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Re: [Qemu-devel] Migration dirty bitmap: should only mark pages as dirty

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] Migration dirty bitmap: should only mark pages as dirty after they have been sent
Date: Mon, 26 Sep 2016 12:23:50 +0100
User-agent: Mutt/1.7.0 (2016-08-17)

* Chunguang Li (address@hidden) wrote:
> Hi all!
> I have some confusion about the dirty bitmap during migration. I have digged 
> into the code. I figure out that every now and then during migration, the 
> dirty bitmap will be grabbed from the kernel space through 
> ioctl(KVM_GET_DIRTY_LOG), and then be used to update qemu's dirty bitmap. 
> However I think this mechanism leads to resendness of some NON-dirty pages.
> Take the first iteration of precopy for instance, during which all the pages 
> will be sent. Before that during the migration setup, the 
> ioctl(KVM_GET_DIRTY_LOG) is called once, so the kernel begins to produce the 
> dirty bitmap from this moment. When the pages "that haven't been sent" are 
> written, the kernel space marks them as dirty. However I don't think this is 
> correct, because these pages will be sent during this and the next iterations 
> with the same content (if they are not written again after they are sent). It 
> only makes sense to mark the pages which have already been sent during one 
> iteration as dirty when they are written.
> Am I right about this consideration? If I am right, is there some advice to 
> improve this?

I think you're right that this can happen; to clarify I think the
case you're talking about is:

  Iteration 1
    sync bitmap
    start sending pages
    page 'n' is modified - but hasn't been sent yet
    page 'n' gets sent
  Iteration 2
    sync bitmap
       'page n is shown as modified'
    send page 'n' again

So you're right that is wasteful; I guess it's more wasteful
on big VMs with slow networks where the length of each iteration
is large.

Fixing it is not easy, because you have to be really careful
never to miss a page modification, even if the page is sent
about the same time it's dirtied.

One way would be to sync the dirty log from the kernel
in smaller chunks.


> Thanks,
> Chunguang Li
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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