From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Roland Dreier <rdreier@cisco.com>
Subject: Re: [ofa-general] Re: [GIT PULL] please pull ummunotify
Cc: kosaki.motohiro@jp.fujitsu.com,
       Jason Gunthorpe <jgunthorpe@obsidianresearch.com>,
       Brice Goglin <Brice.Goglin@inria.fr>, linux-rdma@vger.kernel.org,
       linux-kernel@vger.kernel.org, general@lists.openfabrics.org,
       akpm@linux-foundation.org, torvalds@linux-foundation.org
In-Reply-To: <adaljklifkt.fsf@cisco.com>
References: <20090911064019.GZ4973@obsidianresearch.com> <adaljklifkt.fsf@cisco.com>
Message-Id: <20090915155231.DB86.A69D9226@jp.fujitsu.com>
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Date: Tue, 15 Sep 2009 16:03:04 +0900 (JST)
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> 
>  > So.. What is the problem with fork? The semantics of what should
>  > happen seem natural enough to me, the PD doesn't get copied to the
>  > child, so the MR stays with the parent. COW events on the pinned
>  > region must be resolved so that the physical page stays with the
>  > process that has pinned it - the pin is logically released in the
>  > child because the MR doesn't exist because the PD doesn't exist.
> 
> This is getting away from the problem that ummunotify is solving, but
> handling a COW fault generated by the parent by doing the copy in the
> child seems like a pretty major, tricky change to make.  The child may
> have forked 100 more times in the meantime, meaning we now have to
> change 101 memory maps ... the cost of page faults goes through the roof
> probably...

Ummm...
Perhaps my first question was wrong. I'm not intent to NAK your patch.
I merely want to know your patch detail...

ok, I ask you again as another word.

 - I guess you have your MPI implementaion w/ ummunotify, right?
 - I guess you have test sevaral pattern, right?
   if so, can we see your test result?
 - I think you can explain your MPI advantage/disadvantage against
   current OpenMPI (or mpich et al).
 - I guess your patch dramatically improve MPI implementaion, but
   it's not free. it request some limitation to MPI application, right?
 - I imagine multi thread and fork. Is there another linmitaion?
 - In past discuttion, you said ummunotify user should not use
   multi threading. you also think user should not fork?


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