Re: [PATCH 1/5] mm: Add support for unaccepted memory

Andi Kleen
2021-08-10T19:23:54Z

> But, not everyone is going to agree with me.

Both the Intel TDX and the AMD SEV side independently came to opposite
conclusions. In general people care a lot about boot time of VM guests.

>
> This also begs the question of how folks know when this "blip" is over.
> Do we have a counter for offline pages? Is there any way to force page
> acceptance? Or, are we just stuck allocating a bunch of memory to warm
> up the system?
>
> How do folks who care about these new blips avoid them?

It's not different than any other warmup. At warmup time you always have
lots of blips until the working set stabilizes. For example in
virtualization first touch of a new page is usually an EPT violation
handled to the host. Or in the native case you may need to do IO or free
memory. Everybody who based their critical latency percentiles around a
warming up process would be foolish, the picture would be completely
distorted.

So the basic operation is adding some overhead, but I don't think
anything is that unusual compared to the state of the art.

Now perhaps the locking might be a problem if the other operations all
run in parallel, causing unnecessary serialization If that's really a
problem I guess we can optimize later. I don't think there's anything
fundamental about the current locking.

-Andi