RE: [SSI-devel] Re: get shared memory and user space talking

[MAASK G. <maa...@ya...>]

Hi,
Stevie, Sorry for this delayed reply ... was on leave
for a few days.

--- "Walker, Bruce J" <bru...@hp...> wrote:

> The second feature migshm has is that it will
> re-join clones that have
> both migrated to the same node (so they share their
> process address
> space again).  What I was hoping they had was a way
> to actually
> coherently have the clones on different nodes.  I'm
> pretty sure that is
> not provided.  
--- That is wrong Bruce. This is in fact provided in
migShm, provided that multi-threaded programs also use
semaphores for synchronization. 

This constraint(for shared memory processes as well as
clones) come in the picture because we need some event
for handling consistency. Any calls in the kernel can
be used for this. We decided upon semaphores as that
came to our mind first when we talk of shared memory.
As accesses to shared memory(or cloned memory map)
cannot be detected, we do not know when to sync up all
the copies ... so we had to put this constraint. 

>I believe we could provide the memory
> re-join capability
> pretty trivially in the current code base  (I'll
> check with John Byrne
> on this).  The concern we had about migrating clones
> was making sure
> they all migrated at the same time and to the same
> node.  I don't know
> if openMosix does anything to guarantee that.  
--- Once again, this is not needed as migShm can have
two clones running on different nodes with consistency
handled as the time of release of the semaphore.

Regards,
Asmita

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