Request/discussion: BufferedReader reading using async API while providing sync API

Brunoais brunoaiss at
Tue Oct 25 21:16:27 UTC 2016

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:*

 1. 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.
 2. 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:

 1. The stream ended.
 2. 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> 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> wrote:
>>>> Here's my poc/prototype:
>>>> 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.
>>>> 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> 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 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> 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?
>> <><>

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