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Re: [Qemu-devel] Performance regression using KVM/ARM

From: Christoffer Dall
Subject: Re: [Qemu-devel] Performance regression using KVM/ARM
Date: Fri, 22 Apr 2016 12:02:00 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Hi Laszlo,

On Thu, Apr 21, 2016 at 11:58:07PM +0200, Laszlo Ersek wrote:
> On 04/21/16 18:23, Christoffer Dall wrote:
> > Hi,
> > 
> > Commit 9fac18f (oslib: allocate PROT_NONE pages on top of RAM,
> > 2015-09-10) had the unfortunate side effect that memory slots registered
> > with KVM no longer contain a userspace address that is aligned to a 2M
> > boundary, causing the use of THP to fail in the kernel.
> > 
> > I fail to see where in the QEMU code we should be asking for a 2M
> > alignment of our memory region.  Can someone help pointing me to the
> > right place to fix this or suggest a patch?
> > 
> > This causes a performance regssion of hackbench on KVM/ARM of about 62%
> > compared to the workload running with THP.
> > 
> > We have verified that this is indeed the cause of the failure by adding
> > various prints to QEMU and the kernel, but unfortunatley my QEMU
> > knowledge is not sufficient for me to fix it myself.
> > 
> > Any help would be much appreciated!
> Can you please test the attached series?
> (Note that I'm only interested in solving this problem as a productive
> distraction, so if the patches don't work, or require a lot of massaging
> for merging, I'll just drop them (or, preferably, give them to someone
> else).)

I like your procrastination methods!

Unfortunately this fix wasn't the right one either.


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