sun.nio.ch.DirectBuffer and jdk9/jigsaw

Thu Feb 23 21:39:07 UTC 2017

Hi Vitaly, 

> De: "Vitaly Davidovich" <vitalyd at gmail.com>
> À: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "Uwe Schindler" <uschindler at apache.org>, "jigsaw-dev"
> <jigsaw-dev at openjdk.java.net>
> Envoyé: Jeudi 23 Février 2017 19:17:57
> Objet: Re: sun.nio.ch.DirectBuffer and jdk9/jigsaw

> Hey Remi,

[...] 

> P.S. We've nicely sidetracked a bit from my original question, but I suppose the
> between-the-lines answer is that there's no plan to expose some of the guts of
> a direct buffer in a supported manner.

yes, 
sun.nio.ch.DirectBuffer is not part of the exported API as you already know. 

Rémi 

> On Thu, Feb 23, 2017 at 1:05 PM, Remi Forax < forax at univ-mlv.fr > wrote:

>> Hi Vitaly,
>> I believe that what you are looking for is MethodHandles.byteBufferViewVarHandle
>> [1] 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.

>> cheers,
>> Rémi

>> http://download.java.net/java/jdk9/docs/api/java/lang/invoke/MethodHandles.html#byteBufferViewVarHandle-java.lang.Class-java.nio.ByteOrder-

>> ----- 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 >
>> > wrote:

>> >> Hi,

>> >> 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
>> > chain.

>> >> Uwe

>> >> -----
>> >> Uwe Schindler
>> >> uschindler at apache.org
>> >> ASF Member, Apache Lucene PMC / Committer
>> >> Bremen, Germany
>> >> http://lucene.apache.org/

>> >> > -----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 >
>> >> wrote:
>> >> > > >> ...
>> >> > > > The buffers are reused by having them point to different native
>> >> memory
>> >> > > > block addresses; those blocks are managed by native code. As
>> >> > mentioned,
>> >> > > > the ByteBuffer (DirectByteBuffer concretely) is used as the Java
>> >> level
>> >> > > > 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
>> >> no
>> >> > 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,
>> >> that's
>> >> > 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
>> >> where
>> >> > 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 -
>> >> the
>> >> > 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,
>> >> although
>> >> > 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
>> >> manner
>> >> > above (obviously callers will need to know what mode they're using).

>> >> > > -Chris.