From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH v2 2/2] virtio_balloon: replace oom notifier with shrinker Date: Wed, 1 Aug 2018 13:34:44 +0200 Message-ID: <20180801113444.GK16767__49120.2788231478$1533123164$gmane$org@dhcp22.suse.cz> References: <1532683495-31974-1-git-send-email-wei.w.wang@intel.com> <1532683495-31974-3-git-send-email-wei.w.wang@intel.com> <20180730090041.GC24267@dhcp22.suse.cz> <5B619599.1000307@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <5B619599.1000307@intel.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Wei Wang Cc: virtio-dev@lists.oasis-open.org, mst@redhat.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-mm@kvack.org, akpm@linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org 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? 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? -- Michal Hocko SUSE Labs