Messages in this thread | | | Date | Fri, 03 Aug 2018 13:35:25 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker |
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On 08/02/2018 07:47 PM, Michal Hocko wrote: > On Thu 02-08-18 18:32:44, 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. >> >>> Let's say >>> you have a medium page cache workload which triggers kswapd to do a >>> light reclaim? Hardcoded shrinking sounds quite dubious to me but I have >>> no idea how people expect this to work. Shouldn't this be more >>> adaptive? How precious are those pages anyway? >> Those pages are given to host to use usually because the guest has enough >> free memory, and host doesn't want to waste those pieces of memory as they >> are not used by this guest. When the guest needs them, it is reasonable that >> the guest has higher priority to take them back. >> But I'm not sure if there would be a more adaptive approach than "gradually >> giving back as the guest wants more". > I am not sure I follow. Let me be more specific. Say you have a trivial > stream IO triggering reclaim to recycle clean page cache. This will > invoke slab shrinkers as well. Do you really want to drop your batch of > pages on each invocation? Doesn't that remove them very quickly? Just > try to dd if=large_file of=/dev/null and see how your pages are > disappearing. Shrinkers usually scale the number of objects they are > going to reclaim based on the memory pressure (aka targer to be > reclaimed).
Thanks, I think it looks better to make it more adaptive. I'll send out a new version for review.
Best, Wei
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