Re: [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG

[Wei Wang]

On 07/12/2017 08:40 PM, Wei Wang wrote:
> Add a new feature, VIRTIO_BALLOON_F_SG, which enables to
> transfer a chunk of ballooned (i.e. inflated/deflated) pages using
> scatter-gather lists to the host.
>
> The implementation of the previous virtio-balloon is not very
> efficient, because the balloon pages are transferred to the
> host one by one. Here is the breakdown of the time in percentage
> spent on each step of the balloon inflating process (inflating
> 7GB of an 8GB idle guest).
>
> 1) allocating pages (6.5%)
> 2) sending PFNs to host (68.3%)
> 3) address translation (6.1%)
> 4) madvise (19%)
>
> It takes about 4126ms for the inflating process to complete.
> The above profiling shows that the bottlenecks are stage 2)
> and stage 4).
>
> This patch optimizes step 2) by transferring pages to the host in
> sgs. An sg describes a chunk of guest physically continuous pages.
> With this mechanism, step 4) can also be optimized by doing address
> translation and madvise() in chunks rather than page by page.
>
> With this new feature, the above ballooning process takes ~491ms
> resulting in an improvement of ~88%.


I found a recent mm patch, bb01b64cfab7c22f3848cb73dc0c2b46b8d38499
, zeros all the ballooned pages, which is very time consuming.

Tests show that the time to balloon 7G pages is increased from ~491 ms to
2.8 seconds with the above patch.

How about moving the zero operation to the hypervisor? In this way, we
will have a much faster balloon process.


Best,
Wei