RFR 8243491: Implementation of Foreign-Memory Access API (Second Incubator)

Tue Apr 28 22:08:12 UTC 2020

On 4/28/20 10:49 PM, Maurizio Cimadamore wrote:
>
> On 28/04/2020 17:12, Peter Levart wrote:
>> Hi Maurizio,
>>
>> I'm checking out the thread-confinement in the parallel stream case. 
>> I see the Spliterator.trySplit() is calling AbstractMemorySegmentImpl's:
>>
>>  102     private AbstractMemorySegmentImpl asSliceNoCheck(long 
>> offset, long newSize) {
>>  103         return dup(offset, newSize, mask, owner, scope);
>>  104     }
>>
>> ...so here the "owner" of the slice is still the same as that of 
>> parent segment...
>>
>> But then later in tryAdvance or forEachRemaining, the segment is 
>> acquired/closed for each element of the stream (in case of 
>> tryAdvance) or for the whole chunk to the end of spliterator (in case 
>> of forEachRemaining). So some pipelines will be more optimal than 
>> others...
>
> Not sure I follow here - you have to create a new segment for each 
> element of the stream since you don't know what thread is gonna 
> process it anyway no?

When forEachRemaining is called for all remaining elements, you only 
have to acquire one child scope, since your loop will process all 
elements in the same thread. You do create slices for each element, but 
slices don't acquire new child scope and close it afterwards. But when 
tryAdvance is called for each element from the pipeline, you have to 
acquire new scope and close it after each element. And this is the point 
of contention since multiple threads will be doing the same 
concurrently... So depending on whether the pipeline might shortcut the 
stream or not, execution could be more optimal or less, maybe to the 
point where contention is so large that it is prohibitive.

Peter

>
> Maurizio
>
>>
>> So I'm thinking. Would it be possible to "lazily" acquire scope just 
>> once in tryAdvance and then re-use the scope until the end? 
>> Unfortunately Spliterator does not have a close() method to be called 
>> when the pipeline is done with it. Perhaps it could be added to the 
>> API? This is not the 1st time I wished Spliterator had a close 
>> method. I had a similar problem when trying to create a Spliterator 
>> with a database backend. When using JDBC API a separate transaction 
>> (Connection) is typically required for each thread of execution since 
>> several frameworks bind it to the ThreadLocal.
>>
>> WDYT?
>>
>> Regards, Peter
>>
>>
>> On 4/23/20 10:33 PM, Maurizio Cimadamore wrote:
>>> Hi,
>>> time has come for another round of foreign memory access API 
>>> incubation (see JEP 383 [3]). This iteration aims at polishing some 
>>> of the rough edges of the API, and adds some of the functionalities 
>>> that developers have been asking for during this first round of 
>>> incubation. The revised API tightens the thread-confinement 
>>> constraints (by removing the MemorySegment::acquire method) and 
>>> instead provides more targeted support for parallel computation via 
>>> a segment spliterator. The API also adds a way to create a custom 
>>> native segment; this is, essentially, an unsafe API point, very 
>>> similar in spirit to the JNI NewDirectByteBuffer functionality [1]. 
>>> By using this bit of API,  power-users will be able to add support, 
>>> via MemorySegment, to *their own memory sources* (e.g. think of a 
>>> custom allocator written in C/C++). For now, this API point is 
>>> called off as "restricted" and a special read-only JDK property will 
>>> have to be set on the command line for calls to this method to 
>>> succeed. We are aware there's no precedent for something like this 
>>> in the Java SE API - but if Project Panama is to remain true about 
>>> its ultimate goal of replacing bits of JNI code with (low level) 
>>> Java code, stuff like this has to be *possible*. We anticipate that, 
>>> at some point, this property will become a true launcher flag, and 
>>> that the foreign restricted machinery will be integrated more neatly 
>>> into the module system.
>>>
>>> A list of the API, implementation and test changes is provided 
>>> below. If you have any questions, or need more detailed 
>>> explanations, I (and the rest of the Panama team) will be happy to 
>>> point at existing discussions, and/or to provide the feedback required.
>>>
>>> Thanks
>>> Maurizio
>>>
>>> Webrev:
>>>
>>> http://cr.openjdk.java.net/~mcimadamore/8243491_v1/webrev
>>>
>>> Javadoc:
>>>
>>> http://cr.openjdk.java.net/~mcimadamore/8243491_v1/javadoc
>>>
>>> Specdiff:
>>>
>>> http://cr.openjdk.java.net/~mcimadamore/8243491_v1/specdiff/overview-summary.html 
>>>
>>>
>>> CSR:
>>>
>>> https://bugs.openjdk.java.net/browse/JDK-8243496
>>>
>>>
>>>
>>> API changes
>>> ===========
>>>
>>> * MemorySegment
>>>   - drop support for acquire() method - in its place now you can 
>>> obtain a spliterator from a segment, which supports divide-and-conquer
>>>   - revamped support for views - e.g. isReadOnly - now segments have 
>>> access modes
>>>   - added API to do serial confinement hand-off 
>>> (MemorySegment::withOwnerThread)
>>>   - added unsafe factory to construct a native segment out of an 
>>> existing address; this API is "restricted" and only available if the 
>>> program is executed using the -Dforeign.unsafe=permit flag.
>>>   - the MemorySegment::mapFromPath now returns a MappedMemorySegment
>>> * MappedMemorySegment
>>>   - small sub-interface which provides extra capabilities for mapped 
>>> segments (load(), unload() and force())
>>> * MemoryAddress
>>>   - added distinction between *checked* and *unchecked* addresses; 
>>> *unchecked* addresses do not have a segment, so they cannot be 
>>> dereferenced
>>>   - added NULL memory address (it's an unchecked address)
>>>   - added factory to construct MemoryAddress from long value (result 
>>> is also an unchecked address)
>>>   - added API point to get raw address value (where possible - e.g. 
>>> if this is not an address pointing to a heap segment)
>>> * MemoryLayout
>>>   - Added support for layout "attributes" - e.g. store metadata 
>>> inside MemoryLayouts
>>>   - Added MemoryLayout::isPadding predicate
>>>   - Added helper function to SequenceLayout to rehape/flatten 
>>> sequence layouts (a la NDArray [4])
>>> * MemoryHandles
>>>   - add support for general VarHandle combinators (similar to MH 
>>> combinators)
>>>   - add a combinator to turn a long-VH into a MemoryAddress VH (the 
>>> resulting MemoryAddress is also *unchecked* and cannot be dereferenced)
>>>
>>> Implementation changes
>>> ======================
>>>
>>> * add support for VarHandle combinators (e.g. IndirectVH)
>>>
>>> The idea here is simple: a VarHandle can almost be thought of as a 
>>> set of method handles (one for each access mode supported by the var 
>>> handle) that are lazily linked. This gives us a relatively simple 
>>> idea upon which to build support for custom var handle adapters: we 
>>> could create a VarHandle by passing an existing var handle and also 
>>> specify the set of adaptations that should be applied to the method 
>>> handle for a given access mode in the original var handle. The 
>>> result is a new VarHandle which might support a different carrier 
>>> type and more, or less coordinate types. Adding this support was 
>>> relatively easy - and it only required one low-level surgery of the 
>>> lambda forms generated for adapted var handle (this is required so 
>>> that the "right" var handle receiver can be used for dispatching the 
>>> access mode call).
>>>
>>> All the new adapters in the MemoryHandles API (which are really 
>>> defined inside VarHandles) are really just a bunch of MH adapters 
>>> that are stitched together into a brand new VH. The only caveat is 
>>> that, we could have a checked exception mismatch: the VarHandle API 
>>> methods are specified not to throw any checked exception, whereas 
>>> method handles can throw any throwable. This means that, 
>>> potentially, calling get() on an adapted VarHandle could result in a 
>>> checked exception being thrown; to solve this gnarly issue, we 
>>> decided to scan all the filter functions passed to the VH 
>>> combinators and look for direct method handles which throw checked 
>>> exceptions. If such MHs are found (these can be deeply nested, since 
>>> the MHs can be adapted on their own), adaptation of the target VH 
>>> fails fast.
>>>
>>>
>>> * More ByteBuffer implementation changes
>>>
>>> Some more changes to ByteBuffer support were necessary here. First, 
>>> we have added support for retrieval of "mapped" properties 
>>> associated with a ByteBuffer (e.g. the file descriptor, etc.). This 
>>> is crucial if we want to be able to turn an existing byte buffer 
>>> into the "right kind" of memory segment.
>>>
>>> Conversely, we also have to allow creation of mapped byte buffers 
>>> given existing parameters - which is needed when going from (mapped) 
>>> segment to a buffer. These two pieces together allow us to go from 
>>> segment to buffer and back w/o losing any information about the 
>>> underlying memory mapping (which was an issue in the previous 
>>> implementation).
>>>
>>> Lastly, to support the new MappedMemorySegment abstraction, all the 
>>> memory mapped supporting functionalities have been moved into a 
>>> common helper class so that MappedMemorySegmentImpl can reuse that 
>>> (e.g. for MappedMemorySegment::force).
>>>
>>> * Rewritten memory segment hierarchy
>>>
>>> The old implementation had a monomorphic memory segment class. In 
>>> this round we aimed at splitting the various implementation classes 
>>> so that we have a class for heap segments (HeapMemorySegmentImpl), 
>>> one for native segments (NativeMemorySegmentImpl) and one for memory 
>>> mapped segments (MappedMemorySegmentImpl, which extends from 
>>> NativeMemorySegmentImpl). Not much to see here - although one 
>>> important point is that, by doing this, we have been able to speed 
>>> up performances quite a bit, since now e.g. native/mapped segments 
>>> are _guaranteed_ to have a null "base". We have also done few tricks 
>>> to make sure that the "base" accessor for heap segment is sharply 
>>> typed and also NPE checked, which allows C2 to speculate more and 
>>> hoist. With these changes _all_ segment types have comparable 
>>> performances and hoisting guarantees (unlike in the old 
>>> implementation).
>>>
>>> * Add workarounds in MemoryAddressProxy, AbstractMemorySegmentImpl 
>>> to special case "small segments" so that VM can apply bound check 
>>> elimination
>>>
>>> This is another important piece which allows to get very good 
>>> performances out of indexes memory access var handles; as you might 
>>> know, the JIT compiler has troubles in optimizing loops where the 
>>> loop variable is a long [2]. To make up for that, in this round we 
>>> add an optimization which allows the API to detect whether a segment 
>>> is *small* or *large*. For small segments, the API realizes that 
>>> there's no need to perform long computation (e.g. to perform bound 
>>> checks, or offset additions), so it falls back to integer logic, 
>>> which in turns allows bound check elimination.
>>>
>>> * renaming of the various var handle classes to conform to "memory 
>>> access var handle" terminology
>>>
>>> This is mostly stylistic, nothing to see here.
>>>
>>> Tests changes
>>> =============
>>>
>>> In addition to the tests for the new API changes, we've also added 
>>> some stress tests for var handle combinators - e.g. there's a flag 
>>> that can be enabled which turns on some "dummy" var handle 
>>> adaptations on all var handles created by the runtime. We've used 
>>> this flag on existing tests to make sure that things work as expected.
>>>
>>> To sanity test the new memory segment spliterator, we have wired the 
>>> new segment spliterator with the existing spliterator test harness.
>>>
>>> We have also added several micro benchmarks for the memory segment 
>>> API (and made some changes to the build script so that native 
>>> libraries would be handled correctly).
>>>
>>>
>>> [1] - 
>>> https://docs.oracle.com/en/java/javase/14/docs/specs/jni/functions.html#newdirectbytebuffer
>>> [2] - https://bugs.openjdk.java.net/browse/JDK-8223051
>>> [3] - https://openjdk.java.net/jeps/383
>>> [4] - 
>>> https://docs.scipy.org/doc/numpy/reference/generated/numpy.reshape.html#numpy.reshape
>>>
>>>
>>