NMT: Weird merging of regions

[coleen.phillimore at oracle.com]

Yeah, I don't remember why we checked for null.   Probably just to 
simplify the code path.

The patch looks good.  There should be a test in the test system to 
verify it (see the bug that Zhengyu pointed to).

Coleen

On 1/24/18 10:43 AM, Zhengyu Gu wrote:
> Hi Stefan,
>
> It looks like a bug that subsequent small regions were not merged, 
> they all have the same stacks. This part of code was last touched by 
> JDK-8138705 (https://bugs.openjdk.java.net/browse/JDK-8138705), not 
> sure if it is the cause.
>
> Your patch looks good to me.
>
> Thanks,
>
> -Zhengyu
>
> On 01/24/2018 10:17 AM, Stefan Karlsson wrote:
>> Hi Zhengyu (all),
>>
>> I was looking at the NMT output from one of our stress tests and saw 
>> something unexpected. It looks like the first number commits of small 
>> ZPages get merged into one region, until we allocate a medium ZPage 
>> which ends up at the end of the virtual address space. After the 
>> medium ZPage has been allocated non of the subsequent small ZPages 
>> get merged.
>>
>> I traced the committing of memory:
>>
>> [0.079s][info][gc     ] nmt_commit   0x0000040000000000 514048K
>> [2.523s][info][gc     ] nmt_commit   0x000004001f600000 2048K
>> [2.528s][info][gc     ] nmt_commit   0x000004001f800000 2048K
>> [2.533s][info][gc     ] nmt_commit   0x000004001fa00000 2048K
>> [2.535s][info][gc     ] nmt_commit   0x000004001fc00000 2048K
>> [2.537s][info][gc     ] nmt_commit   0x000004001fe00000 2048K
>> [2.538s][info][gc     ] nmt_commit   0x0000040020000000 2048K
>> [2.543s][info][gc     ] nmt_commit   0x0000040020200000 2048K
>> [2.547s][info][gc     ] nmt_commit   0x0000040020400000 2048K
>> ...
>> [3.714s][info][gc       ] nmt_commit   0x000004003c200000 2048K
>> [3.715s][info][gc       ] nmt_commit   0x000004003c400000 2048K
>> [3.731s][info][gc       ] nmt_commit   0x000007fffe000000 32768K
>> [3.737s][info][gc       ] nmt_commit   0x000007fffc000000 32768K
>> [3.737s][info][gc       ] nmt_commit   0x000004003c600000 2048K
>> [3.741s][info][gc       ] nmt_commit   0x000004003c800000 2048K
>>
>> The output from NMT is:
>>
>> [0x0000040000000000 - 0x000004001f600000] committed 514048KB from
>>      [0x00002b9e6209618b] 
>> ZPreMappedMemory::ZPreMappedMemory(ZVirtualMemoryManager&, 
>> ZPhysicalMemoryManager&, unsigned long)+0xfb
>>      [0x00002b9e6208e277] ZPageAllocator::ZPageAllocator(unsigned 
>> long, unsigned long, unsigned long)+0x67
>>      [0x00002b9e620706fc] ZHeap::ZHeap()+0xac
>>      [0x00002b9e6206c2c9] 
>> ZCollectedHeap::ZCollectedHeap(ZCollectorPolicy*)+0x49
>>
>> [0x000004001f600000 - 0x000004003c600000] committed 475136KB from
>>      [0x00002b9e620907bb] ZPageAllocator::alloc_page(unsigned char, 
>> unsigned long, ZAllocationFlags)+0x9b
>>      [0x00002b9e620712e7] ZHeap::alloc_page(unsigned char, unsigned 
>> long, ZAllocationFlags)+0x17
>>      [0x00002b9e6207faa4] 
>> ZObjectAllocator::alloc_object_in_shared_page(ZPage**, unsigned char, 
>> unsigned long, unsigned long, ZAllocationFlags)+0x114
>>      [0x00002b9e62081853] ZObjectAllocator::alloc_object(unsigned 
>> long)+0x103
>>
>> [0x000004003c600000 - 0x000004003c800000] committed 2048KB from
>>      [0x00002b9e620907bb] ZPageAllocator::alloc_page(unsigned char, 
>> unsigned long, ZAllocationFlags)+0x9b
>>      [0x00002b9e620712e7] ZHeap::alloc_page(unsigned char, unsigned 
>> long, ZAllocationFlags)+0x17
>>      [0x00002b9e6207faa4] 
>> ZObjectAllocator::alloc_object_in_shared_page(ZPage**, unsigned char, 
>> unsigned long, unsigned long, ZAllocationFlags)+0x114
>>      [0x00002b9e62081853] ZObjectAllocator::alloc_object(unsigned 
>> long)+0x103
>>
>> [0x000004003c800000 - 0x000004003ca00000] committed 2048KB from
>>      [0x00002b9e620907bb] ZPageAllocator::alloc_page(unsigned char, 
>> unsigned long, ZAllocationFlags)+0x9b
>>      [0x00002b9e620712e7] ZHeap::alloc_page(unsigned char, unsigned 
>> long, ZAllocationFlags)+0x17
>>      [0x00002b9e6207faa4] 
>> ZObjectAllocator::alloc_object_in_shared_page(ZPage**, unsigned char, 
>> unsigned long, unsigned long, ZAllocationFlags)+0x114
>>      [0x00002b9e62081853] ZObjectAllocator::alloc_object(unsigned 
>> long)+0x103
>>
>>
>> As can be seen all pages between:
>> [2.523s][info][gc     ] nmt_commit   0x000004001f600000 2048K
>>
>> and:
>> [3.715s][info][gc       ] nmt_commit   0x000004003c400000 2048K
>>
>> are merged into:
>> [0x000004001f600000 - 0x000004003c600000] committed 475136KB from
>>
>> However the small page commits after the medium page commits all get 
>> separate entries.
>>
>> I looked at the code in add_committed_region and this seems to be 
>> intentionally:
>>        // special case to expand prior region if there is no next region
>>
>> but I don't understand the _reason_ why we don't merge the added 
>> region with adjacent next regions.
>>
>> Zhengyu, do you know/remember the motivation behind this?
>>
>>
>> I created a patch to try to merge with the next region:
>>   http://cr.openjdk.java.net/~stefank/zgc/zNMTMerging/webrev.01/
>>
>> This yields NMT output like the following:
>>
>> [0x0000040000000000 - 0x000004001f600000] committed 514048KB from
>>      [0x00002b26cda232db] 
>> ZPreMappedMemory::ZPreMappedMemory(ZVirtualMemoryManager&, 
>> ZPhysicalMemoryManager&, unsigned long)+0xfb
>>      [0x00002b26cda1b3c7] ZPageAllocator::ZPageAllocator(unsigned 
>> long, unsigned long, unsigned long)+0x67
>>      [0x00002b26cd9fd84c] ZHeap::ZHeap()+0xac
>>      [0x00002b26cd9f9419] 
>> ZCollectedHeap::ZCollectedHeap(ZCollectorPolicy*)+0x49
>>
>> [0x000004001f600000 - 0x0000040380800000] committed 14174208KB from
>>      [0x00002b26cda1d90b] ZPageAllocator::alloc_page(unsigned char, 
>> unsigned long, ZAllocationFlags)+0x9b
>>      [0x00002b26cd9fe437] ZHeap::alloc_page(unsigned char, unsigned 
>> long, ZAllocationFlags)+0x17
>>      [0x00002b26cda0cbf4] 
>> ZObjectAllocator::alloc_object_in_shared_page(ZPage**, unsigned char, 
>> unsigned long, unsigned long, ZAllocationFlags)+0x114
>>      [0x00002b26cda0e9a3] ZObjectAllocator::alloc_object(unsigned 
>> long)+0x103
>>
>> [0x000007ffce000000 - 0x0000080000000000] committed 819200KB from
>>      [0x00002b26cda1d90b] ZPageAllocator::alloc_page(unsigned char, 
>> unsigned long, ZAllocationFlags)+0x9b
>>      [0x00002b26cd9fe437] ZHeap::alloc_page(unsigned char, unsigned 
>> long, ZAllocationFlags)+0x17
>>      [0x00002b26cda0cbf4] 
>> ZObjectAllocator::alloc_object_in_shared_page(ZPage**, unsigned char, 
>> unsigned long, unsigned long, ZAllocationFlags)+0x114
>>      [0x00002b26cda0e935] ZObjectAllocator::alloc_object(unsigned 
>> long)+0x95
>>
>> StefanK