Memory Mapped Segment with offsets into the underlying file

[Johannes Lichtenberger]

Hi,

is it currently possible to specify a start offset somehow to map a
specific region, despite the number of bytes to map? Im using Java 14 as of
now.

As already mentioned, my application is always appending data to a file and
needs to read randomly. For writing I thought I could use a
RandomAccessFile or Channel based implementation and only mmap the file for
read-only operations. However, somehow, when I write a long offset into the
position 0 and afterwards try to read it via:

dataFileSegment =
    MemorySegment.mapFromPath(checkNotNull(dataFile),
dataFile.toFile().length(), FileChannel.MapMode.READ_ONLY);

final MemoryAddress baseAddress = dataFileSegment.baseAddress();

uberPageReference.setKey((long) LONG_VAR_HANDLE.get(baseAddress));


the key I'm setting seems to be way off. However, it clearly might be
a bug in my implementation or it's somehow not synchronized when
something is written to the file without the Foreign Memory API or it
might well be an anti-pattern.

Now I thought I could just map a region, which starts at the end of
the current file for appending operations and spans maybe 1Gb for
instance.

Then I could save in a field the real length of the written data and
adapt it everytime. When the mapped segment doesn't have enough space
simply a new segment is created. Before closing the segment I'd have
to truncate the size to the real length, however.

Furthermore, I think it would be great if the JVM would have support for
madvise to get rid of the prefetching of pages done by the Kernel in my
case (random reads, as it's a tree of tries... but basically as in every
index structure).

And as suggested I now try to map large regions of the file (or the whole
file) plus setting VarHandles as static final fields :-)

kind regards
Johannes