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Subject: Re: [OMPI devel] Notes from mem hooks call today
From: Roland Dreier (rdreier_at_[hidden])
Date: 2008-05-28 17:07:44

> - gleb asks: don't we want to avoid the system call when possible?
> - patrick: a single syscall can be/is cheaper than a reg cache
> lookup in user space

This doesn't really make sense -- syscall + cache lookup in kernel is
"obviously" more expensive than cache lookup in userspace with no
context switch (I don't see any tricks the kernel can do that make the
cache lookup cheaper there).

However the solution I proposed a long time ago (when Pete Wyckoff
originally did his work on having the kernel track this -- and as a side
note, it's not clear to me whether MMU notifiers really help what Pete
did) is for userspace to provide a pointer to a flag when registering
memory with the kernel, and then the kernel can mark the flag if the
mapping changes -- ie keep the userspace cache but have the kernel
manage invalidation "perfectly" without any malloc hooks.

 - R.