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" <[email protected]> > À: "Uwe Schindler" <[email protected]> > Cc: "jigsaw-dev" <[email protected]> > 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 <[email protected]> > 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 >> [email protected] >> ASF Member, Apache Lucene PMC / Committer >> Bremen, Germany >> http://lucene.apache.org/ >> >> > -----Original Message----- >> > From: jigsaw-dev [mailto:[email protected]] On Behalf >> > Of Vitaly Davidovich >> > Sent: Thursday, February 23, 2017 5:30 PM >> > To: Chris Hegarty <[email protected]> >> > Cc: jigsaw-dev <[email protected]> >> > Subject: Re: sun.nio.ch.DirectBuffer and jdk9/jigsaw >> > >> > On Thu, Feb 23, 2017 at 11:10 AM, Chris Hegarty >> > <[email protected]> >> > wrote: >> > >> > > >> > > > On 23 Feb 2017, at 11:30, Vitaly Davidovich <[email protected]> >> 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. >> > > >> > > >>
