pages to be used when doing
> >> // so would not cause fragmentation; in particular, a single page can
> >> // be used when region_min_size == region_max_size == a supported page
> >> // size.
> >> 
> >> Reducing fragmentation of large pages is good but the varying return
> >> value makes it very hard to depend on os::page_size_for_region in
> >> calculations (for example when sizing the heap).
> >> 
> >> This patch removes the fragmentation prevention mechanism, simplifies
> >> the function and adds a regression tests:
> >> 
> >> http://cr.openjdk.java.net/~ehelin/8027915/webrev.00/
> >> 
> >> Testing:
> >> - JPRT
> >> 
> >> Thanks,
> >> Erik