[ovs-dev] [PATCH v2] Simplify kernel sFlow implementation

Jesse Gross jesse at nicira.com
Thu Aug 18 11:15:38 UTC 2011


On Aug 18, 2011 12:21 AM, "Ben Pfaff" <blp at nicira.com> wrote:
>
> [bringing Neil McKee into the conversation since he knows more about
> this than me]
>
> On Wed, Aug 17, 2011 at 02:17:03PM +0800, Jesse Gross wrote:
> > The one piece that I'm the least sure of is how to handle stats.  In
> > my ideal world, we would drop the sample pool altogether and have
> > userspace figure it out for itself.  The information is clearly
> > present in userspace to calculate any packet count that may be
> > required but it might not be instantaneously accurate.  I'm unsure of
> > the consequences of this. I think it's reasonable to report some kind
> > of stats (current flow stats seems to fit) but if you're trying to get
> > the current port counters then that isn't really useful.  Directly
> > reporting port count seems a little out of place to me.  Ben, (or
>            ^^^^^^^^^ packet count?

Yes, sorry, I meant packet count on the port (as opposed to the flow), which
is essentially what the sample pool is.

> > anyone else) do you have any thoughts?
>
> My understanding is that the sflow sample pool should be
> asymptotically correct, that is, it should be within O(1) of the
> correct value as n increases to infinity.  So I think that it's OK if
> the stats are always instantaneously behind a bit.

OK, I buy that and it means that userspace already has all the information
it needs.
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