Pre-RFR 8236847 CDS archive with 4K alignment unusable on machines with 64k pages

[Ioi Lam]

Hi Dmitry,

The goal of my patch is to make it possible to use the CDS archive 
anywhere regardless of the OS page size of the build machine. In my  v01 
patch, I added SharedRegionAlignment for flexibility, but I guess that's 
more of a hassle.

Here's a simplified v02 patch that removes the SharedRegionAlignment flag:

http://cr.openjdk.java.net/~iklam/jdk15/8236847-SharedRegionAlignment.v02/

Now, on AARCH64, we always dump with 64K alignment, even if the host has 
4K pages -- see MetaspaceShared::init_alignments(). At run time, the 
check in MetaspaceShared::map_archive() is relaxed to allow mapping of 
64K-aligned archives on a host with 4K page size.

Could you try this and see if it works for you?

I don't have access to aarch64 hosts with 64k pages, so I tested on x64 
by changing the #if line in init_alignments to "#if 1". All tests passed.

Thanks
- Ioi


On 1/30/20 12:34 AM, Dmitry Samersoff wrote:
> Hello Ioi,
>
> I'm not sure that explicit specification of SharedRegionAlignment=64k 
> helps.  The goal is to build JDK on a build machine then use it 
> smoothly on different test/production machines with both 4k and 64k 
> pages, ever if it's the same JDK that resides in a shared network 
> location.
>
> For a long term we may consider to revisit entire logic of default 
> archive dumping and dump CDS archive of JDK classes on the first JVM 
> run to $HOME/.cache but it requires more work and is out of scope of 
> this issue.
>
> So as a short term solution I would propose to implement following logic:
>
> 1. Without any parameters
>
>     - try to use existing CDS archive
>
>    - if alignment doesn't match - disable CDS and continue execution.
>
>         - if debug or fastdebug  - issue a meaningful warning.
>
> 2. With explicit -Xshare:auto or -Xshare:on
>
>     - try to use existing CDS archive
>
>     - if alignment doesn't match - terminate VM with meaningful message.
>
> -Dmitry
>
>
> On 29.01.20 08:37, Ioi Lam wrote:
>> https://bugs.openjdk.java.net/browse/JDK-8236847
>> http://cr.openjdk.java.net/~iklam/jdk15/8236847-SharedRegionAlignment.v01/ 
>>
>>
>> Hi Dmitry,
>>
>> Here's a proposal to make it possible to generate CDS archives that 
>> can be used on machines with different OS page alignments: generate 
>> the CDS archive with
>>
>>      java -Xshare:dump -XX:SharedRegionAlignment=64k
>>
>> I want to be conservative, as this seems to affect only 
>> Linux/aarch64, so I have limited the range of this flag to be no more 
>> than 64KB. You can see checks for other invalid values in the new 
>> test case.
>>
>> If SharedRegionAlignment is not specified (as for most platforms), 
>> os::vm_allocation_granularity() will be used.
>>
>> For simplicity, -XX:SharedRegionAlignment can only be set when 
>> dumping the base archive (-Xshare:dump). The dynamic archive 
>> (-XX:DynamicDumpSharedSpaces) will always use the value as stored in 
>> the base archive.
>>
>> Please let me know if this solution will work for your situation. If 
>> so, I will file a CSR for the new flag and also write more test cases.
>>
>> (Also, the default CDS archive is created at JDK build time. I'll 
>> leave it to the ARM porting folks to do the makefile changes to pass the
>> -XX:SharedRegionAlignment=64k flag to BUILD_CDS_ARCHIVE in Images.gmk)
>>
>> Thanks
>> - Ioi