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

[Jan Kiszka]

On 2012-11-05 09:12, Avi Kivity wrote:
> 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.

OK - what should v2 do differently?

Jan

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