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


From: Wei Wang
Subject: Re: [Qemu-devel] [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG
Date: Fri, 28 Jul 2017 16:25:19 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

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





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