MemoryMappedBuffer flushing

[Bobrowski, Maciej]

Thank you very much Alan for your prompt reply. I understand that force flushes all the dirty pages, but that does not answer my question. Let me give a more verbose example:

http://psy-lob-saw.blogspot.co.uk/2013/02/alignment-concurrency-and-torture-x86.html - firstly I went through this article, which describes concurrent access to direct memory, which even for putInt/Long might not be guaranteed (even for aligned accesses). This sounds quite scary to me tbh.

Now, let's think more in the case of MappedByteBuffer and IPC. Let's assume I put 100 bytes on each call to put i.e. place 100 bytes in the memory mapped file in Process A. Given the above, do I have any guarantees, that the page synchronozation will not  happen half way through my process writing to the page, so that the underlying file will reflect an update of 50 bytes only? This is especially crucial if you assume there is another process, periodically reading in the same page and acting upon it, seeing partial write per se.

My gut feeling tells me there is no such guarantees and the O/S can synch at any point in time. So, shrinking down the window of potential disaster, does a single putInt brings any atomicity guarantees, such that O/S synch will either happen before putInt or after?

Thanks,
Maciej

From: Alan Bateman [mailto:Alan.Bateman at oracle.com]
Sent: 23 September 2014 12:51
To: Bobrowski, Maciej (ICT); nio-dev at openjdk.java.net
Subject: Re: MemoryMappedBuffer flushing

On 23/09/2014 11:02, Bobrowski, Maciej wrote:
Hi All,


Given that I am using a memory-mapped file, with an underlying O/S page sie of e.g. 4KB, are there any guarantees about when the flush (page synch) will be performed with respect to writes?

No, you'll need to use the force method to ensure that the underlying pages are written back to the file system.

-Alan


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