Marcelo Tosatti <mtosa...@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.
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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