Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping free pages

[Li, Liang Z]

> > > > > > Agree. Current balloon just send 256 PFNs a time, that's too
> > > > > > few and lead to too many times of virtio transmission, that's
> > > > > > the main reason for the
> > > > > bad performance.
> > > > > > Change the VIRTIO_BALLOON_ARRAY_PFNS_MAX to a large value
> can
> > > > > improve
> > > > > > the performance significant. Maybe we should increase it
> > > > > > before doing the further optimization, do you think so ?
> > > > >
> > > > > We could push it up a bit higher: 256 is 1kbyte in size, so we
> > > > > can make it 3x bigger and still fit struct virtio_balloon is a
> > > > > single page. But if we are going to add the bitmap variant
> > > > > anyway, we probably
> > > shouldn't bother.
> > > > >
> > > > > > > > c. address translation and madvise() operation (24%,
> > > > > > > > 1423ms)
> > > > > > >
> > > > > > > How is this split between translation and madvise?  I
> > > > > > > suspect it's mostly madvise since you need translation when
> > > > > > > using bitmap as
> > > well.
> > > > > > > Correct? Could you measure this please?  Also, what if we
> > > > > > > use the new MADV_FREE instead?  By how much would this help?
> > > > > > >
> > > > > > For the current balloon, address translation is needed.
> > > > > > But for live migration, there is no need to do address translation.
> > > > >
> > > > > Well you need ram address in order to clear the dirty bit.
> > > > > How would you get it without translation?
> > > > >
> > > >
> > > > If you means that kind of address translation, yes, it need.
> > > > What I want to say is, filter out the free page can be done by
> > > > bitmap
> > > operation.
> > > >
> > > > Liang
> > >
> > > OK so I see that your patches use block->offset in struct RAMBlock
> > > to look up bits in guest-supplied bitmap.
> > > I don't think that's guaranteed to work.
> >
> > It's part of the bitmap operation, because the latest change of the
> ram_list.dirty_memory.
> > Why do you think so? Could you tell me the reason?
> >
> > Liang
> 
> Sorry, why do I think what? That ram_addr_t is not guaranteed to equal GPA
> of the block?
> 

I mean why do you think that's can't guaranteed to work.
Yes, ram_addr_t is not guaranteed to equal GPA of the block. But I didn't use 
them as
GPA. The code in the filter_out_guest_free_pages() in my patch just follow the 
style of
the latest change of  ram_list.dirty_memory[].

The free page bitmap got from the guest in my RFC patch has been filtered out 
the
'hole', so the bit N of the free page bitmap and the bit N in 
ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]->blocks are corresponding to
the same guest page.  Right?
If it's true, I think I am doing the right thing?


Liang

> E.g. HACKING says:
>       Use hwaddr for guest physical addresses except pcibus_t
>       for PCI addresses.  In addition, ram_addr_t is a QEMU internal
> address
>       space that maps guest RAM physical addresses into an intermediate
>       address space that can map to host virtual address spaces.
> 
> 
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