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SubjectRe: The performance and behaviour of the anti-fragmentation related patches
> But if you don't require a lot of higher order allocations anyway, then
> guest fragmentation caused by ballooning doesn't seem like much problem.

If you only need to allocate 1 page size and smaller allocations then no it's not a
problem. As soon as you go above that it will be. You don't need to go all the way
up to MAX_ORDER size to see an impact, it's just increasingly more severe as you get
away from 1 page and towards MAX_ORDER.

>
> If you need higher order allocations, then ballooning is bad because of
> fragmentation, so you need memory unplug, so you need higher order
> allocations. Goto 1.

Yes, it's a closed loop. But hotplug isn't the only one that needs higher order
allocations. In fact it's pretty far down the list. I look at it like this, a lot
of users need high order allocations for better performance and things like on-demand
hugepages. As a bonus you get memory hot-remove.

> Balooning probably does skew memory management stats and watermarks, but
> that's just because it is implemented as a module. A couple of hooks
> should be enough to allow things to be adjusted?

That is a good idea independent of the current discussion.

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