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

David S. Ahern
2008-06-23T14:19:12Z

Avi Kivity wrote:
> David S. Ahern wrote:
>> Avi:
>>
>> We did not get a chance to do this at the Forum. I'd be interested in
>> whatever options you have for reducing the scan time further (e.g., try
>> to get scan time down to 1-2 seconds).
>>
>>
>
> I'm unlikely to get time to do this properly for at least a week, as
> this will be quite difficult and I'm already horribly backlogged.
> However there's an alternative option, modifying the source and getting
> it upstreamed, as I think RHEL 3 is still maintained.
>
> The attached patch (untested) should give a 3X boost for kmap_atomics,
> by folding the two accesses to set the pte into one, and by dropping the
> access that clears the pte. Unfortunately it breaks the ABI, since
> external modules will inline the original kmap_atomic() which expects
> the pte to be cleared.
>
> This can be worked around by allocating new fixmap slots for kmap_atomic
> with the new behavior, and keeping the old slots with the old behavior,
> but we should first see if the maintainers are open to performance
> optimizations targeting kvm.
>
RHEL3 is in Maintenance mode (for an explanation see
http://www.redhat.com/security/updates/errata/) which means performance
enhancement patches will not make it in.

Also, I'm going to be out of the office for a couple of weeks in July,
so I will need to put this aside until mid-August or so. I'll reevaluate
options then.

david