Re: [dpdk-dev] dpdk/vhost-user and VM migration

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, Oct 14, 2015 at 12:16:29AM +0300, Michael S. Tsirkin wrote:
> Hello!
> I am currently looking at how using dpdk on host, accessing VM memory
> using the vhost-user interface, interacts with VM migration.
> 
> The issue is that any changes made to VM memory need to be tracked so
> that updates can be sent from migration source to destination.
> 
> At the moment, there's a proposal of an interface extension to
> vhost-user which adds ability to do this tracking through shared memory.
> dpdk would then be responsible for tracking these updates using atomic
> operations to set bits (per page written) in a memory bitmap.
> 
> This only needs to happen during migration, at other times there could
> be a jump to skip this logging.
> 
> Is this a reasonable approach?

Hi Michael,

As I stated in another email, adding dpdk/vhost-user vm migration
support is my second TODO. However, I barely know anything about
vm migration so far, that I can't tell now.

I will re-visit this question when I finished my first item and
after some more investigation.

	--yliu

> Would performance degradation during
> migration associated with atomics affect the performance to a level
> where it's no longer useful?  Pls note these logs aren't latency
> sensitive, so can be done on a separate core, and can be batched.
> 
> 
> One alternative I'm considering is extending linux kernel so it can do
> this tracking automatically, by marking pages read-only, detecting a
> pagefault and logging the write, then making the pages writeable.  This
> would mean higher worst-case overhead (pagefaults are expensive) but
> lower average one (not extra code after the first fault).  Not sure how
> feasible this is yet, this would be harder to implement and it will only
> be apply to newer host kernels.
> 
> Any feedback would be appreciated.
> 
> -- 
> MST