RFR: 8191821: Finer granularity for GC verification

sangheon.kim sangheon.kim at oracle.com
Thu Nov 30 17:52:19 UTC 2017

Hi Stefan,

On 11/29/2017 07:43 AM, Stefan Johansson wrote:
> Thanks for reviewing Thomas,
> Updated webrevs:
> Inc:  http://cr.openjdk.java.net/~sjohanss/8191821/01-02/
> Full: http://cr.openjdk.java.net/~sjohanss/8191821/02/
02 version looks good.


> Comments inline.
> On 2017-11-29 12:16, Thomas Schatzl wrote:
>> Hi,
>> On Tue, 2017-11-28 at 17:25 +0100, Stefan Johansson wrote:
>>> Hi,
>>> Please review this change for enhancement:
>>> https://bugs.openjdk.java.net/browse/JDK-8191821
>>> Webrev:
>>> http://cr.openjdk.java.net/~sjohanss/8191821/00/
>>> Summary:
>>> Heap verification is a very good way to track down GC bugs, but it
>>> comes
>>> with a lot of overhead. Using VerifyBeforeGC, VerifyDuringGC and
>>> VerifyAfterGC often slows down the execution more than is needed
>>> since
>>> we sometimes only want to verify certain types of GCs. This change
>>> adds
>>> this feature for G1 by adding a new diagnostic flag VerifyGCType.
>>> The
>>> new flag currently only works with G1 but can easily be added for
>>> more
>>> GCs if needed. The type of the flag is ccstrlist which means the
>>> option
>>> can be used multiple times to allow more than one type to be
>>> verified.
>>> The types available for G1 is, young, mixed, remark, cleanup and
>>> full.
>>> If the flag is not specified all GCs are verified.
>>> Note that Verify/Before/After/During/GC is still needed to decide
>>> what
>>> to verify, VerifyGCType only describes when.
>>> Testing:
>>> * Added new Gtest for G1HeapVerifier functionality.
>>> * Added new Jtreg test for basic command line functionality.
>>> * Executed Jtreg tests through mach5 to make sure it passes on all
>>> platforms.
>>    - I would like to see a special verification type for initial mark
>> gcs, since these are part of the concurrent cycle. I mean, the change
>> does not allow to verify the whole concurrent cycle alone without
>> enabling verification for *all* young-only gcs.
> Fixed.
>>    - please change G1VerifyYoung to G1VerifyYoungOnly. Mixed gcs are
>> also young gcs. We should be exact in our internal naming.
> Fixed.
>>    - in g1HeapVerifier.hpp consider aligning the flag values below each
>> other.
> Fixed.
>>    - gcArguments.cpp:89: the warning message should print the given
>> type. Not in "VerifyGCType %s", as that would be confusing, potentially
>> only indicating that "type" is not supported.
> Updated per Poonams request to only be printed once and with this I 
> think I can be left as is.
>>    - gcArguments.cpp:109: are these really the only delimiters for
>> arguments. Not sure if e.g. \t is also a delimiter. Isn't there some
>> existing argument processing method available (or at least this
>> constant) elsewhere?
> So I basically used the same that were available for VerifySubSet but 
> also added \n since this is the delimiter added if the flag is used 
> multiple times. I think the expected way to use ccstrlist is to have 
> one value per usage like:
> -XX:VerifyGCType=full -XX:VerifyGCType=young
> But since VerifySubSet allows comma separated values I decided to do 
> so as well. I people start using tabs I'm not sure we want to support 
> that. I would then rather just support "\n" and have one value per flag.
>>    - gcArguments.hpp:52: maybe some documentation what this is supposed
>> to do, and that the accepted types are GC specific.
> Fixed.
>>    - globals.hpp:2276: the list of available types is not complete
>> (remark, cleanup?). Also there is no indication that the supported
>> types are collector specific. Or just leave them out here, and document
>> the available strings in the collector's parsing code header file (in
>> the enum?)
> Good point, will remove this listing and update the text.
>>    - TestVerifyGCType.java: instead of using a GarbageProducer you can
>> directly trigger specific GCs using whitebox. This would make the tests
>> a lot more specific imho.
> Agreed and fixed. Also added test for mixed and initial-mark now.
> Thanks,
> Stefan
>> Thanks,
>>    Thomas

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