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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-11-05 13:33, Avi Kivity wrote:
> On 11/05/2012 10:51 AM, Jan Kiszka wrote:
>> 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 <jan.kiszka@siemens.com>
>>>>>>
>>>>>> 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?
>>
> 
> As I noted, init and destroy cannot cause a topology update.

Ah, right. Why are we wrapping them in transaction_begin/commit at all then?

Jan

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