RFR (S) 8211980: Remove ThreadHeapSampler enable/disable/enabled methods

David Holmes david.holmes at oracle.com
Thu Oct 11 08:10:20 UTC 2018

On 11/10/2018 3:10 AM, Hohensee, Paul wrote:
> There used to be a rule against using “namespace”. Don’t know if that’s 
> still true or not: I believe it is. Hence the “static const <whatever>”.
> You only need OrderAccess if there could be a race on accesses to 
> whatever you’re guarding. Looks like the old code wanted to make sure 
> that log_table was initialized and its content available to other 
> threads before the _enabled flag was set. I.e., the _enabled flag acted 
> as a store visibility barrier for log_table, and possibly other 
> ThreadHeapSampler data structures. If no thread can ever see a partially 
> initialized log_table, then no need for ordering. The old code used a 
> MutexLocker in init_log_table(), which former has a fence in its 
> destructor and probably (haven’t dived into the code) guards all 
> accesses to log_table, so the release_store() on _enabled in enable() 
> was redundant.

The release_store and load_acquire were necessary to ensure the 
lock-free enabled() check ensured visibility of the initialization of 
the data structures in the ensuing code. Otherwise you'd need to grab 
the lock on the enabled() checks, which is too heavy-weight. The lock is 
only used to ensure single-threaded initialization of the log_table, 
actual accesses are again lock-free.


  Same with the release_store() in disable(), unless there
> was some reason to make sure all threads saw previous stores to 
> ThreadHeapSampler related memory before _enabled was set to zero. The 
> load_acquire in enabled() may not have been needed either, because it 
> only prevents subsequent loads from being executed before the load from 
> _enabled, so if _enabled was being used to guard access only to 
> ThreadHeapSampler data such as log_table, the release_store() on 
> _enabled would guarantee that all necessary stores would be done before 
> _enabled was set to one and seen by enabled().
> Yeah, that’s hard to follow, and I wrote it. :) It comes down to what 
> you’re guarding with OrderAccess. If it’s only ThreadHeapSampler data, 
> and since only a Thread has one, and since ThreadHeapSampler statics are 
> initialized before construction of the first _heap_sampler, and since 
> the construction of a Thread is guarded by multiple mutexes which will 
> force visibility of any ThreadHeapSampler statics before a Thread is 
> used, you don’t need OrderAccess.
> I’d put anything to do with ThreadHeapSampler into its class definition 
> rather than define them at file scope in threadHeapSampler.cpp. I.e., 
> all of FastLogNumBits, FastLogMask, and internal_log_table (and name it 
> back to that log_table). File scope data is a no-no.
> Hope this helps,
> Paul
> *From: *serviceability-dev <serviceability-dev-bounces at openjdk.java.net> 
> on behalf of JC Beyler <jcbeyler at google.com>
> *Date: *Tuesday, October 9, 2018 at 11:58 PM
> *To: *"serviceability-dev at openjdk.java.net" 
> <serviceability-dev at openjdk.java.net>
> *Subject: *RFR (S) 8211980: Remove ThreadHeapSampler 
> enable/disable/enabled methods
> Hi all,
> When talking with Serguei about JDK-8201655 
> <https://bugs.openjdk.java.net/browse/JDK-8201655>, we talked about why 
> ThreadHeapSampler has an enabled/disabled when we could have just used 
> the should_post_sampled_object_alloc to begin with.
> Could I get a review for this:
> Webrev: http://cr.openjdk.java.net/~jcbeyler/8211980/webrev.00/ 
> <http://cr.openjdk.java.net/%7Ejcbeyler/8211980/webrev.00/>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8211980
> This passed my testing on my dev machine in release and fastdebug.
> The question I would like to raise here at the same time (in order to 
> reduce email spam and because it should be included in the review I 
> believe) is:
>    - When I did the enable/disable, I used OrderAccess to do so after a 
> reviewer asked for it
>    - From what I can tell, JVMTI_SUPPORT_FLAG does not use it and does 
> instead:
> #define JVMTI_SUPPORT_FLAG(key)                                           \
>    private:                                                                \
>    static bool  _##key;                                                    \
>    public:                                                                 \
>    inline static void set_##key(bool on) {                                 \
>      JVMTI_ONLY(_##key = (on != 0));                                       \
>      NOT_JVMTI(report_unsupported(on));                                    \
>    }                                                                       \
>    inline static bool key() {                                              \
>      JVMTI_ONLY(return _##key);                                            \
>      NOT_JVMTI(return false);                                              \
>    }
> Should it (ie in a future bug/webrev)?
> Thanks,
> Jc

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