On Fri, Jul 28, 2017 at 04:25:19PM +0800, Wei Wang wrote: > 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. Sounds like it should be reverted. Post a revert pls and we'll discuss. > How about moving the zero operation to the hypervisor? In this way, we > will have a much faster balloon process. > > > Best, > Wei Or in other words hypervisors should not be stupid and should not try to run ksm on DONTNEED pages. -- MST
