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Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default
Date: Wed, 29 Nov 2017 06:17:40 +0200

On Tue, Nov 28, 2017 at 11:20:27PM +0300, Denis V. Lunev wrote:
> > There's one thing I don't understand in your test case: if you
> > just found out that Linux will behave worse if it assumes that
> > the VCPUs are sharing a L3 cache, why are you configuring a
> > 8-core VCPU topology explicitly?
> >
> > Do you still see a difference in the numbers if you use "-smp 8"
> > with no "cores" and "threads" options?
> >
> This is quite simple. A lot of software licenses are bound to the amount
> of CPU __sockets__. Thus it is mandatory in a lot of cases to set topology
> with 1 socket/xx cores to reduce the amount of money necessary to
> be paid for the software.

This answers the first question but not the second one.

If the answer to the second one is negative, then I don't understand why
changing the default makes sense.

I would expect qemu by default to be as close to emulating a physical
system as possible.  If one has to deviate from that for some workloads,
that is fine, but probably not a good default.


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