On 12/03/2011 06:37 AM, Takuya Yoshikawa wrote: > Avi Kivity <a...@redhat.com> 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. -- error compiling committee.c: too many arguments to function
