Proposal for adding O_DIRECT support into JDK 9

[Jon V.]

Why not call aligned_alloc for doing page alignment?  It should waste much
less memory.

http://en.cppreference.com/w/c/memory/aligned_alloc

On Thu, Nov 3, 2016 at 8:14 AM, David M. Lloyd <david.lloyd at redhat.com>
wrote:

> 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
>