Re: [HACKYSTAT-DEV-L] Weak/Soft/Phantom References

Tue, 19 Sep 2006 12:54:24 -0700

My bad. It's already SoftReference. I don't why I had the pre-conception that WeakHashMap was used in ThreeKeyCache implementation.
I will probably do some research on the internal implementation of Sun GC on my 18-hour trip from here to NY, and report back if I have any useful finding. 


By the way, GC in client and server JVM behaves differently. On client 
JVM, it's supposed to do small memory sweeps frequently in order to hide 
the latency from a user.


Cheers,

Cedric



Philip Johnson wrote:

Hmm... did I inadvertantly type 'weak' reference somewhere?  That's a mistake. 
The only non-strong reference (in the ThreeKeyCache class, anyway) is a soft 
reference. And even soft references were definitely reclaimed too fast, at 
least in my trials under JDK 1.4 this summer when I was developing the 
FileMetric DPD redesign.

What I've been reading recently in various articles is:

(a) there aren't supposed to be major differences between 1.4 and 1.5 with 
respect to this issue; and
(b) The JDK 1.5 GC does a better job of 'waiting' to reclaim SoftReferences 
than prior GCs.


When I eventually get some spare cycles, I will profile Hackystat and figure out what's currently the case. 


Cheers,
Philip

----- Original Message -----
From: "(Cedric) Qin ZHANG" <[EMAIL PROTECTED]>
Date: Monday, September 18, 2006 10:57 pm
Subject: [HACKYSTAT-DEV-L] Weak/Soft/Phantom References
To: [email protected]


  

Philip mentioned that Java GC reclaims weakly referenced objects 
too 
fast, often times before the cached object could be reused. 
According to 
the IBM article listed below, it seems that it's just a matter of 
replacing WeakReference with SoftReference in Hackystat code. I am 
quite 
interested in knowing why there need to be more than one type of 
non-strong reference in Java. Do we have any Java GC expert on this 
list 
that can probably shed light on this topic?


Thanks.

Cedric



Philip Johnson (JIRA) wrote:

    

    [ http://hackydev.ics.hawaii.edu:8080/browse/HACK-

      

796?page=comments#action_11837 ] 
    

Philip Johnson commented on HACK-796:
-------------------------------------


I found a couple more links that might of use when doing further 
      

research on this issue:

    

Shows a soft reference based hashmap:


      

http://www.javaspecialists.co.za/archive/newsletter.do?issue=098&locale=en_US>

    

An article about soft references:
http://www-128.ibm.com/developerworks/java/library/j-jtp01246.html


A bug report indicating that maybe soft references don't get GC'd 
      

so fast in JDK 1.5:

    

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6349930



  
      

NullPointerException in DailyProjectFileMetrics
-----------------------------------------------

         Key: HACK-796
         URL: http://hackydev.ics.hawaii.edu:8080/browse/HACK-796
     Project: Hackystat
        Type: Bug

    
        

  
      

    Reporter: Qin Zhang
    Assignee: Philip Johnson
    Priority: Critical
     Fix For: 7.5

    
        

  
      

To reproduce: use telemetry wall to hit the server hard.  It 
        

seems that DailyProjectFileMetric is not written in a thread-safe way.

    

09/17 13:13:18 DailyProjectFileMetric failed to initialize!
Hackystat-7 09-Sep-2006
java.lang.NullPointerException
        at java.util.LinkedList.remove(LinkedList.java:647)
        at java.util.LinkedList.removeLast(LinkedList.java:144)

        at 
        

org.hackystat.core.kernel.cache.ThreeKeyCache.put(ThreeKeyCache.java:252)>>     
    at 
org.hackystat.core.kernel.sensordata.SensorDataCache.getSensorData(SensorDataCache.java:48)

    

        at 
        

org.hackystat.sdt.filemetric.dailyproject.FileMetricRawDataMap.<init>(FileMetricRawDataMap.java:91)>>
         at 
org.hackystat.sdt.filemetric.dailyproject.DailyProjectFileMetric.initialize(DailyProjectFileMetric.java:117)

    

        at 
        

org.hackystat.sdt.filemetric.dailyproject.DailyProjectFileMetric.getAllFileTypes(DailyProjectFileMetric.java:152)>>
         at 
org.hackystat.sdt.filemetric.reducer.LanguageFileMetricReducer.getAllFileTypes(LanguageFileMetricReducer.java:101)

    

        at 
        

org.hackystat.sdt.filemetric.reducer.LanguageFileMetricReducer.compute(LanguageFileMetricReducer.java:72)>>
         at 
org.hackystat.core.telemetry.reducer.TelemetryReducerManager.compute(TelemetryReducerManager.java:141)

    

        at 
        

org.hackystat.core.telemetry.evaluator.TelemetryEvaluator.resolveReducerCall(TelemetryEvaluator.java:380)>>
         at 
org.hackystat.core.telemetry.evaluator.TelemetryEvaluator.resolveFunctionCall(TelemetryEvaluator.java:322)

    

        at 
        

org.hackystat.core.telemetry.evaluator.TelemetryEvaluator.resolveExpression(TelemetryEvaluator.java:285)>>
         at 
org.hackystat.core.telemetry.evaluator.TelemetryEvaluator.evaluate(TelemetryEvaluator.java:56)

    

        at 
        

org.hackystat.core.telemetry.evaluator.TelemetryEvaluator.evaluate(TelemetryEvaluator.java:140)>>
         at 
org.hackystat.core.telemetry.evaluator.TelemetryEvaluator.evaluate(TelemetryEvaluator.java:259)

    

        at 
        

org.hackystat.core.telemetry.analysis.TelemetryReportAnalysis.prepareReportForJspRendering(TelemetryReportAnalysis.java:133)>>
         at 
org.hackystat.core.telemetry.analysis.TelemetryExpertAnalysis.process(TelemetryExpertAnalysis.java:146)

    

        at 
        

org.hackystat.core.kernel.mvc.Controller.doPost(Controller.java:271)>>         
at org.hackystat.core.kernel.mvc.Controller.doGet(Controller.java:300)

    

        at 
        

javax.servlet.http.HttpServlet.service(HttpServlet.java:689)>>      
  at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
    

        at 
        

org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)>>
         at 
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)

    

        at 
        

org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)>>
         at 
org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)

    

        at 
        

org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)>> 
        at 
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)

    

        at 
        

org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)>>
         at 
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)

    

        at 
        

org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)>>    
     at 
org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)

    

        at 
        

org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)>>
         at 
org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)

    

        at 
        

org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)>>
         at java.lang.Thread.run(Thread.java:595)

    

    
        

  
      






















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