Proposal for adding O_DIRECT support into JDK 9

[Lu, Yingqi]

I think DirectByteBuffer do not use large page. Please refer to https://bugs.openjdk.java.net/browse/JDK-8021829 for more details.

Our code changes are done for alignment. I think they should not impact the usage of either small pages or large pages. 

Thanks,
Lucy

>-----Original Message-----
>From: David M. Lloyd [mailto:david.lloyd at redhat.com]
>Sent: Thursday, November 03, 2016 5:15 AM
>To: Lu, Yingqi <yingqi.lu at intel.com>; Alan Bateman <Alan.Bateman at oracle.com>
>Cc: nio-dev at openjdk.java.net; Kharbas, Kishor <kishor.kharbas at intel.com>;
>core-libs-dev at openjdk.java.net; Kaczmarek, Eric <eric.kaczmarek at intel.com>
>Subject: Re: Proposal for adding O_DIRECT support into JDK 9
>
>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