On Thu, Jul 18, 2019 at 1:49 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Thu, Jul 18, 2019 at 01:34:03PM -0700, Alexander Duyck wrote: > > On Thu, Jul 18, 2019 at 1:24 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > > > On Thu, Jul 18, 2019 at 08:34:37AM -0700, Alexander Duyck wrote: > > > > > > > For example we allocate pages until shrinker kicks in. > > > > > > > Fair enough but in fact many it would be better to > > > > > > > do the reverse: trigger shrinker and then send as many > > > > > > > free pages as we can to host. > > > > > > > > > > > > I'm not sure I understand this last part. > > > > > > > > > > Oh basically what I am saying is this: one of the reasons to use page > > > > > hinting is when host is short on memory. In that case, why don't we use > > > > > shrinker to ask kernel drivers to free up memory? Any memory freed could > > > > > then be reported to host. > > > > > > > > Didn't the balloon driver already have a feature like that where it > > > > could start shrinking memory if the host was under memory pressure? If > > > > so how would adding another one add much value. > > > > > > Well fundamentally the basic balloon inflate kind of does this, yes :) > > > > > > The difference with what I am suggesting is that balloon inflate tries > > > to aggressively achieve a specific goal of freed memory. We could have a > > > weaker "free as much as you can" that is still stronger than free page > > > hint which as you point out below does not try to free at all, just > > > hints what is already free. > > > > Yes, but why wait until the host is low on memory? > > It can come about for a variety of reasons, such as > other VMs being aggressive, or ours aggressively caching > stuff in memory. > > > With my > > implementation we can perform the hints in the background for a low > > cost already. So why should we wait to free up memory when we could do > > it immediately. Why let things get to the state where the host is > > under memory pressure when the guests can be proactively freeing up > > the pages and improving performance as a result be reducing swap > > usage? > > You are talking about sending free memory to host. > Fair enough but if you have drivers that aggressively > allocate memory then there won't be that much free guest > memory without invoking a shrinker. So then what we really need is a way for the host to trigger the shrinker via a call to drop_slab() on the guest don't we? Then we could automatically hint the free pages to the host.
