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Java memory mapped files and swap


Java memory mapped files and swap

By : user3862239
Date : November 22 2020, 07:01 PM
wish of those help Linux doesn't really distinguish between anonymous and memory-mapped pages. They all get demand-loaded anyway via page faults.
You can think of anonymous memory as if it was a private memory mapping of /dev/zero.
code :


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Memory-mapped files in Java

Memory-mapped files in Java


By : Oliver Erxleben
Date : March 29 2020, 07:55 AM
it fixes the issue Did anyone actually check to see if ByteBuffers created by memory mapping support invoking .array() in the first place, regardless of readonly/readwrite?
From my poking around, as far as I can tell, the answer is NO. A ByteBuffer's ability to return a direct byte[] array via ByteBuffer.array() is goverened by the presence of ByteBuffer.hb (byte[]), which is always set to null when a MappedByteBuffer is created.
Java Memory-mapped files?

Java Memory-mapped files?


By : Cathy
Date : March 29 2020, 07:55 AM
around this issue It uses the OS support for memory mapped files.
I'm trying to find documentation to back that up, but I haven't found anything conclusive yet. However, various bits of the docs do say things like this:
Moving memory in memory mapped swap file

Moving memory in memory mapped swap file


By : AeroKale
Date : March 29 2020, 07:55 AM
hop of those help? Ok, I found a way to do this by using the filechannel that is mapped by the MappedByteBuffer to transfer memory within itself.
code :
RandomAccessFile raFile = new RandomAccessFile(MyFile, "rw");
FileChannel fileChannel = raFile.getChannel();
fileChannel.position(srcPosition);
fileChannel.transferFrom(fileChannel, dstPosition, count);
Memory mapped files in java

Memory mapped files in java


By : rogerarce22
Date : March 29 2020, 07:55 AM
wish helps you Memory mapped files are much faster then regular ByteBuffer version but it will allocate whole memory for example if you map 4MB file operating system will create 4MB file on filesystem that map file to a memory and you can directly write to file just by writing to memory. This is handy when you know exactly how much of data you want to write as if you write less then specified rest of the data array will be filled with zeros. Also Windows will lock the file (can't be deleted until JVM exits), this is not the case on Linux.
Below is the example of appending to a file with memory mapped buffer, for position just put the file size of file that you are writing to:
code :
int BUFFER_SIZE = 4 * 1024 * 1024; // 4MB
String mainPath = "C:\\temp.txt";
SeekableByteChannel dataFileChannel = Files.newByteChannel("C:\\temp.txt", EnumSet.of(StandardOpenOption.WRITE, StandardOpenOption.CREATE, StandardOpenOption.APPEND));
MappedByteBuffer writeBuffer = dataFileChannel.map(FileChannel.MapMode.READ_WRITE, FILE_SIZE, BUFFER_SIZE);
writeBuffer.write(arrayOfBytes);
Java NIO - Memory mapped files

Java NIO - Memory mapped files


By : Ketan
Date : March 29 2020, 07:55 AM
it fixes the issue I recently came across this article which provided a nice intro to memory mapped files and how it can be shared between two processes. Here is the code for a process that reads in the file: , There are several ways.
code :
MappedByteBuffer bb;
…
// write your data

bb.force();// ensure completion of all writes
bb.put(specialPosition, specialMarkerValue);
bb.force();// ensure visibility of the marker
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