Re: [Qemu-devel] [PATCH] memory: Reintroduce dirty flag to optimize changes on disabled regions

[Avi Kivity]

On 11/05/2012 08:26 AM, Jan Kiszka wrote:
> On 2012-11-04 20:21, Avi Kivity wrote:
> > On 11/04/2012 10:30 AM, Jan Kiszka wrote:
> >> From: Jan Kiszka <address@hidden>
> >>
> >> Cirrus is triggering this, e.g. during Win2k boot: Changes only on
> >> disabled regions require no topology update when transaction depth drops
> >> to 0 again.
> > 
> > 817dcc5368988b0 (pci: give each device its own address space) mad this
> > much worse by multiplying the number of address spaces.  Each change is
> > now evaluated N+2 times, where N is the number of PCI devices.  It also
> > causes a corresponding expansion in memory usage.
>
> I know... But this regression predates your changes, is already visible
> right after 02e2b95fb4.
>
> > 
> > I want to address this by caching AddressSpaceDispatch trees with the
> > key being the contents of the FlatView for that address space.  This
> > will drop the number of distinct trees to 2-4 (3 if some devices have
> > PCI_COMMAND_MASTER disabled, 4 if the PCI address space is different
> > from the cpu memory address space) but will fail if we make each address
> > space different (for example filtering out the device's own BARs).
> > 
> > If this change also improves cpu usage sufficiently, then it will be
> > better than your patch, which doesn't recognize changes in an enabled
> > region inside a disabled or hidden region.
>
> True, though the question is how common such scenarios are. This one
> (cirrus with win2k) is already special.
>
> >  In other words, your patch
> > fits the problem at hand but isn't general.  On the other hand my
> > approach doesn't eliminate render_memory_region(), just the exec.c stuff
> > and listener updates.  So we need to understand where the slowness comes
> > from.
>
> I would just like to have some even intermediate solution for 1.3. We
> can still make it more perfect later on if required.
>

I think we should apply a v2 then, the more general optimizations will
take some time.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.