Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

Xiao Guangrong
2011-06-29T12:39:04Z

On 06/29/2011 08:27 PM, Avi Kivity wrote:
> On 06/29/2011 03:28 PM, Xiao Guangrong wrote:
>> On 06/29/2011 08:18 PM, Avi Kivity wrote:
>> > On 06/29/2011 02:50 PM, Xiao Guangrong wrote:
>> >> >> >
>> >> >> > I think we should do this unconditionally. The cost of ping-ponging the shared cache line containing reader_counter will increase with large smp counts. On the other hand, zap_page is very rare, so it can be a little slower. Also, less code paths = easier to understand.
>> >> >> >
>> >> >>
>> >> >> On soft mmu, zap_page is very frequently, it can cause performance regression in my test.
>> >> >
>> >> > Any idea what the cause of the regression is? It seems to me that simply deferring freeing shouldn't have a large impact.
>> >> >
>> >>
>> >> I guess it is because the page is freed too frequently, i have done the test, it shows
>> >> about 3219 pages is freed per second
>> >>
>> >> Kernbench performance comparing:
>> >>
>> >> the origin way: 3m27.723
>> >> free all shadow page in rcu context: 3m30.519
>> >
>> > I don't recall seeing such a high free rate. Who is doing all this zapping?
>> >
>> > You may be able to find out with the function tracer + call graph.
>> >
>>
>> I looked into it before, it is caused by "write flood" detected, i also noticed
>> some pages are zapped and allocation again and again, maybe we need to improve
>> the algorithm of detecting "write flood".
>
> Ok. Let's drop the two paths, and put this improvement on the TODO instead.
>

Avi, i am sorry, i do not understand it clearly, it means keep the patch as the
original way and do the improvement after it merged?