On Mon, Feb 27, 2017 at 04:43:04PM +0100, Michal Hocko wrote:

On Mon 27-02-17 12:25:10, Heiko Carstens wrote:

quoted

On Mon, Feb 27, 2017 at 11:02:09AM +0100, Vitaly Kuznetsov wrote:

quoted

A couple of other thoughts:
1) Having all newly added memory online ASAP is probably what people
want for all virtual machines.

This is not true for s390. On s390 we have "standby" memory that a guest
sees and potentially may use if it sets it online. Every guest that sets
memory offline contributes to the hypervisor's standby memory pool, while
onlining standby memory takes memory away from the standby pool.

The use-case is that a system administrator in advance knows the maximum
size a guest will ever have and also defines how much memory should be used
at boot time. The difference is standby memory.

Auto-onlining of standby memory is the last thing we want.

quoted

Unfortunately, we have additional complexity with memory zones
(ZONE_NORMAL, ZONE_MOVABLE) and in some cases manual intervention is
required. Especially, when further unplug is expected.

This also is a reason why auto-onlining doesn't seem be the best way.

Can you imagine any situation when somebody actually might want to have
this knob enabled? From what I understand it doesn't seem to be the
case.

I can only speak for s390, and at least here I think auto-online is always
wrong, especially if you consider the added complexity that you may want to
online memory sometimes to ZONE_NORMAL and sometimes to ZONE_MOVABLE.

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