linux-kernel - Re: [PATCH 3/5] vhost: support upto 509 memory regions

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Date:	Mon, 22 Jun 2015 11:45:59 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, andrey@...l.ru
Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions



On 22/06/2015 09:10, Igor Mammedov wrote:
> So far HVA is unusable even if we will make this assumption and let guest crash.
> virt_net doesn't work with it anyway,
> translation of GPA to HVA for descriptors works as expected (correctly)
> but vhost+HVA hack backed virtio still can't send/received packets.
> 
> That's why I prefer to merge kernel solution first as a stable and
> not introducing any issues solution. And work on userspace approach on
> top of that.

Also, let's do some math.

Let's assume 3 network devices per VM, one vhost device per queue, one
queue per VCPU per network device.  Let's assume the host is
overcommitted 3:1.

Thus we have 3*3=9 times vhost devices as we have physical CPUs.

We're thus talking about 108K per physical CPU.

>From a relative point of view, and assuming 1 GB of memory per physical
CPU (pretty low amount if you're overcommitting CPU 3:1), this is 0.01%
of the total memory.

>From an absolute point of view, it takes a system with 60 physical CPUs
to reach the same memory usage as the vmlinuz binary of a typical distro
kernel (not counting the modules).

Paolo

> Hopefully it could be done but we still would need time
> to iron out side effects/issues it causes or could cause so that
> fix became stable enough for production.
> 
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