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

[Amnon Ilan <ailan@redhat.com>]

----- Original Message -----
> From: "Yuanhan Liu" <yuanhan.liu@linux.intel.com>
> To: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: dev@dpdk.org
> Sent: Friday, October 16, 2015 10:37:38 AM
> Subject: Re: [dpdk-dev] dpdk/vhost-user and VM migration
> 
> 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.

Yuanhan, 

Live-migration for vhost-user is now available upstream.
Do you need some guidance on how to implement it in DPDK?

Amnon

> 
> 	--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
>