VarHandle instance methods performance

[Martin Buchholz]

There's a good chance COWAL access to the array can be optimized as you
suggest using VarHandles.
Write using release; read using acquire, or plain if holding the lock.
Doesn't buy much on x86 but performance improvement should be measurable on
ppc.

On Wed, Apr 24, 2019 at 10:33 PM Frank Yuan <frank.yuan at oracle.com> wrote:

> >
> > Hi Frank,
> > a VarHandle is a glorified integer value that correspond to the number
> of bytes from a pointer to a field address,
> > so you give it a reference and you have access to the field at the
> address: reference + shift, given that the shift value is the same for all
> the
> > instances of the same class,
>
> I know, but I thought they were ready once the VarHandle instance was
> constructed...
>
> Actually, I was looking up some way to improve library performance by
> VarHandle API.
> For example, CopyOnWriteArrayList uses a volatile variable referring to
> Object[] array, the writing procedure updates the volatile variable once it
> makes a new array completely, and then the reading procedure can read the
> new array by the volatile variable. Here, I think in theory, we can replace
> the volatile reading by getOpaque, which only requires the program does
> read the variable from the memory.
>
> But the actual performance of getOpaque is not better than volatile
> reading due to the overhead(with using static VH instance), I have to hold
> on my idea now.
>
> Thanks
> YC
>
>
> > it should be declared static (and final so it's a constant for the VM).
> >
> > Rémi
> >
> > ----- Mail original -----
> > > De: "Frank Yuan" <frank.yuan at oracle.com>
> > > À: "Aleksey Shipilev" <shade at redhat.com>
> > > Cc: "core-libs-dev" <core-libs-dev at openjdk.java.net>
> > > Envoyé: Mercredi 24 Avril 2019 12:11:11
> > > Objet: RE: VarHandle instance methods performance
> >
> > >> On 4/24/19 11:51 AM, Frank Yuan wrote:
> > >> > My test code is as below:
> > >> > ...
> > >> >     final VarHandle vhf;
> > >> >     final VarHandle vhvf;
> > >> > ...
> > >>
> > >> Make these two "static final", initialize them in class initializer,
> then try
> > >> again.
> > >>
> > >> VarHandle (like Atomic*FieldUpdaters, MethodHandle, etc), have
> internal checks
> > >> that can only be
> > >> folded away when the VH/MH/A*FU instance is constant.
> > >>
> > >
> > > Thank you, it works!
> > >
> > > YC
> > >
> > > > -Aleksey
>
>