Re: How to use memory over 4GB

[Roberto Fichera]

At 14.56 16/05/2005, Michael Tokarev wrote:

> Roberto Fichera wrote:
> > Hi All,
> >
> > I've a dual Xeon 3.2GHz HT with 8GB of memory running kernel 2.6.11.
> > I whould like to know the way how to use all the memory in a single
> > process, the application is a big simulation which needs big memory
> > chuncks.
> > I have readed about hugetlbfs, shmfs and tmpfs, but don't understand how
> > I can access
> > the whole memory. Ok! I can create a big file on tmpfs using shm_open() and
> > than map it by using mmap() or mmap2() but how can I access over 4GB using
> > standard pointers (if I had to use it)?
>
> There's no "standard" and simple way to utilize more than 4Gb memory on
> i386 hardware, especially in a userspace.  That is, the size of a pointer
> is 32bits, which is 4GB addresspace maximum.  i386 architecture just can't
> have a pointer of greather size.
>
> All "extra" (>4GB) space can be used like a file in a filesystem, not like
> a plain memory.  Think of read()/write() (or pread()/pwrite() for that 
> matter),
> but much faster ones compared to disk-based storage -- in tmpfs.  You can
> also mmap() *parts* of such a file, but will be still limited to 4GB at
> once -- in order to have more, you will have to unmap() something.
>
> All the "large applications" (most notable large database systems such as
> Oracle) can't use more than 4GB memory directly, but can utilize it for
> database cache.  In directly-addressible space there's a "table of content"
> of cached buffers is keept, and when a buffer is needed, it is mmap()'ed
> into the application's address space, and unmapped right away when it isn't
> needed anymore (but it is still in memory).

I was thinking to create as many object I need by the usual shm_open(), 
than mmap()
it on a process until I get ENOMEM. So, when I get a ENOMEM I start to
munmap() objects in order to free some user space memory and create the needed
space to complete mmap() for the requested object.

> Ofcourse you can't have
> usual pointers into that memory, but you can use something like
> (block-number,offset) instead of a pointer (pagetables).

Could you explain better this approch?


> /mjt
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Roberto Fichera. 

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