RFR (S) 8211980: Remove ThreadHeapSampler enable/disable/enabled methods
JC Beyler
jcbeyler at google.com
Thu Oct 11 22:20:17 UTC 2018
I'm 100% in agreement with David :)
Inlined are my comments:
On Thu, Oct 11, 2018 at 3:13 PM David Holmes <david.holmes at oracle.com>
wrote:
> On 12/10/2018 3:44 AM, Hohensee, Paul wrote:
> > So, given that the lock was only used to protect log_table
> initialization, and that the patch moves that into the C++ "class
> initializer" which is run when the first Thread object is constructed, we
> don't need any locking/memory ordering anymore, right?
>
> Right so:
>
> - ThreadHeapSampler_lock can be removed
> - The load-acquire/release-store of _sampling_interval seem to serve no
> purpose, but _sampling_interval should at least be marked volatile (from
> a documentation perspective if nothing else). If the intent was for a
> change in sampling_interval to be immediately visible then a fence()
> would be needed.
>
Right, I would leave them in place (or at least postpone the conversation
about them to a later webrev if someone feels really strongly about them; I
believe it does not hurt, this will never be critical code but at least is
semantically safe).
>
> The _log_table_initialized flag is not needed from the perspective of an
> initialization check - you can't run code until after the static
> initializers have run (though I'm unclear how C++ manages this from a
> concurrency perspective). But the flag may be needed just as a means to
> separate the allocation of the table from the initialization of it -
> again I'm unclear how C++ static initialization works in detail (you
> have to be very careful with such initialization to ensure there are
> zero dependencies on anything done as part of VM initialization).
>
>
Exactly. I cannot force the initialization of a static array via a method
without initialization *something*. It can be to initialize a pointer to
the array and I just use the pointer in the class; that was what the first
webrev one was doing. Using a boolean such as here seems to be the lesser
evil and allows us to add an assert that all is well in the world at the
only usage point of the array in assert mode.
So I'm happy with the current form (I'm biased since I sent the webrev for
review :-)), any LGTM or other comments?
Jc
David
>
> > Paul
> >
> > On 10/11/18, 4:11 AM, "David Holmes" <david.holmes at oracle.com> wrote:
> >
> > 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.
> >
> > David
> >
> > 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
> > >
> >
> >
>
--
Thanks,
Jc
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