On Thu, Aug 02, 2018 at 06:32:44PM +0800, Wei Wang wrote: > On 08/01/2018 07:34 PM, Michal Hocko wrote: > > On Wed 01-08-18 19:12:25, Wei Wang wrote: > > > On 07/30/2018 05:00 PM, Michal Hocko wrote: > > > > On Fri 27-07-18 17:24:55, Wei Wang wrote: > > > > > The OOM notifier is getting deprecated to use for the reasons mentioned > > > > > here by Michal Hocko: https://lkml.org/lkml/2018/7/12/314 > > > > > > > > > > This patch replaces the virtio-balloon oom notifier with a shrinker > > > > > to release balloon pages on memory pressure. > > > > It would be great to document the replacement. This is not a small > > > > change... > > > OK. I plan to document the following to the commit log: > > > > > > The OOM notifier is getting deprecated to use for the reasons: > > > - As a callout from the oom context, it is too subtle and easy to > > > generate bugs and corner cases which are hard to track; > > > - It is called too late (after the reclaiming has been performed). > > > Drivers with large amuont of reclaimable memory is expected to be > > > released them at an early age of memory pressure; > > > - The notifier callback isn't aware of the oom contrains; > > > Link: https://lkml.org/lkml/2018/7/12/314 > > > > > > This patch replaces the virtio-balloon oom notifier with a shrinker > > > to release balloon pages on memory pressure. Users can set the amount of > > > memory pages to release each time a shrinker_scan is called via the > > > module parameter balloon_pages_to_shrink, and the default amount is 256 > > > pages. Historically, the feature VIRTIO_BALLOON_F_DEFLATE_ON_OOM has > > > been used to release balloon pages on OOM. We continue to use this > > > feature bit for the shrinker, so the shrinker is only registered when > > > this feature bit has been negotiated with host. > > Do you have any numbers for how does this work in practice? > > It works in this way: for example, we can set the parameter, > balloon_pages_to_shrink, to shrink 1GB memory once shrink scan is called. > Now, we have a 8GB guest, and we balloon out 7GB. When shrink scan is > called, the balloon driver will get back 1GB memory and give them back to > mm, then the ballooned memory becomes 6GB. > > When the shrinker scan is called the second time, another 1GB will be given > back to mm. So the ballooned pages are given back to mm gradually. I think what's being asked here is a description of tests that were run. Which workloads see improved behaviour? Our behaviour under memory pressure isn't great, in particular it is not clear when it's safe to re-inflate the balloon, if host attempts to re-inflate it too soon then we still get OOM. It would be better if VIRTIO_BALLOON_F_DEFLATE_ON_OOM would somehow mean "it's ok to ask for almost all of memory, if guest needs memory from balloon for apps to function it can take it from the balloon". -- MST
