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SubjectRe: [RFC PATCH 0/3] Weight-balanced binary tree + KVM growable memory slots using wbtree
On 03/01/2011 08:20 PM, Alex Williamson wrote:
> > > It seems like we need a good mixed workload benchmark. So far we've
> > > only tested worst case, with a pure emulated I/O test, and best case,
> > > with a pure memory test. Ordering an array only helps the latter, and
> > > only barely beats the tree, so I suspect overall performance would be
> > > better with a tree.
> >
> > But if we cache the missed-all-memslots result in the spte, we eliminate
> > the worst case, and are left with just the best case.
>
> There's potentially a lot of entries between best case and worst case.

The mid case is where we have a lot of small slots which are
continuously flushed. That would be (ept=0 && new mappings continuously
established) || (lots of small mappings && lots of host paging
activity). I don't know of any guests that continuously reestablish BAR
mappings; and host paging activity doesn't apply to device assignment.
What are we left with?

> >
> > The problem here is that all workloads will cache all memslots very
> > quickly into sptes and all lookups will be misses. There are two cases
> > where we have lookups that hit the memslots structure: ept=0, and host
> > swap. Neither are things we want to optimize too heavily.
>
> Which seems to suggest that:
>
> A. making those misses fast = win
> B. making those misses fast + caching misses = win++
> C. we don't care if the sorted array is subtly faster for ept=0
>
> Sound right? So is the question whether cached misses alone gets us 99%
> of the improvement since hits are already getting cached in sptes for
> cases we care about?

Yes, that's my feeling. Caching those misses is a lot more important
than speeding them up, since the cache will stay valid for long periods,
and since the hit rate will be very high.

Cache+anything=O(1)
no-cache+tree=O(log(n))
no-cache+array=O(n)

--
error compiling committee.c: too many arguments to function



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