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

Xiao Guangrong
2011-06-29T12:28:37Z

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".