In the article appeared in Vector, online version at
http://www.jsoftware.com/papers/mmf.htm
it said:
Memory-mapped files can be much larger than system RAM, yet they can
be accessed without page thrashing.
The content of mapped files are never duplicated in physical ram so
that it appeared that the restriction is that the total size of view
of all mapped file must fit into vas. Even the view is the entire
file, it should pose no problem for 64-bit system.
I'm puzzled too.
On Tue, 18 Nov 2008, Don Guinn wrote:
> After reading this thread, I'm a little confused on how mapped files are
> really handled. Years ago I worked on CDC mainframes(205) and on them mapped
> files were nothing unusual. When a file was opened as mapped it simply
> became an extension of the page (swap) file and was assigned a virtual
> address range to cover the file. So references to the file simply were done
> as virtual addresses and were paged into and out of real memory as needed
> directly from and to the file. There were no restrictions on the size of
> real memory other than performance considerations and limits of the virtual
> address range. I thought that mapped files on the PC (Windows, at least)
> worked the same way.
>
> It seems to me that the only real problem with large mapped files in
> J should be the ease in which an entire file can be processed. We have to go
> to a lot of trouble to break files into pieces. Most other languages process
> files a record at a time so for them there is seldom a need of keeping an
> entire mapped file in real memory.
>
> Are you saying that mapping a file on a PC copies the entire file into real
> memory then to the swap file as needed? If so then it must make a second
> write to the file itself for updates. Sounds pretty awkward.
--
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