NMT: Weird merging of regions

[Zhengyu Gu]

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