sun.nio.ch.DirectBuffer and jdk9/jigsaw
forax at univ-mlv.fr
Thu Feb 23 18:05:37 UTC 2017
I believe that what you are looking for is MethodHandles.byteBufferViewVarHandle  that creates a VarHandle that allows to do aligned/unaligned access on a ByteBuffer.
It should be a little slower that using Unsafe because of the bound check (and the nullcheck but it is usually removed by the VM).
That's said, unaligned access are faster with a VarHandle because sun.misc.Unsafe (which is stuck in the Java 8 world unlinke jdk.internal.misc.Unsafe) does not provide a way to do an unaligned access so you have do multiple read which is slower.
----- Mail original -----
> De: "Vitaly Davidovich" <vitalyd at gmail.com>
> À: "Uwe Schindler" <uschindler at apache.org>
> Cc: "jigsaw-dev" <jigsaw-dev at openjdk.java.net>
> Envoyé: Jeudi 23 Février 2017 18:05:36
> Objet: Re: sun.nio.ch.DirectBuffer and jdk9/jigsaw
> On Thu, Feb 23, 2017 at 11:54 AM, Uwe Schindler <uschindler at apache.org>
>> Why do you need the address at all in the Java code? Java code can use the
>> official ByteBuffer methods to access the memory you are wrapping. In Java
>> 9 that’s optimized very good by Hotspot and should be almost as fast as
>> array accesses (we proved that in Apache Lucene - congrats to the Hotspot
>> committers). If you need special access modes like volatile access, then
>> you can use Java 9's VarHandles. You can get a VarHandle to the backing
>> direct buffer using the MethodHandles API.
> I mentioned this upthread - the base address is used for index calculations
> to read/write data using Unsafe directly. I don't know about Java 9 as
> I've not tried it yet, but the generated assembly for using BB methods vs
> Unsafe did not favor BB. There are also various safety checks in
> DBB/Buffer internals that won't get optimized away.
> Also, my general experience with looking at C2 optimizations has led me to
> the conclusion that the optimizations are "unstable" - innocent code
> changes, differences in order of how classes are loaded, differences in
> what callsites trigger compilation first, and a bunch of other otherwise
> benign things can interfere with inlining decisions, which is typically the
> reason things go sideways in terms of optimization.
> As for MethodHandles and VarHandles, that's one possibility I'm considering
> as a way to migrate off using DirectBuffer (although I'd still like to know
> if there's any plan to standardize/formalize some notion of a direct
> buffer). However, my understanding is that using MH will still require me
> to crack into jdk code (to get access to the DBB and friends) and thus
> require addExports. DirectBuffer is still accessible if using addExports,
> but it's a wrinkle I was hoping to iron out, hence what started this email
>> Uwe Schindler
>> uschindler at apache.org
>> ASF Member, Apache Lucene PMC / Committer
>> Bremen, Germany
>> > -----Original Message-----
>> > From: jigsaw-dev [mailto:jigsaw-dev-bounces at openjdk.java.net] On Behalf
>> > Of Vitaly Davidovich
>> > Sent: Thursday, February 23, 2017 5:30 PM
>> > To: Chris Hegarty <chris.hegarty at oracle.com>
>> > Cc: jigsaw-dev <jigsaw-dev at openjdk.java.net>
>> > Subject: Re: sun.nio.ch.DirectBuffer and jdk9/jigsaw
>> > On Thu, Feb 23, 2017 at 11:10 AM, Chris Hegarty
>> > <chris.hegarty at oracle.com>
>> > wrote:
>> > >
>> > > > On 23 Feb 2017, at 11:30, Vitaly Davidovich <vitalyd at gmail.com>
>> > > >> ...
>> > > > The buffers are reused by having them point to different native
>> > > > block addresses; those blocks are managed by native code. As
>> > mentioned,
>> > > > the ByteBuffer (DirectByteBuffer concretely) is used as the Java
>> > > > interface/view of native memory, allowing Java and native code to
>> > > > communicate.
>> > >
>> > > So a DBB, under your code, may report a different address at some time
>> > > in the future, to that of what it currently reports?
>> > Correct.
>> > > I was not aware of this
>> > > usecase. Is any similar code available on the web, or elsewhere, so we
>> > > could try to determine why this is being done?
>> > >
>> > Unfortunately it's not open source code, and I don't immediately know of
>> > anything similar on the web (or otherwise). However, the gist is the
>> > following:
>> > 1) Allocate a 0-size DBB (i.e. ByteBuffer.allocateDirect(0)). This gives
>> > you a Java "handle", if you will, to some native memory. But, since this
>> > DBB will be attached/reattached to different memory dynamically, there's
>> > need for an actual allocation.
>> > 2) Native code wants to expose a segment of memory to Java. In JNI, it
>> > sets the address and capacity of this DBB to the pointer where the native
>> > memory segment starts, and to the capacity (it knows how big the native
>> > segment is). Java code asks for this DBB to be "attached" to, say, some
>> > sort of message, and the JNI/native code perform these functions.
>> > 3) Java gets the attached DBB back, and can then use its API
>> > (getXXX/setXXX) to read/write that native block. Once the operation
>> > completes, the DBB is recycled for reuse (i.e. can be attached to a
>> > different native segment again).
>> > Obviously, we can use
>> > http://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions
>> > html#GetDirectBufferAddress
>> > to get the address and then expose that via a JNI helper - in fact,
>> > what was done before. But, there's a JNI call penalty here for what is
>> > otherwise a memory read. DirectBuffer::address() solves that nicely, and
>> > also plays well with the C2 JIT (as mentioned) because the callsites
>> > this is used only see DBB, and then the whole invokeinterface call is
>> > devirtualized and inlined into a quick type check and Java field read -
>> > performance of this is, as you can imagine, significantly better than the
>> > JNI approach.
>> > If you think of what a DBB really is, it's pretty much what it's name
>> > suggests - it's an API to read/write to native memory, rather than Java
>> > heap memory (i.e. HeapByteBuffer). But, there's no reason the native
>> > memory backing the DBB has to also be allocated via Unsafe itself,
>> > that's the more common scenario.
>> > On the Java side, consumers of this have a common and conventional API
>> > over
>> > a byte buffer, i.e. ByteBuffer, which can optionally be used in the
>> > above (obviously callers will need to know what mode they're using).
>> > > -Chris.
>> > >
>> > >
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