linux-kernel - Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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Date:   Tue, 10 Dec 2019 11:02:11 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

On Tue, Dec 10, 2019 at 02:31:54PM +0100, Paolo Bonzini wrote:
> On 10/12/19 14:25, Michael S. Tsirkin wrote:
> >> There is no new infrastructure to track the dirty pages---it's just a
> >> different way to pass them to userspace.
> > Did you guys consider using one of the virtio ring formats?
> > Maybe reusing vhost code?
> 
> There are no used/available entries here, it's unidirectional
> (kernel->user).

Agreed.  Vring could be an overkill IMHO (the whole dirty_ring.c is
100+ LOC only).

> 
> > If you did and it's not a good fit, this is something good to mention
> > in the commit log.
> > 
> > I also wonder about performance numbers - any data here?
> 
> Yes some numbers would be useful.  Note however that the improvement is
> asymptotical, O(#dirtied pages) vs O(#total pages) so it may differ
> depending on the workload.

Yes.  I plan to give some numbers when start to work on the QEMU
series (after this lands).  However as Paolo said, those numbers would
probably only be with some special case where I know the dirty ring
could win.  Frankly speaking I don't even know whether we should
change the default logging mode when the QEMU work is done - I feel
like the old logging interface is still good in many major cases
(small vms, or high dirty rates).  It could be that we just offer
another option when the user could consider to solve specific problems.

Thanks,

-- 
Peter Xu