Proposal for adding O_DIRECT support into JDK 9

[David M. Lloyd]

What happens if you're using large pages?  Are the direct buffers always 
allocated off of small pages?

On 11/02/2016 06:34 PM, Lu, Yingqi wrote:
> Hi All,
>
> Our most recent DirectIO patch is available at http://cr.openjdk.java.net/~igraves/8164900-3/
>
> In this version, we made following changes:
>
> 1. Removed the flag "direct" from FileDescriptor class. Instead, moved it to the FileChannelImpl class.
>
> 2. Provided a way for user to allocate a page aligned direct ByteBuffer.
>     1) Added a constructor DirectByteBuffer(int cap, boolean direct) to allocate a direct ByteBuffer that is aligned to the page size.
>     2) Added Util.getTemporaryAlignedDirectBuffer(int size)
>     3) Added DirectByteBuffer.isAligned(int pos) to check if the buffer is aligned before doing native IO with it.
>
> 3. Moved all the alignment check from C code to Java code (mainly FileChannelImpl and IOUtil.java).
>
> 4. Made the DirectIO functionality consistent between read and write operations. With current version of the patch, user would be responsible for the alignment with file offset and IO size.
>
> 5. Made the API for DirectIO more extensible to all the supporting platforms.
>     1) Unix OS specific code are done through UnixConstants.java.template and FileChannelImpl.c.
>     2) Coded and tested for Linux and OS X (OS X testing is done through VirtualBox with OS X VM on top of Linux OS).
>     3) Coded for Solaris. We do not have environment to test it so that we commented the changes out.
>
> 6. We added 4 test cases following the existing non-direct IO examples.
>
> 7. We did jtreg test for the entire nio package and no errors were found due to our changes.
>
> Please let us know your feedback and comment. Thank you very much for your time and consideration!
>
> Thanks,
> Lucy
>
>> -----Original Message-----
>> From: Alan Bateman [mailto:Alan.Bateman at oracle.com]
>> Sent: Monday, October 17, 2016 7:59 AM
>> To: Lu, Yingqi <yingqi.lu at intel.com>
>> Cc: nio-dev at openjdk.java.net; Kaczmarek, Eric <eric.kaczmarek at intel.com>;
>> Kharbas, Kishor <kishor.kharbas at intel.com>
>> Subject: Re: Proposal for adding O_DIRECT support into JDK 9
>>
>> On 12/10/2016 17:41, Lu, Yingqi wrote:
>>
>>> :
>>>
>>> You are correct about the "extra copy" with DirectIO. Will it be acceptable if we
>> add a function "Util.getAlignedTemporaryDirectBuffer" and use that for the
>> DirectIO operation? In this case, I think we should be able to avoid the additional
>> copy?
>>>
>> Yes, that should work but it still lacks a way for the user to get an aligned buffer
>> and so you will always be copying in and out of an aligned buffer. The other thing
>> is the sizing of the I/O operation where I think you will also need a way to expose
>> the multiple (or block size) to the user.
>>
>> -Alan.

-- 
- DML