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SubjectRe: [PATCH 3/4 changelog-v2] KVM: Switch to srcu-less get_dirty_log()
On Thu, Mar 08, 2012 at 10:35:45AM +0900, Takuya Yoshikawa wrote:
> Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> > What is worrying are large memory cases: think of the 50GB slot case.
> > 100ms hold time is pretty bad (and reacquiring the lock is relatively
> > simple).
> >
>
> OK, I agree basically.
>
> But let me explain one thing before deciding what I should do next.
>
> With my method, even when we use a 50GB slot, the hold time will be under
> 10ms -- not 100ms -- if the memory actually updated from the last time is
> 1GB (256K dirty pages).
>
> > > 8747274.0 10973.3 33.3 -31% -3% 256K
> Note that this unit-test was done on my tiny core-i3 32-bit host.
> On servers which can install more than 50GB memory, this will become
> much faster: actually my live migration tests done on Xeon saw much
> better numbers. Unit-test has been tuned for the worst case.
>
> I admit that if the dirty memory size is more than 10GB, we may see over
> 100ms hold time you are worrying about.
>
> For that I was proposing introducing a new GET_DIRTY_LOG API which can
> restrict the number of dirty pages we get the log - but this is a long
> term goal.
>
>
> So, I am OK to try to introduce cond_resched_lock_cb() as you suggested.
> But, as I explained above, my current implementation does not introduce
> any real regression concerning to mmu_lock hold time:
>
> Before we could see 10ms hold time in real workloads:
> > funcgraph_entry: ! 9783.060 us | kvm_mmu_slot_remove_write_access();
>
> I have never seen ms hold time with my method in the same workloads.
>
> So, it would be helpful if you can apply the patch series and I can work
> on top of that: although I cannot use servers with 100GB memory now,
> migrating a guest with 16GB memory or so may be possible later: I need
> to reserve servers for that.

Makes sense.

It looks good to me, Avi can you review & ack please?

> I also want to know "mmu_lock -- TLB flush"-decoupling plan. We will not
> need to introduce cond_resched_lock_cb() in sched.h if that is possible.
>
> Thanks,
> Takuya


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