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

Avi Kivity
2011-06-29T12:27:58Z

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.

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