Messages in this thread | | | Date | Fri, 12 Jan 2018 17:14:09 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v21 2/5 RESEND] virtio-balloon: VIRTIO_BALLOON_F_SG |
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On 01/11/2018 07:06 PM, Tetsuo Handa wrote: > Wei Wang wrote: >> Michael, could we merge patch 3-5 first? > No! I'm repeatedly asking you to propose only VIRTIO_BALLOON_F_SG changes. > Please don't ignore me. > > > > Patch 4 depends on patch 2. Thus, back to patch 2.
There is not strict dependence per se. I plan to split the two features into 2 series, and post out 3-5 first, and the corresponding hypervisor code. After that's done, I'll get back to the discussion of patch 2.
> Now, proceeding to patch 4. > > Your patch is trying to call add_one_sg() for multiple times based on > > ---------------------------------------- > + /* > + * This is expected to never fail: there is always at least 1 entry > + * available on the vq, because when the vq is full the worker thread > + * that adds the sg will be put into sleep until at least 1 entry is > + * available to use. > + */
This will be more clear in the new version which is not together with patch 2.
> > Now, I suspect we need to add VIRTIO_BALLOON_F_FREE_PAGE_VQ flag. I want to see > the patch for the hypervisor side which makes use of VIRTIO_BALLOON_F_FREE_PAGE_VQ > flag because its usage becomes tricky. Between the guest kernel obtains snapshot of > free memory blocks and the hypervisor is told that some pages are currently free, > these pages can become in use. That is, I don't think > > The second feature enables the optimization of the 1st round memory > transfer - the hypervisor can skip the transfer of guest free pages in the > 1st round. > > is accurate. The hypervisor is allowed to mark pages which are told as "currently > unused" by the guest kernel as "write-protected" before starting the 1st round. > Then, the hypervisor performs copying all pages except write-protected pages as > the 1st round. Then, the 2nd and later rounds will be the same. That is, > VIRTIO_BALLOON_F_FREE_PAGE_VQ requires the hypervisor to do 0th round as > preparation. Thus, I want to see the patch for the hypervisor side. > > Now, what if all free pages in the guest kernel were reserved as ballooned pages? > There will be no free pages which VIRTIO_BALLOON_F_FREE_PAGE_VQ flag would help. > The hypervisor will have to copy all pages because all pages are either currently > in-use or currently in balloons. After ballooning to appropriate size, there will > be little free memory in the guest kernel, and the hypervisor already knows which > pages are in the balloon. Thus, the hypervisor can skip copying the content of > pages in the balloon, without using VIRTIO_BALLOON_F_FREE_PAGE_VQ flag. > > Then, why can't we do "inflate the balloon up to reasonable level (e.g. no need to > wait for reclaim and no need to deflate)" instead of "find all the free pages as of > specific moment" ? That is, code for VIRTIO_BALLOON_F_DEFLATE_ON_OOM could be reused > instead of VIRTIO_BALLOON_F_FREE_PAGE_VQ ? >
I think you misunderstood the work, which seems not easy to explain everything from the beginning here. I wish to review patch 4 (I'll send out a new independent version) with Michael if possible. I'll discuss with you about patch 2 later. Thanks.
Best, Wei
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