Re: Memory-Mapping with LFS

[Christoph Lameter <clameter@engr.sgi.com>]

On Fri, 26 Aug 2005, Andreas Baer wrote:

> What are the current file size limits for memory mapping via glibc's
> mmap() function on linux:

Its 2^wordsize - areas used for other purposes. 1-3GB on i386. A couple of 
petabytes(?) on ia64.

> - I doubt that the full 64-Bit (something within Exabyte) are available
> in practical use. Right or wrong?

You can likely map a 512 M section starting at 6GB into your 
address space.

> I've also found an old kernel-list e-mail from 2004 that says:
> "There is a limit per process in the kernel vm that prevent from
> mmapping more than 512GB of data."
> 
> - Is this still true for the current kernel?

That depends on the architecture. Certainly no problem on Itanium. 

> Let's presume the following case. I have an 8 GB file, 1 GB physical
> memory and I want to use memory mapping for that file using LFS on a
> 32-Bit machine.
> - Is it possible?
Yes. I am not sure why one would have to use LFS. This should work on any 
filesystem.

> If yes, let's presume that I have 2 or more pointers, that are
> frequently pointing to completely different places and switch the data
> they are pointing to.
> 
> - How is it managed (by the kernel)? Through the pages, that are
> mentioned in the glibc documentation above? Are these page operations
> really faster than normal random file access (lseek etc)?

Kernel gets a fault if the page pointed to is not present and reads it in 
from the file. And yes its faster because I/O is aligned and the page does 
not need to be copied from kernel to user space.
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