Frequently dereferenced weak references never get cleared

[Peter Booth]

Michal,

I read your email with interest and my immediate reaction was “you’ve done well”. If I understand you correctly, you have built, and successfully used for over a decade, a framework that depends upon the prompt collection of weakly referenced objects. My strong reaction was because I hadn’t realized such a thing was practical. About a dozen years ago I’d concluded that weak references were much less useful than they sounded, because they seemed to require an additional GC cycle to be collected compared to unreferenced objects. Perhaps this wasn’t an obstacle in your scenario?

Regardless, my knee-jerk reaction when I read your message was “why don’t they change their framework to remove it’s use of weak references?” 
I’ve used C4 for about seven years now, both on Vega hardware and the Zing product, and I’ve enjoyed the benefits. I’m currently working on apps that use CMS and G1 and it feels like a step backwards. ZGC is an interesting alternative, in theory.

Peter




Sent from my iPhone

> On Mar 14, 2019, at 5:26 PM, Ing. Michal Frajt (Luxonit s.r.o.) <michal.frajt at luxonit.com> wrote:
> 
> Hi all,
> 
> 
> 
> We are evaluating ZGC for financial applications and so far have seen
> promising results. However there seems to be an issue with handling of weak
> references that is affecting our custom data distribution framework.
> 
> 
> 
> Since 2005 we are mainly using the CMS collector for financial applications
> based on a custom data distribution framework. The framework is based on
> weak references used similar way as smart pointers in C++. Application code
> holds strong references where framework provides all data via weak
> references only. A weak reference clear acts as an indication for the
> framework that data is not required by the application code anymore. The
> framework provided data are always coupled with a network resource
> (receiving market updates) or CPU resource (computing
> aggregations/risk/etc). The framework has huge interest to get the weak
> reference cleared by the GC as it can deregister network subscription (less
> data to read, less data to parse and handle) or stop CPU intensive
> computations. As long as the weak reference is not cleared (reported via the
> reference queue) the framework must permanently update provided data which
> simply requires to dereference weak references regularly. Such dereferencing
> happens on an average once per second for each weakly referenced object. 
> 
> 
> 
> In order to get the weak references cleared regularly we were using the
> incremental CMS (iCMS) which was always running in the background and
> scanning the complete heap. When the iCMS got announced to be deprecated we
> started working on a Hotspot extension and introduced the new
> CMSTriggerInterval parameter into 8u40
> (https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8038265) to allow to
> specify a maximum time between CMS collections. This way all our weak
> references (which are not strongly referenced) are cleared within one or two
> CMS iterations invoked by the CMSTriggerInterval or by another reason
> (occupancy or explicit concurrent GC invoke for example). Usually we
> configure the CMSTriggerInterval between 5 and 10 minutes.
> 
> 
> 
> Some years back (2012) we evaluated Azul C4 collector and recognized an
> issue with the weak references processing. After some discussion we were
> finally told by Gil Tene that C4 will never clear a weak reference if
> dereferenced within the C4 major cycle (that time around each 5 minutes). We
> were offered a C4 extension where a weak reference might be dereferenced but
> the strong reference cannot be stored to any object, only kept at the
> thread/call stack level. If stored, even for a short moment, it will again
> require the full C4 major cycle without dereferencing it to get it cleared.
> Because of this and other reasons our client finally did not purchased the
> Azul C4 collector. We don't know how the C4 is addressing this issue in the
> current implementation. 
> 
> 
> 
> Today we got very pleased with the ZGC initial testing results. The test
> scenario with CMS has ParNew STW 150ms every 10 seconds. It changed to 1ms
> only every 5 minutes when using ZGC (if understanding the reporting
> correctly). We immediately tested the weak references processing and
> unfortunately observed the same behaviour as with the C4 collector. It seems
> that weak references which are frequently dereferenced are never cleared.
> Invoking the explicit concurrent GC by the jcmd GC.run does not help either.
> 
> 
> 
> Could you please explain us the ZGC weak references handling related to the
> described scenario? Is there a way out for us? 
> 
> 
> 
> Your input would be much appreciated.
> 
> 
> 
> Best regards, 
> Michal Frajt
> Luxonit
> 
> 
>