RFR (round 3), JDK-8214259: Implementation: JEP 189: Shenandoah: A Low-Pause Garbage Collector (original) (raw)

Per Liden per.liden at oracle.com
Fri Nov 30 09:34:19 UTC 2018

Hi Roman,

On 11/30/18 10:09 AM, Roman Kennke wrote:

Hi Per,

Thanks for taking time to look at this! We will take care of all the issues you mentioned. Regarding the flags, I think they are useful as they are now. Shenandoah can be run in 'passive' mode, which doesn't involve any concurrent GC (iow, it runs kindof like Parallel GC). In this mode we can selectively turn on/off different kinds of barriers. This is useful: - for debugging. if we see a crash and we suspect it's caused by a certain type of barrier, we can turn on/off those barriers to narrow down - for performance experiments: passive w/o any barriers give a good baseline against which we can measure impact of types of barriers. Debugging may or may not be useful in develop mode (some bugs only show up in product build), performance work quite definitely is not useful in develop builds, and therefore I think it makes sense to keep them as diagnostic, because that is what they are: diagnostic flags. Makes sense?

I can see how these flags can be useful. But I think you might be in violating-spec-territory here, if you're allowing users to turn on flags that crash the VM. If they are safe to use in 'passive' mode, then I think there should be code in shenandoahArguments.cpp that rejects/corrects flag combinations that are unstable.

I think the correct way to think about this is: We should pass the TCK, regardless of which features are enabled/disabled (unless the VM barfs at start-up because the chosen combination of flags are incompatible or isn't supported in the current environment, etc).

CC:ing Mikael here, who might be able to shed some light on what's ok and not ok here.

cheers, Per

Thanks, Roman

On 11/29/18 9:41 PM, Roman Kennke wrote: [...] http://cr.openjdk.java.net/~rkennke/shenandoah-upstream/03/ Some comments (I didn't look at the compiler part):

src/hotspot/share/gc/shared/barrierSetConfig.hpp src/hotspot/share/gc/shared/barrierSetConfig.inline.hpp ------------------------------------------------------- Please insert the Shenandoah lines so that the lists remain alphabetically sorted on GC names. src/hotspot/share/gc/shared/gcCause.cpp --------------------------------------- Add the Shenandoah stuff after dcmdgcrun to match the order in the header file. src/hotspot/share/gc/shared/gcConfig.cpp ---------------------------------------- Please insert the Shenandoah lines so that the lists remain alphabetically sorted on GC names. Only the last change is in the right place now. src/hotspot/share/gc/shared/gcTrace.hpp --------------------------------------- Please move this new class to a gc/shenandoah/shanandoahTrace.hpp file (or something similar). src/hotspot/share/gc/shared/gcglobals.hpp src/hotspot/share/gc/shared/vmStructsgc.hpp ------------------------------------------ Please insert the INCLUDESHENANDOAHGC/SHENANDOAHGCONLY stuff so that the lists remain alphabetically sorted on GC names. src/hotspot/share/utilities/globalDefinitions.hpp ------------------------------------------------- I think you want to call the new macro UINT64FORMATXW, to maintain similarity to it's sibling UINT64FORMATX. Also please adjust the indentation/alignment so the list of macros remains nicely structured. src/hotspot/share/utilities/macros.hpp -------------------------------------- Please insert the Shenandoah lines so that the lists remain alphabetically sorted on GC names. src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/gc/shenandoah/ShenandoahHeap.java src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/gc/shenandoah/ShenandoahHeapRegion.java ------------------------------------------------------------------------------- Since the heap walking is now disabled, it looks like there's quite a bit of code here that can be removed. test/hotspot/jtreg/* -------------------- As Aleksey and I discussed in a separate thread. Using 'vm.gc.Shenandoah' does not mean the same thing as 'vm.gc == "Shenandoah"', and you typically want to use 'vm.gc == "Shenandoah"' in most cases (I did't check all), like in tests that aren't Shenandoah specific. src/hotspot/share/gc/shenandoah/shenandoahglobals.hpp ------------------------------------------------------ I don't think you want to expose the following flags. Setting any of them to false will crash the VM, right? If you really want to keep them I'd suggest you make them develop-flags.  _346   diagnostic(bool, ShenandoahSATBBarrier, true,          _  _347           "Turn on/off SATB barriers in Shenandoah")          _  _348          _  _349   diagnostic(bool, ShenandoahKeepAliveBarrier, true,          _  _350           "Turn on/off keep alive barriers in Shenandoah")          _  _351          _  _352   diagnostic(bool, ShenandoahWriteBarrier, true,          _  _353           "Turn on/off write barriers in Shenandoah")          _  _354          _  _355   diagnostic(bool, ShenandoahReadBarrier, true,          _  _356           "Turn on/off read barriers in Shenandoah")          _  _357          _  _358   diagnostic(bool, ShenandoahStoreValEnqueueBarrier, false,          _  359           "Turn on/off enqueuing of oops for storeval barriers") __  _360          _  _361   diagnostic(bool, ShenandoahStoreValReadBarrier, true,          _  362           "Turn on/off store val read barriers in Shenandoah") __  _363          _  _364   diagnostic(bool, ShenandoahCASBarrier, true,          _  _365           "Turn on/off CAS barriers in Shenandoah")          _  _366          _  _367   diagnostic(bool, ShenandoahAcmpBarrier, true,          _  _368           "Turn on/off acmp barriers in Shenandoah")          _  _369          _  _370   diagnostic(bool, ShenandoahCloneBarrier, true,          _  _371           "Turn on/off clone barriers in Shenandoah")          _ cheers, Per

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