On 10/23/2017 01:13 AM, Michael S. Tsirkin wrote:
On Fri, Oct 20, 2017 at 07:54:26PM +0800, Wei Wang wrote:
This patch forces the cease of the inflating work when OOM occurs.
The fundamental idea of memory ballooning is to take out some guest
pages when the guest has low memory utilization, so it is sensible to
inflate nothing when the guest is already under memory pressure.
On the other hand, the policy is determined by the admin or the
orchestration layer from the host. That is, the host is expected to
re-start the memory inflating request at a proper time later when
the guest has enough memory to inflate, for example, by checking
the memory stats reported by the balloon.
Is there any other way to do it? And if so can't we just have guest do
it automatically? Maybe the issue is really that fill attempts to
allocate memory aggressively instead of checking availability.
Maybe with deflate on oom it should check availability?
I think it might not be easy to do it in the guest in practice.
For example, the host asks for 4G from the guest, and the guest checks
that it has 4G that can be inflated at that point. While it is inflating
and 2G
is done inflating, another new task on the guest comes out and
takes the remaining 2G to use. Now the guest has nothing to inflate.
This would raise the questions:
1) what is the point of checking the availability?
Maybe we could just let the guest inflate as much as it can, that is, till
balloon_page_enqueue() returns NULL, then stop inflating.
2) How long would the host has to wait for this guest to get the
remaining 2G?
If I understand "guest do it automatically" correctly: now the guest is
responsible
for giving another 2G, which he owes to the host in this case - not
giving up inflating
whenever there is some free memory. Maybe in the next 1 hour it wouldn't
have any
memory available to give to the host. The time seems non-deterministic.
If we leave it to the host to define the policy, I think it would be easier.
Once the host finds that the guest can only offer 2G, then it can just
give up asking
for memory from this guest, and continue to check other guests to see if
it can get
some memory there to satisfy the needs.
Best,
Wei
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