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

[Dmitry Samersoff]

Hello Ioi,

Sorry for delay. I'll test it early next week.

-Dmitry


On 14.02.20 00:59, Ioi Lam wrote:
> Hi Dmitry,
>
> Thanks for the review. Have you tried the patch on your 4k/64k page 
> size hosts to see if the CDS archives are compatible with each other?
>
> Here's the latest version to address comments from you and Calvin:
>
> http://cr.openjdk.java.net/~iklam/jdk15/8236847-SharedRegionAlignment.v03/ 
>
> http://cr.openjdk.java.net/~iklam/jdk15/8236847-SharedRegionAlignment.v03-delta/ 
>
>
>
> On 2/10/2020 12:34 AM, Dmitry Samersoff wrote:
>> Hello Ioi,
>>
>> The changes looks good for me, besides few nits below.
>>
>> cds.h
>>
>> 56: extra brackets in comments
>>
>
> Fixed.
>
>> dynamicArchive.cpp
>>
>> 727: Is it possible to add here a comment explaining 3 * 
>> MetaspaceShared::region_alignment()
>
> I changed the code like this:
>
>   size_t total = estimate_archive_size();
>   // We may need to round up the size of each core region for proper 
> alignment.
>   total += MetaspaceShared::num_core_region * 
> MetaspaceShared::region_alignment();
>
>
>>
>> metaspaceShared.cpp
>>
>> 237:  "on Linux/aarch64," - twice in comments
>>
>
> Fixed
>
>> 242: if  is_power_of_2 is not necessary here
>>
>
> Removed.
>
> Thanks
> - Ioi
>> -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
>>>
>