> > > > > > 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.
>
> > > > 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
> --
> MST
_______________________________________________
Virtualization mailing list
Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
