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

[Avi Kivity]

On 11/30/2011 07:15 AM, Takuya Yoshikawa wrote:
> (2011/11/30 14:02), Takuya Yoshikawa wrote:
>
>> IIUC, even though O(1) is O(1) at the timing of GET DIRTY LOG, it
>> needs O(N) write
>> protections with respect to the total number of dirty pages:
>> distributed, but
>> actually each page fault, which should be logged, does some write
>> protection?
>
> Sorry, was not precise.  It depends on the level, and not completely
> distributed.
> But I think it is O(N), and the total number of costs will not change
> so much,
> I guess.

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.

-- 
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