Design question: why not reuse the CMS slots in mark word?

[Christine Flood]

Make sure you don't break object serialization.

I really think that incremental update (adding the post write barrier value to the marking set instead of SATB) would be a better bang for your buck.


Christine


----- Original Message -----
> From: "Roman Kennke" <rkennke at redhat.com>
> To: "Christine Flood" <chf at redhat.com>
> Cc: "Aleksey Shipilev" <shade at redhat.com>, shenandoah-dev at openjdk.java.net
> Sent: Tuesday, September 20, 2016 8:15:26 AM
> Subject: Re: Design question: why not reuse the CMS slots in mark word?
> 
> There's a well defined way to access the heap and objects, it's called
> JVMTI and JNI. They give you access to anything that such tools would
> ever need, I think. Those are under our control.
> 
> Do you really think there are tools out there that do something more
> esotheric like directly accessing memory to inspect objects (I want to
> know them..)? It sounds to me like they already subscribed to a world
> of unnecessary pain. Why should I care to make their world less
> painful? :-D ;-) :-P
> 
> Roman
> 
> Am Dienstag, den 20.09.2016, 07:43 -0400 schrieb Christine Flood:
> > I wouldn't.
> > 
> > There are a bazillion third party heap walking tools that assume they
> > know the object layout.
> > Even if you got it working inside of Hotspot, the villagers would
> > come for you with pitchforks and flaming torches.
> > 
> > 
> > 
> > Christine
> > 
> > 
> > ----- Original Message -----
> > > 
> > > From: "Roman Kennke" <rkennke at redhat.com>
> > > To: "Christine Flood" <chf at redhat.com>, "Aleksey Shipilev" <shade at r
> > > edhat.com>
> > > Cc: shenandoah-dev at openjdk.java.net
> > > Sent: Tuesday, September 20, 2016 7:37:27 AM
> > > Subject: Re: Design question: why not reuse the CMS slots in mark
> > > word?
> > > 
> > > Am Dienstag, den 20.09.2016, 07:28 -0400 schrieb Christine Flood:
> > > > 
> > > > Here's some recent work on the cost of barriers in GC.
> > > > 
> > > > http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.231.3040
> > > > &rep
> > > > =rep1&type=pdf
> > > > 
> > > > You can see that a conditional read barrier is three times as
> > > > expensive as a non-conditional read barrier.  Roman didn't
> > > > believe me
> > > > and insisted on testing it for himself and found the same result
> > > > empirically ;-)
> > > 
> > > Yep :-)
> > > 
> > > Notice that this also precludes any form of side-table based
> > > lookup.
> > > 
> > > All that said, I would love to see the brooks ptr 'inlined' in the
> > > regular object layout. It may not actually be *that* complex
> > > assuming
> > > all code nicely respects the various base-offsets of object layout
> > > (and
> > > they must, right?) My idea would be to insert the brooks ptr right
> > > before the object/array fields start (i.e. after the klass*). This
> > > puts
> > > it 2 words nearer to them and thus make it more likely to warm um
> > > cache
> > > for following field accesses. Plus, we'd loose less space with
> > > compressed oops because we could squeeze a compressed brooks ptr
> > > into
> > > the unused 32bits of compressed klass* for regular objects (non-
> > > arrays), and otherwise benefit from alignment.
> > > 
> > > I think I'll spend a day or two exploring that idea a little more.
> > > 
> > > Roman
> > > 
> > > > 
> > > > 
> > > > 
> > > > Christine
> > > > 
> > > > 
> > > > ----- Original Message -----
> > > > > 
> > > > > 
> > > > > From: "Aleksey Shipilev" <shade at redhat.com>
> > > > > To: "Christine Flood" <chf at redhat.com>
> > > > > Cc: shenandoah-dev at openjdk.java.net
> > > > > Sent: Tuesday, September 20, 2016 4:53:36 AM
> > > > > Subject: Re: Design question: why not reuse the CMS slots in
> > > > > mark
> > > > > word?
> > > > > 
> > > > > On 09/19/2016 11:00 PM, Christine Flood wrote:
> > > > > > 
> > > > > > 
> > > > > > The theory was to make the read barrier as efficient as
> > > > > > possible.
> > > > > > It's always just a single read of a fixed offset.  If we
> > > > > > tried to
> > > > > > reuse the mark word we'd have to deal with locked objects and
> > > > > > hash
> > > > > > codes and all that nonsense potentially making read barriers
> > > > > > much
> > > > > > more expensive.
> > > > > > 
> > > > > > If you have a protocol that uses the mark word and doesn't
> > > > > > introduce
> > > > > > extra overhead in the common case (a conditional in a read
> > > > > > barrier
> > > > > > would be deadly) I'd love to hear it.
> > > > > 
> > > > > Okay, that makes sense, thanks.
> > > > > 
> > > > > Is this a hunch about the read barriers cost, or we actually
> > > > > tried
> > > > > more
> > > > > complicated read barriers? Trying to understand if potentially
> > > > > worth
> > > > > investigating, or this road was taken already.
> > > > > 
> > > > > Thanks,
> > > > > -Aleksey
> > > > > 
> > > > > 
> > > 
>