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SubjectRe: [RFC][PATCH] Cross Memory Attach
On Wed, 15 Sep 2010 10:02:35 +0200
Ingo Molnar <> wrote:
> What did those OpenMPI facilities use before your patch - shared
> memory or sockets?

This comparison is against OpenMPI using the shared memory btl.

> I have an observation about the interface:
> A small detail: 'int flags' should probably be 'unsigned long flags'
> - it leaves more space.


> Also, note that there is a further performance optimization possible
> here: if the other task's ->mm is the same as this task's (they share
> the MM), then the copy can be done straight in this process context,
> without GUP. User-space might not necessarily be aware of this so it
> might make sense to express this special case in the kernel too.


> More fundamentally, wouldnt it make sense to create an iovec
> interface here? If the Gather(v) / Scatter(v) / AlltoAll(v) workloads
> have any fragmentation on the user-space buffer side then the copy of
> multiple areas could be done in a single syscall. (the MM lock has to
> be touched only once, target task only be looked up only once, etc.)

yes, I think so. Currently where I'm using the interface in OpenMPI I
can't take advantage of this, but it could be changed in the future- and
its likely other MPI's could take advantage of it already.

> Plus, a small naming detail, shouldnt the naming be more IO like:
> sys_process_vm_read()
> sys_process_vm_write()

Yes, that looks better to me. I really wasn't sure how to name them.



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