Request/discussion: BufferedReader reading using async API while providing sync API
Bernd Eckenfels
ecki at zusammenkunft.net
Thu Oct 27 09:28:30 UTC 2016
Hello,
the NIO2 code for files is pretty disappointing as it
does not use file completion ports on Windows or AIO on Linux.
You use the file channel, in that case it is blocking. It does not
implement SelectableChannel.
AsyncFileChannel uses Java threads in the background for the
completion handling (and last time I checked they are not signaled by
the OS). Performance with that was not very good for sequential
processing in my tests.
Gruss
Bernd
Am Thu,
27 Oct 2016 07:20:20 +0100 schrieb Brunoais <brunoaiss at gmail.com>:
> Did you read the C code?
> Have you got any idea how many functions Windows or Linux (nearly all
> flavors) have for the read operation towards a file?
>
> I have already done that homework myself. I may not have read JVM's
> source code but I know well that there's functions on both Windows
> and Linux that provide such interface I mentioned although they
> require a slightly different treatment (and different constants).
>
>
> On 27/10/2016 00:06, Vitaly Davidovich wrote:
> >
> >
> > On Wednesday, October 26, 2016, Brunoais <brunoaiss at gmail.com
> > <mailto:brunoaiss at gmail.com>> wrote:
> >
> > It is actually based on the premise that:
> >
> > 1. The first call to ReadableByteChannel.read(ByteBuffer) sets
> > the OS buffer size to fill in as the same size as ByteBuffer.
> >
> > Why do you say that? AFAICT, it issues a read syscall and that will
> > block if the data isn't in page cache.
> >
> > 2. The consecutive calls to ReadableByteChannel.read(ByteBuffer)
> > orders
> > the JVM to order the OS to execute memcpy() to copy from its
> > memory to the shared memory created at ByteBuffer instantiation (in
> > java 8)
> > using Unsafe and then for the JVM to update the ByteBuffer
> > fields.
> >
> > I think subsequent reads just invoke the same read syscall, passing
> > the current file offset maintained by the file channel instance.
> >
> > 3. The call will not block waiting for I/O and it won't take
> > longer than the JNI interface if no new data exists. However, it
> > will block
> > waiting for the OS to execute memcpy() to the shared memory.
> >
> > So why do you think it won't block?
> >
> >
> > Is my premise wrong?
> >
> > If I read correctly, if I don't use a DirectBuffer, there would
> > be even another intermediate buffer to copy data to before giving it
> > to the "user" which would be useless.
> >
> > If you use a HeapByteBuffer, then there's an extra copy from the
> > native buffer to the Java buffer.
> >
> >
> >
> > On 26/10/2016 11:57, Pavel Rappo wrote:
> >
> > I believe I see where you coming from. Please correct me if
> > I'm wrong.
> >
> > Your implementation is based on the premise that a call to
> > ReadableByteChannel.read()
> > _initiates_ the operation and returns immediately. The OS
> > then continues to fill
> > the buffer while there's a free space in the buffer and the
> > channel hasn't encountered EOF.
> >
> > Is that right?
> >
> > On 25 Oct 2016, at 22:16, Brunoais <brunoaiss at gmail.com>
> > wrote:
> >
> > Thank you for your time. I'll try to explain it. I hope
> > I can clear it up.
> > First of it, I made a meaning mistake between
> > asynchronous and non-blocking. This implementation uses a
> > non-blocking algorithm internally while providing a blocking-like
> > algorithm on the surface. It is single-threaded and not
> > multi-threaded where one thread fetches data and blocks
> > waiting and the other accumulates it and provides to
> > whichever wants it.
> >
> > Second of it, I had made a mistake of going after
> > BufferedReader instead of going after
> > BufferedInputStream. If you want me to go after BufferedReader it's
> > ok but I only thought that going after BufferedInputStream would be
> > more generically useful than BufferedReaderwhen I
> > started the poc.
> >
> > On to my code:
> > Short answers:
> > • The sleep(int) exists because I don't know how
> > to wait until more data exists in the buffer which is
> > part of read()'s contract.
> > • The ByteBuffer gives a buffer that is filled
> > by the OS (what I believe Channels do) instead of getting
> > data only by demand (what I believe Streams do).
> > Full answers:
> > The blockingFill(boolean) method is a method for a busy
> > wait for a fill which is used exclusively by the read()
> > method. All other methods use the version that does not
> > sleep (fill(boolean)).
> > blockingFill(boolean)'s existance like that is only
> > because the read() method must not return unless either:
> >
> > • The stream ended.
> > • The next byte is ready for reading.
> > Additionally, statistically, that while loop will rarely
> > evaluate to true as reads are in chunks so readPos will
> > be behind writePos most of the time.
> > I have no idea if an interrupt will ever happen, to be
> > honest. The main reasons why I'm using a sleep is
> > because I didn't want a hog onto the CPU in a full thread usage
> > busy wait and because I didn't find any way of doing a
> > thread sleep in order to wake up later when the buffer
> > managed by native code has more data.
> > The Non-blocking part is managed by the buffer the OS
> > keeps filling most if not all the time. That buffer is
> > the field
> >
> > ByteBuffer readBuffer
> > That's the gaining part against the plain old Buffered
> > classes.
> >
> >
> > Did that make sense to you? Feel free to ask anything
> > else you need.
> >
> > On 25/10/2016 20:52, Pavel Rappo wrote:
> >
> > I've skimmed through the code and I'm not sure I can
> > see any asynchronicity
> > (you were pointing at the lack of it in
> > BufferedReader). And the mechanics of this is very puzzling to me,
> > to be honest:
> > void blockingFill(boolean forced) throws
> > IOException {
> > fill(forced);
> > while (readPos == writePos) {
> > try {
> > Thread.sleep(100);
> > } catch (InterruptedException e) {
> > // An interrupt may mean more data
> > is available
> > }
> > fill(forced);
> > }
> > }
> > I thought you were suggesting that we should utilize
> > the tools which OS provides
> > more efficiently. Instead we have something that
> > looks very similarly to a
> > "busy loop" and... also who and when is supposed to
> > interrupt Thread.sleep()?
> > Sorry, I'm not following. Could you please explain
> > how this is supposed to work?
> >
> > On 24 Oct 2016, at 15:59, Brunoais
> > <brunoaiss at gmail.com>
> > wrote:
> > Attached and sending!
> > On 24/10/2016 13:48, Pavel Rappo wrote:
> >
> > Could you please send a new email on this
> > list with the source attached as a
> > text file?
> >
> > On 23 Oct 2016, at 19:14, Brunoais
> > <brunoaiss at gmail.com>
> > wrote:
> > Here's my poc/prototype:
> >
> > http://pastebin.com/WRpYWDJF
> >
> > I've implemented the bare minimum of the
> > class that follows the same contract of
> > BufferedReader while signaling all
> > issues I think it may have or has in comments.
> > I also wrote some javadoc to help
> > guiding through the class.
> > I could have used more fields from
> > BufferedReader but the names were so
> > minimalistic that were confusing me. I
> > intent to change them before sending
> > this to openJDK.
> > One of the major problems this has is
> > long overflowing. It is major because it is
> > hidden, it will be extremely rare and it
> > takes a really long time to reproduce.
> > There are different ways of dealing with
> > it. From just documenting to actually
> > making code that works with it.
> > I built a simple test code for it to
> > have some ideas about performance and correctness.
> >
> > http://pastebin.com/eh6LFgwT
> >
> > This doesn't do a through test if it is
> > actually working correctly but I see no
> > reason for it not working correctly
> > after fixing the 2 bugs that test found.
> > I'll also leave here some conclusions
> > about speed and resource consumption I
> > found. I made tests with default buffer sizes,
> > 5000B 15_000B and 500_000B. I noticed
> > that, with my hardware, with the 1 530
> > 000 000B file, I was getting around:
> > In all buffers and fake work: 10~15s
> > speed improvement ( from 90% HDD speed to 100%
> > HDD speed)
> > In all buffers and no fake work: 1~2s
> > speed improvement ( from 90% HDD speed
> > to 100% HDD speed)
> > Changing the buffer size was giving
> > different reading speeds but both were
> > quite equal in how much they would
> > change when changing the buffer size.
> > Finally, I could always confirm that I/O
> > was always the slowest thing while this
> > code was running.
> > For the ones wondering about the file
> > size; it is both to avoid OS cache and
> > to make the reading at the main use-case
> > these objects are for (large streams of
> > bytes).
> > @Pavel, are you open for discussion now
> > ;)? Need anything else?
> > On 21/10/2016 19:21, Pavel Rappo wrote:
> >
> > Just to append to my previous email.
> > BufferedReader wraps any Reader out
> > there. Not specifically FileReader. While
> > you're talking about the case of
> > effective reading from a file.
> > I guess there's one existing
> > possibility to provide exactly what
> > you need (as I
> > understand it) under this method:
> > /**
> > * Opens a file for reading,
> > returning a {@code BufferedReader}
> > to read text
> > * from the file in an efficient
> > manner...
> > ...
> > */
> > java.nio.file.Files#newBufferedReader(java.nio.file.Path)
> > It can return _anything_ as long as
> > it is a BufferedReader. We can do it, but it
> > needs to be investigated not only
> > for your favorite OS but for other OSes as
> > well. Feel free to prototype this
> > and we can discuss it on the list later.
> > Thanks,
> > -Pavel
> >
> > On 21 Oct 2016, at 18:56,
> > Brunoais <brunoaiss at gmail.com>
> > wrote:
> > Pavel is right.
> > In reality, I was expecting such
> > BufferedReader to use only a
> > single buffer and have that
> > Buffer being filled asynchronously, not
> > in a different Thread.
> > Additionally, I don't have the
> > intention of having a larger
> > buffer than before unless stated
> > through the API (the
> > constructor). In my idea, internally, it is
> > supposed to use
> > java.nio.channels.AsynchronousFileChannel
> > or equivalent.
> > It does not prevent having two
> > buffers and I do not intent to
> > change BufferedReader itself.
> > I'd do an BufferedAsyncReader of sorts
> > (any name suggestion is welcome
> > as I'm an awful namer).
> > On 21/10/2016 18:38, Roger Riggs
> > wrote:
> >
> > Hi Pavel,
> > I think Brunoais asking for
> > a double buffering scheme in
> > which the implementation of
> > BufferReader fills (a second
> > buffer) in parallel with the
> > application reading from the
> > 1st buffer
> > and managing the swaps and
> > async reads transparently.
> > It would not change the API
> > but would change the
> > interactions between the
> > buffered reader
> > and the underlying stream.
> > It would also increase memory
> > requirements and processing
> > by introducing or using a
> > separate thread and the
> > necessary synchronization.
> > Though I think the formal
> > interface semantics could be
> > maintained, I have doubts
> > about compatibility and its
> > unintended consequences on
> > existing subclasses,
> > applications and libraries.
> > $.02, Roger
> > On 10/21/16 1:22 PM, Pavel
> > Rappo wrote:
> >
> > Off the top of my head,
> > I would say it's not
> > possible to change the
> > design of an
> > _extensible_ type that
> > has been out there for 20 or
> > so years. All these I/O
> > streams from java.io
> > <http://java.io> were
> > designed for simple
> > synchronous use case.
> > It's not that their
> > design is flawed in some way,
> > it's that they doesn't
> > seem to suit your needs. Have you
> > considered using
> > java.nio.channels.AsynchronousFileChannel
> > in your applications?
> > -Pavel
> >
> > On 21 Oct 2016, at
> > 17:08, Brunoais
> > <brunoaiss at gmail.com>
> > wrote:
> > Any feedback on
> > this? I'm really interested
> > in implementing such
> > BufferedReader/BufferedStreamReader
> > to allow speeding up
> > my applications
> > without having to
> > think in an
> > asynchronous way or
> > multi-threading
> > while programming with it.
> > That's why I'm
> > asking this here.
> > On 13/10/2016 14:45,
> > Brunoais wrote:
> >
> > Hi,
> > I looked at
> > BufferedReader
> > source code for
> > java 9 long with
> > the source code
> > of the
> > channels/streams
> > used. I noticed
> > that, like in
> > java 7, BufferedReader
> > does not use an
> > Async API to
> > load data from files,
> > instead, the
> > data loading is all
> > done
> > synchronously even when the OS
> > allows
> > requesting a file to be read
> > and getting a
> > warning later
> > when the file is
> > effectively
> > read. Why Is
> > BufferedReader
> > not async while
> > providing a
> > sync API?
> >
> > <BufferedNonBlockStream.java><Tests.java>
> >
> >
> >
> >
> >
> > --
> > Sent from my phone
>
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