Re: [kvm-devel] performance with guests running 2.4 kernels (specifically RHEL3)

[Andrea Arcangeli <andrea@qumranet.com>]

On Wed, May 28, 2008 at 08:13:44AM -0600, David S. Ahern wrote:
> Weird. Could it be something about the hosts?

Note that the VM itself will never make use of kmap. The VM is "data"
agonistic. The VM has never any idea with the data contained by the
pages. kmap/kmap_atomic/kunmap_atomic are only need to access _data_.

Only I/O (if not using DMA, or because of bounce buffers) and page
faults triggered in user process context, or other operations again
done from user process context will call into kmap or kmap_atomic.

And if KVM is inefficient in handling kmap/kmap_atomic that will lead
to the user process running slower, and in turn generating less
pressure to the guest and host VM if something. Guest will run slower
than it should if KVM isn't optimized for the workload but it
shouldn't alter any VM kernel thread CPU usage, only the CPU usage of
the guest process context and host system time in qemu task should go
up, nothing else. This is again because the VM will never care about
the data contents and it'll never invoked kmap/kmap_atomic.

So I never found a relation to the symptom reported of VM kernel
threads going weird, with KVM optimal handling of kmap ptes.