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

[Stefan Johansson]

On 2014-06-17 13:15, Erik Helin wrote:
> Hi Stefan,
>
> sorry for the late reply, please see my comments inline.
>
> On Tuesday 13 May 2014 14:31:10 PM Stefan Johansson wrote:
>> On 2014-05-08 17:34, 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.
>> Good fix.
> Thanks!
>
> On Tuesday 13 May 2014 14:31:10 PM Stefan Johansson wrote:
>> Regarding the testing, I looked at the code and the _page_sizes array is
>> public. You could start of the testing with recording all "real" values
>> and then set up an array that allows more testing. It would be nice to
>> have a case in the alignment test that makes sure for example that for a
>> 10M region you can get 2M pages but not 4M pages. If doing this you need
>> to make sure the values in the _page_sizes array are set back to normal
>> after the test. This would also allow the test to run even if
>> UseLargePages isn't set.
> I agree that this is a good idea, but I'm not fond of modifying the
> _page_sizes array. Even though the VM is quite idle when the internal
> vm tests are running, there is a possibility that threads running
> concurrently might pick up a strange value from the _page_sizes
> array. Diagnosing an eventual crash due to this might be tricky :)
>
> What do you think of this?
I agree, and as we discussed off-line, this kind of tests would be 
easier to write if we had a better unit-testing framework. With that in 
mind I'm ok with leaving the test as is for now.

Thanks,
Stefan
>
> On Tuesday 13 May 2014 14:31:10 PM Stefan Johansson wrote:
>>> 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.
>> Great!
> Thanks,
> Erik
>
>>> 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).
>> I agree that this is probably the correct solution, but it would be nice
>> to hear what the compiler guys think. As you say, this is a little more
>> strict and there might be cases where we now stop getting large pages
>> where we used to before.
>>
>> To summarize, I think the changes are good but wouldn't mind some
>> improved tests for it.
>>
>> Stefan
>>
>>> 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