linux-kernel - Re: [RFC][PATCH] Cross Memory Attach

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Date:	Wed, 15 Sep 2010 22:50:19 +0930
From:	Christopher Yeoh <cyeoh@....ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] Cross Memory Attach

On Wed, 15 Sep 2010 10:02:35 +0200
Ingo Molnar <mingo@...e.hu> 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.

ok.

> 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.

ok.

> 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.

Regards,

Chris
-- 
cyeoh@...ibm.com
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