Re: [patch] SMP alternatives

From: Andi Kleen
Date: Wed Nov 23 2005 - 16:37:12 EST

On Wed, Nov 23, 2005 at 10:05:40PM +0000, Alan Cox wrote:
> > Might be a bit costly to rewrite all the page tables for that case
> > just to change the PAT index. A bit is nicer for that.
> CPU insert/remove is performed how many times a second ? Or for that
> matter why not just reload the PAT register and keep the index the
> same ?

For user space the primary trigger event would be "has any shared
writable mappings or multiple threads". Even on a real MP systems it's
perfectly ok to run a program with no writable shared mappings with LOCK off.
Depending on the workload this transistion could happen quite often.
Especially there is a worst case of an application allocating a few
GB of memory and then starting a new thread.

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