Re: linux vm page sharing

Re: linux vm page sharing

[Mark_H_Johnson]

>i am trying to implement a system where different processes can share
>pages, this means that not only same executables, but different
>executables , but when the pages required are same.

Why not use one of the existing memory management functions such as:
 - mmap, munmap to map a file into memory (this also provides persistence)
 - shmget, shmat, shmdt, shmctl to create, attach, detach, and control
shared memory segments. The status of these is also available through the
ipcs command line program. Also look at "man 5 ipc" to get a summary of
interprocess communications methods (semaphores, shared memory, message
queues).
Note that the base address of the memory segments are not necessarily the
same in each executable. This means you should always use offsets (relative
to the base of shared memory) to locate items within the shared memory
area. For example, you code would compute the "real address" as the "base
address" plus "offset" before dereferencing pointers. Trying to force the
base address of the shared memory is "hard" and prone to problems due to
the relative size of the executables.

>[snip - looking at page structure]
I would use just the documented interfaces for shared memory.

--Mark H Johnson
  <mailto:Mark_H_Johnson@raytheon.com>

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Re: linux vm page sharing

[Raghu R. Arur]

  I think the idea you are proposing is already present in linux. If two 
executables are using the same library file, then there wont be two 
instances of the page of that library file..

 The address_space in the page maps to the the inode of the file that the 
page is mapped to (in case of a file mapped file) or it will be NULL for 
non-mapped pages. In 2.6 mm, the pages that hold the page table entries of 
a process use this mapping to map to the mm_struct of that process.

 HTH,
 Raghu

On Fri, 7 Nov 2003, Sharath Kodi Udupa wrote:

> hi,
> 
> i am trying to implement a system where different processes can share
> pages, this means that not only same executables, but different
> executables , but when the pages required are same.
> but i see in the linux page structure, it keeps a pointer to the
> address_space mapping, but now since if the second process also needs to
> share the page,this wont be the same mapping. so i am planning to add the
> page table entry to the second process, but to leave the
> struct address_space *mapping pointer to whatever it was earlier. I plan
> to do this since, i dont really understand how this is used and also have
> gone through the code to understand it. What significance does this hold?
> 
> any pointers is greatly appreciated
> 
> regards,
> 
> Sharath K Udupa
> Graduate Student,
> Dept. of Computer Science,
> University of Arizona.
> sku@cs.arizona.edu
> http://www.cs.arizona.edu/~sku
> 
> "Sometimes I think the surest sign that intelligent life exists
> elsewhere in the universe is that none of it has tried to contact us."
> --Calvin, The Indispensable Calvin and Hobbes
> 
> 
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> To unsubscribe, send a message with 'unsubscribe linux-mm' in
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linux vm page sharing

[Sharath Kodi Udupa]

hi,

i am trying to implement a system where different processes can share
pages, this means that not only same executables, but different
executables , but when the pages required are same.
but i see in the linux page structure, it keeps a pointer to the
address_space mapping, but now since if the second process also needs to
share the page,this wont be the same mapping. so i am planning to add the
page table entry to the second process, but to leave the
struct address_space *mapping pointer to whatever it was earlier. I plan
to do this since, i dont really understand how this is used and also have
gone through the code to understand it. What significance does this hold?

any pointers is greatly appreciated

regards,

Sharath K Udupa
Graduate Student,
Dept. of Computer Science,
University of Arizona.
sku@cs.arizona.edu
http://www.cs.arizona.edu/~sku

"Sometimes I think the surest sign that intelligent life exists
elsewhere in the universe is that none of it has tried to contact us."
--Calvin, The Indispensable Calvin and Hobbes


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