RFR (S) 8024517: runtime/CDSCompressedKPtrs/XShareAuto.java failed with RuntimeException

[Coleen Phillimore]

Hi David,

On 09/16/2013 11:40 PM, David Holmes wrote:
> On 17/09/2013 12:53 AM, harold seigel wrote:
>> Hi David,
>>
>> Thanks for the review.
>
> Only "attempted review" at that stage :)
>
>> My proposed fix maintains the current behavior for the client VM, which
>> is that CDS is off by default if running on a server class machine.
>> (Fwiw: the test for server class machine (os::is_server_class_machine())
>> usually returns FALSE for c1 because NeverActAsServerClassMachine
>> defaults to TRUE for c1 for arm, x86, ppc, and Sparc.)
>>
>> The fix only changes the behavior for -server.  Previously, CDS was off
>> by default for -server only if the JVM was running on a server class
>> machine.  With this change, CDS will always be off by default when
>> -server is specified.  The fix implements this change by no longer
>> checking for a  server class machine when running the server VM.
>
> Okay I think I have it now. This all seems rather convoluted - not 
> sure why we care about server-class-machine to make a CDS choice?

We don't care about server class machine.  We want to turn CDS off by 
default for -server because it degrades performance.  This 
server-class-machine means -server almost all of the time but in this 
corner case didn't mean -server.   So the gist of Harold's change is to 
turn it off for -server, which we want.

Looking at Harold's change, we should just turn off CDS by default (ie. 
unless specified) under #ifdef COMPILER2 and not check for server class 
machine for the client compiler.

>
>
> That aside what do we document about the CDS behaviour? Does this 
> change need CCC request and/or updates to docs?

I think it's not an interesting corner case and not really intended as a 
change in behavior.  It's what we assumed it was doing in the first place.

I can't comment on the test right now.

Coleen
>
>> The test failure reported in the bug occurred because the test was run
>> with -server on a non server class machine and CDS was not disabled as
>> expected.
>
> Ok. Not sure why the test is 64-bit only though and why does it refer 
> to ObjectAlignmentInBytes? Is this a copy/n/paste error when the test 
> was created? I see this under the "compressed Kptrs" which is 
> presumably 64-bit only, but the tests doesn't see 64-bit related ??
>
> Thanks,
> David
>
>> Thanks, Harold
>>
>> On 9/14/2013 3:41 AM, David Holmes wrote:
>>> Hi Harold,
>>>
>>> Sorry I am a bit confused about the problem and solution. CDS should
>>> be off when using the server VM - right? But this code:
>>>
>>> ! #ifndef COMPILER2
>>> !   if (is_server_class_mach)
>>> ! #endif
>>>
>>> only affects the client VM, so you seem to be saying that CDS should
>>> be off on a client VM when running on a server class machine ???
>>>
>>> David
>>>
>>> On 14/09/2013 5:05 AM, harold seigel wrote:
>>>> Hi,
>>>>
>>>> Please review this small fix for bug 8024517.  This test failed 
>>>> when run
>>>> on a small system because CDS was not disabled by default. Although 
>>>> the
>>>> test was run with '-server', CDS was not disabled because function
>>>> os::is_server_class_machine() returned FALSE.  This fix disables 
>>>> CDS by
>>>> default if either os::is_server_class_machine() returns TRUE or the
>>>> server compiler is running.
>>>>
>>>> Also, the '-server' option was added to the test's '-Xshare:dump'
>>>> command to ensure that the CDS region gets created by the same JVM 
>>>> that
>>>> tries to read it.
>>>>
>>>> The change was tested with the XShareAuto.java test, by hand using the
>>>> NeverActAsServerClassMachine and AlwaysActAsServerClassMachine 
>>>> options,
>>>> the UTE vm quicktests, and JPRT.
>>>>
>>>> Open webrev at: http://cr.openjdk.java.net/~hseigel/bug_8024517/
>>>> <http://cr.openjdk.java.net/%7Ehseigel/bug_8024517/>
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8024517
>>>>
>>>> Thanks! Harold
>>