Re: [Qemu-devel] [PATCH 0/4] KVM: Dirty logging optimization using rmap

[Avi Kivity]

On 12/03/2011 06:37 AM, Takuya Yoshikawa wrote:
> Avi Kivity <address@hidden> wrote:
> > That's true.  But some applications do require low latency, and the
> > current code can impose a lot of time with the mmu spinlock held.
> > 
> > The total amount of work actually increases slightly, from O(N) to O(N
> > log N), but since the tree is so wide, the overhead is small.
> > 
>
> Controlling the latency can be achieved by making the user space limit
> the number of dirty pages to scan without hacking the core mmu code.
>
>       The fact that we cannot transfer so many pages on the network at
>       once suggests this is reasonable.

That is true.  Write protecting everything at once means that there is a
large window between the sampling the dirty log, and transferring the
page.  Any writes within that window cause a re-transfer, even when they
should not.

>
> With the rmap write protection method in KVM, the only thing we need is
> a new GET_DIRTY_LOG api which takes the [gfn_start, gfn_end] to scan,
> or max_write_protections optionally.

Right.

>
>       I remember that someone suggested splitting the slot at KVM forum.
>       Same effect with less effort.
>
> QEMU can also avoid unwanted page faults by using this api wisely.
>
>       E.g. you can use this for "Interactivity improvements" TODO on
>       KVM wiki, I think.
>
> Furthermore, QEMU may be able to use multiple threads for the memory
> copy task.
>
>       Each thread has its own range of memory to copy, and does
>       GET_DIRTY_LOG independently.  This will make things easy to
>       add further optimizations in QEMU.
>
> In summary, my impression is that the main cause of the current latency
> problem is not the write protection of KVM but the strategy which tries
> to cook the large slot in one hand.
>
> What do you think?

I agree.  Maybe O(1) write protection has a place, but it is secondary
to fine-grained dirty logging, and if we implement it, it should be
after your idea, and further measurements.

-- 
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