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

[Erik Helin]

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
>