Re: [Qemu-devel] Are there plans to achieve ram live Snapshot feature?

[Stefan Hajnoczi]

On Thu, Aug 15, 2013 at 10:26:36AM +0800, Wenchao Xia wrote:
> 于 2013-8-14 15:53, Stefan Hajnoczi 写道:
> > On Wed, Aug 14, 2013 at 3:54 AM, Wenchao Xia <address@hidden> wrote:
> >> 于 2013-8-13 16:21, Stefan Hajnoczi 写道:
> >>
> >>> On Tue, Aug 13, 2013 at 4:53 AM, Wenchao Xia <address@hidden>
> >>> wrote:
> >>>>
> >>>> 于 2013-8-12 19:33, Stefan Hajnoczi 写道:
> >>>>
> >>>>> On Mon, Aug 12, 2013 at 12:26 PM, Alex Bligh <address@hidden> wrote:
> >>>>>>
> >>>>>>
> >>>>>> --On 12 August 2013 11:59:03 +0200 Stefan Hajnoczi <address@hidden>
> >>>>>> wrote:
> >>>>>>
> >>>>>>> The idea that was discussed on address@hidden uses fork(2) to
> >>>>>>> capture the state of guest RAM and then send it back to the parent
> >>>>>>> process.  The guest is only paused for a brief instant during fork(2)
> >>>>>>> and can continue to run afterwards.
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> How would you capture the state of emulated hardware which might not
> >>>>>> be in the guest RAM?
> >>>>>
> >>>>>
> >>>>>
> >>>>> Exactly the same way vmsave works today.  It calls the device's save
> >>>>> functions which serialize state to file.
> >>>>>
> >>>>> The difference between today's vmsave and the fork(2) approach is that
> >>>>> QEMU does not need to wait for guest RAM to be written to file before
> >>>>> resuming the guest.
> >>>>>
> >>>>> Stefan
> >>>>>
> >>>>     I have a worry about what glib says:
> >>>>
> >>>> "On Unix, the GLib mainloop is incompatible with fork(). Any program
> >>>> using the mainloop must either exec() or exit() from the child without
> >>>> returning to the mainloop. "
> >>>
> >>>
> >>> This is fine, the child just writes out the memory pages and exits.
> >>> It never returns to the glib mainloop.
> >>>
> >>>>     There is another way to do it: intercept the write in kvm.ko(or other
> >>>> kernel code). Since the key is intercept the memory change, we can do
> >>>> it in userspace in TCG mode, thus we can add the missing part in KVM
> >>>> mode. Another benefit of this way is: the used memory can be
> >>>> controlled. For example, with ioctl(), set a buffer of a fixed size
> >>>> which keeps the intercepted write data by kernel code, which can avoid
> >>>> frequently switch back to user space qemu code. when it is full always
> >>>> return back to userspace's qemu code, let qemu code save the data into
> >>>> disk. I haven't check the exactly behavior of Intel guest mode about
> >>>> how to handle page fault, so can't estimate the performance caused by
> >>>> switching of guest mode and root mode, but it should not be worse than
> >>>> fork().
> >>>
> >>>
> >>> The fork(2) approach is portable, covers both KVM and TCG, and doesn't
> >>> require kernel changes.  A kvm.ko kernel change also won't be
> >>> supported on existing KVM hosts.  These are big drawbacks and the
> >>> kernel approach would need to be significantly better than plain old
> >>> fork(2) to make it worthwhile.
> >>>
> >>> Stefan
> >>>
> >>    I think advantage is memory usage is predictable, so memory usage
> >> peak can be avoided, by always save the changed pages first. fork()
> >> does not know which pages are changed. I am not sure if this would
> >> be a serious issue when server's memory is consumed much, for example,
> >> 24G host emulate 11G*2 guest to provide powerful virtual server.
> > 
> > Memory usage is predictable but guest uptime is unpredictable because
> > it waits until memory is written out.  This defeats the point of
> > "live" savevm.  The guest may be stalled arbitrarily.
> > 
>   I think it is adjustable. There is no much difference with
> fork(), except get more precise control about the changed pages.
>   Kernel intercept the change, and stores the changed page in another
> page, similar to fork(). When userspace qemu code execute, save some
> pages to disk. Buffer can be used like some lubricant. When Buffer =
> MAX, it equals to fork(), guest runs more lively. When Buffer = 0,
> guest runs less lively. I think it allows user to find a good balance
> point with a parameter.
>   It is harder to implement, just want to show the idea.

You are right.  You could set a bigger buffer size to increase guest
uptime.

> > The fork child can minimize the chance of out-of-memory by using
> > madvise(MADV_DONTNEED) after pages have been written out.
>   It seems no way to make sure the written out page is the changed
> pages, so it have a good chance the written one is the unchanged and
> still used by the other qemu process.

The KVM dirty log tells you which pages were touched.  The fork child
process could give priority to the pages which have been touched by the
guest.  They must be written out and marked madvise(MADV_DONTNEED) as
soon as possible.

I haven't looked at the vmsave data format yet to see if memory pages
can be saved in random order, but this might work.  It reduces the
likelihood of copy-on-write memory growth.

Stefan