Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization

[Michael S. Tsirkin]

On Fri, Mar 04, 2016 at 03:49:37PM +0000, Li, Liang Z wrote:
> > > > > > > Only detect the unmapped/zero mapped pages is not enough.
> > > > Consider
> > > > > > the
> > > > > > > situation like case 2, it can't achieve the same result.
> > > > > >
> > > > > > Your case 2 doesn't exist in the real world.  If people could
> > > > > > stop their main memory consumer in the guest prior to migration
> > > > > > they wouldn't need live migration at all.
> > > > >
> > > > > The case 2 is just a simplified scenario, not a real case.
> > > > > As long as the guest's memory usage does not keep increasing, or
> > > > > not always run out, it can be covered by the case 2.
> > > >
> > > > The memory usage will keep increasing due to ever growing caches,
> > > > etc, so you'll be left with very little free memory fairly soon.
> > > >
> > >
> > > I don't think so.
> > 
> > Here's my laptop:
> > KiB Mem : 16048560 total,  8574956 free,  3360532 used,  4113072 buff/cache
> > 
> > But here's a server:
> > KiB Mem:  32892768 total, 20092812 used, 12799956 free,   368704 buffers
> > 
> > What is the difference? A ton of tiny daemons not doing anything, staying
> > resident in memory.
> > 
> > > > > > I tend to think you can safely assume there's no free memory in
> > > > > > the guest, so there's little point optimizing for it.
> > > > >
> > > > > If this is true, we should not inflate the balloon either.
> > > >
> > > > We certainly should if there's "available" memory, i.e. not free but
> > > > cheap to reclaim.
> > > >
> > >
> > > What's your mean by "available" memory? if they are not free, I don't think
> > it's cheap.
> > 
> > clean pages are cheap to drop as they don't have to be written.
> > whether they will be ever be used is another matter.
> > 
> > > > > > OTOH it makes perfect sense optimizing for the unmapped memory
> > > > > > that's made up, in particular, by the ballon, and consider
> > > > > > inflating the balloon right before migration unless you already
> > > > > > maintain it at the optimal size for other reasons (like e.g. a
> > > > > > global resource manager
> > > > optimizing the VM density).
> > > > > >
> > > > >
> > > > > Yes, I believe the current balloon works and it's simple. Do you
> > > > > take the
> > > > performance impact for consideration?
> > > > > For and 8G guest, it takes about 5s to  inflating the balloon. But
> > > > > it only takes 20ms to  traverse the free_list and construct the
> > > > > free pages
> > > > bitmap.
> > > >
> > > > I don't have any feeling of how important the difference is.  And if
> > > > the limiting factor for balloon inflation speed is the granularity
> > > > of communication it may be worth optimizing that, because quick
> > > > balloon reaction may be important in certain resource management
> > scenarios.
> > > >
> > > > > By inflating the balloon, all the guest's pages are still be
> > > > > processed (zero
> > > > page checking).
> > > >
> > > > Not sure what you mean.  If you describe the current state of
> > > > affairs that's exactly the suggested optimization point: skip unmapped
> > pages.
> > > >
> > >
> > > You'd better check the live migration code.
> > 
> > What's there to check in migration code?
> > Here's the extent of what balloon does on output:
> > 
> > 
> >         while (iov_to_buf(elem->out_sg, elem->out_num, offset, &pfn, 4) == 4)
> > {
> >             ram_addr_t pa;
> >             ram_addr_t addr;
> >             int p = virtio_ldl_p(vdev, &pfn);
> > 
> >             pa = (ram_addr_t) p << VIRTIO_BALLOON_PFN_SHIFT;
> >             offset += 4;
> > 
> >             /* FIXME: remove get_system_memory(), but how? */
> >             section = memory_region_find(get_system_memory(), pa, 1);
> >             if (!int128_nz(section.size) || !memory_region_is_ram(section.mr))
> >                 continue;
> > 
> > 
> > trace_virtio_balloon_handle_output(memory_region_name(section.mr),
> >                                                pa);
> >             /* Using memory_region_get_ram_ptr is bending the rules a bit, but
> >                should be OK because we only want a single page.  */
> >             addr = section.offset_within_region;
> >             balloon_page(memory_region_get_ram_ptr(section.mr) + addr,
> >                          !!(vq == s->dvq));
> >             memory_region_unref(section.mr);
> >         }
> > 
> > so all that happens when we get a page is balloon_page.
> > and
> > 
> > static void balloon_page(void *addr, int deflate) { #if defined(__linux__)
> >     if (!qemu_balloon_is_inhibited() && (!kvm_enabled() ||
> >                                          kvm_has_sync_mmu())) {
> >         qemu_madvise(addr, TARGET_PAGE_SIZE,
> >                 deflate ? QEMU_MADV_WILLNEED : QEMU_MADV_DONTNEED);
> >     }
> > #endif
> > }
> > 
> > 
> > Do you see anything that tracks pages to help migration skip the ballooned
> > memory? I don't.
> > 
> 
> No. And it's exactly what I mean. The ballooned memory is still processed during
> live migration without skipping. The live migration code is in migration/ram.c.

So if guest acknowledged VIRTIO_BALLOON_F_MUST_TELL_HOST,
we can teach qemu to skip these pages.
Want to write a patch to do this?

> > 
> > > > > The only advantage of ' inflating the balloon before live
> > > > > migration' is simple,
> > > > nothing more.
> > > >
> > > > That's a big advantage.  Another one is that it does something
> > > > useful in real- world scenarios.
> > > >
> > >
> > > I don't think the heave performance impaction is something useful in real
> > world scenarios.
> > >
> > > Liang
> > > > Roman.
> > 
> > So fix the performance then. You will have to try harder if you want to
> > convince people that the performance is due to bad host/guest interface,
> > and so we have to change *that*.
> > 
> 
> Actually, the PV solution is irrelevant with the balloon mechanism, I just use it
> to transfer information between host and guest. 
> I am not sure if I should implement a new virtio device, and I want to get the answer from
> the community.
> In this RFC patch, to make things simple, I choose to extend the virtio-balloon and use the
> extended interface to transfer the request and free_page_bimap content.
> 
> I am not intend to change the current virtio-balloon implementation.
> 
> Liang

And the answer would depend on the answer to my question above.
Does balloon need an interface passing page bitmaps around?
Does this speed up any operations?
OTOH what if you use the regular balloon interface with your patches?


> > --
> > MST