linux-kernel - Re: [patch 0/2] x86,pat: Reduce contention on the memtype

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Date:	Wed, 24 Mar 2010 03:55:11 -0500
From:	Robin Holt <holt@....com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	holt@....com, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...il.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	x86@...nel.org, Rafael Wysocki <rjw@...ell.com>
Subject: Re: [patch 0/2] x86,pat: Reduce contention on the memtype_lock -V4

On Wed, Mar 24, 2010 at 03:16:14AM +0100, Andi Kleen wrote:
> holt@....com writes:
> 
> > Tracking memtype on x86 uses a single global spin_lock for either reading
> > or changing the memory type.  This includes changes made to page flags
> > which is perfectly parallel.
> >
> > Part one of the patchset makes the page-based tracking use cmpxchg
> > without a need for a lock.
> >
> > Part two of the patchset converts the spin_lock into a read/write lock.
> 
> I'm curious: in what workloads did you see contention?
> 
> For any scalability patches it would be always good to have a description
> of the workload.

It was a job using xpmem (an out of tree kernel module) which uses
vm_insert_pfn to establish ptes.  The scalability issues were shown
in the first patch.  I do not have any test which shows a performance
difference  with the spin_lock to rw_lock conversion.

Robin
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