RFR: 8233299: Implementation: JEP 365: ZGC on Windows

erik.osterlund at oracle.com erik.osterlund at oracle.com
Tue Nov 5 10:40:53 UTC 2019

Hi Stefan,

Awesome. Looks good!


On 10/31/19 11:18 AM, Stefan Karlsson wrote:
> Hi all,
> Please review this patch to add ZGC support on Windows.
> https://cr.openjdk.java.net/~stefank/8233299/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8233299
> As mentioned in the JEP (https://openjdk.java.net/jeps/365), there 
> were some preparation patches that needed to go in to pave the way for 
> this patch:
>     8232601: ZGC: Parameterize the ZGranuleMap table size
>     8232602: ZGC: Make ZGranuleMap ZAddress agnostic
>     8232604: ZGC: Make ZVerifyViews mapping and unmapping precise
>     8232648: ZGC: Move ATTRIBUTE_ALIGNED to the front of declarations
>     8232649: ZGC: Add callbacks to ZMemoryManager
>     8232650: ZGC: Add initialization hooks for OS specific code
>     8232651: Add implementation of os::processor_id() for Windows
> ... they have all been pushed now.
> One important key-point to this implementation is to use the new 
> Windows APIs that support reservation and mapping of memory through 
> "placeholders":  VirtualAlloc2, VirtualFreeEx, MapViewOfFile3, and 
> UnmapViewOfFile2. These functions are available starting from version 
> 1803 of Windows 10 and Windows Server. ZGC will lookup these symbols 
> to determine if the Windows version supports these functions.
> Correlating the text in the JEP with the code:
> * '"Support for multi-mapping memory". ZGC's use of colored pointers 
> requires support for heap multi-mapping, so that the same physical 
> memory can be accessed from multiple different locations in the 
> process address space. On Windows, paging-file backed memory provides 
> physical memory with an identity (a handle), which is unrelated to the 
> virtual address where it is mapped. Using this identity allows ZGC to 
> map the same physical memory into multiple locations.'
> We commit memory via paging file mappings and map views into that memory.
> The function ZMapper::create_and_commit_paging_file_mapping uses 
> CreateFileMappingW with SEC_RESERVE to create this mapping, 
> MapViewOfFile3 to map a temporary view into the mapping, VirtualAlloc2 
> to commit the memory, and then UnmapViewOfFile2 to unmap the view.
> The reason to use SEC_RESERVE and the extra VirtualAlloc2, instead of 
> SEC_COMMIT, is to ensure that the later multi-mappings of committed 
> file mappings don't fail under low-memory situations. Earlier 
> prototypes used SEC_COMMIT and saw these kind of OOME errors when 
> mapping new views to already committed memory. The current 
> platform-independent ZGC code isn't prepared to handle OOME errors 
> when mapping views, so we chose this solution.
> MapViewOfFile3 is then used to multi-map into the committed memory.
> * '"Support for mapping paging-file backed memory into a reserved 
> address space". The Windows memory management API is not as flexible 
> as POSIX's mmap/munmap, especially when it comes to mapping file 
> backed memory into a previously reserved address space region. To do 
> this, ZGC will use the Windows concept of address space placeholders. 
> The placeholder concept was introduced in version 1803 of Windows 10 
> and Windows Server. ZGC support for older versions of Windows will not 
> be implemented.'
> Before the placeholder APIs there was no way to first reserve a 
> specific virtual memory range, and then map a view of a committed 
> paging file over that range. The VirtuaAlloc function could be used to 
> first reserve and then commit anonymous memory, but nothing similar 
> existed for mapped views. Now with placeholders, we can create a 
> placeholder reservation of memory with VirtualAlloc2, and then replace 
> that reservation with MapViewOfFile3. When memory is unmapped, we can 
> use UnmapViewOfFile2 to "preserve" the placeholder memory reservation.
> * '"Support for mapping and unmapping arbitrary parts of the heap". 
> ZGC's heap layout in combination with its dynamic sizing (and 
> re-sizing) of heap pages requires support for mapping and unmapping 
> arbitrary heap granules. This requirement in combination with Windows 
> address space placeholders requires special attention, since 
> placeholders must be explicitly split/coalesced by the program, as 
> opposed to being automatically split/coalesced by the operating system 
> (as on Linux).'
> Half of the preparation patches were put in place to support this. 
> When replacing a placeholder with a view of the backing file, we need 
> to exactly match the address and size of a placeholder. Also, when 
> unmapping a view, we need to exactly match the address and size of the 
> view, and replace it with a placeholder.
> To make it easier to map and unmap arbitrary parts of the heap, we 
> split reserved memory into ZGranuleSize-sized placeholders. So, 
> whenever we perform any of these operations, we know that any given 
> memory range could be dealt with as a number of granules.
> When memory is reserved, but not mapped, it is registered in the 
> ZVirtualMemoryManager. It splits memory into granule-sized placholders 
> when reserved memory is fetched, and coalesces placeholders when 
> reserved memory is handed back.
> * '"Support for committing and uncommitting arbitrary parts of the 
> heap". ZGC can commit and uncommit physical memory dynamically while 
> the Java program is running. To support these operations the physical 
> memory will be divided into, and backed by, multiple paging-file 
> segments. Each paging-file segment corresponds to a ZGC heap granule, 
> and can be committed and uncommitted independently of other segments.'
> Just like we can map and unmap in granules, we want to be able to 
> commit and uncommit memory in granules. You can see how memory is 
> committed and uncommitted in granules in 
> ZBackingFile::commit_from_paging_file and 
> ZBackingFile::uncommit_from_paging_file. Each committed granule is 
> associated with one registered handle. When memory for a granule is 
> uncommitted, the handle is closed. At this point, no views exist to 
> the mapping and the memory is handed back to the OS.
> Final point about ZPhysicalMemoryBacking. We've tried to make this 
> file similar on all OSes, with the hope to be able to combine them 
> when both the Windows and macOS ports have been merged.
> Thanks,
> StefanK

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