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

[Dmitry Samersoff]

Hello Ioi,

The changes looks good for me, besides few nits below.

cds.h

56: extra brackets in comments

dynamicArchive.cpp

727: Is it possible to add here a comment explaining 3 * 
MetaspaceShared::region_alignment()

metaspaceShared.cpp

237:  "on Linux/aarch64," - twice in comments

242:  if  is_power_of_2 is not necessary here

-Dmitry


On 31.01.20 00:02, Ioi Lam wrote:
> 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
>