[ovs-discuss] Kernel-space and user-space flow tables

Ben Pfaff blp at nicira.com
Sat May 5 11:03:08 PDT 2012


No.

What is the use case for adjusting it?

On Sat, May 05, 2012 at 10:58:18AM -0700, Iben Rodriguez wrote:
> Justin - can the idle time be easily adjusted?  Kind of like a TTL?
> 
> I b e n
> 
> On Sat, May 5, 2012 at 10:24 AM, Justin Pettit <jpettit at nicira.com> wrote:
> > If the packets were spaced out enough, the kernel flows may have been
> > already evicted. Idle kernel flows will only stay in the kernel ~5 seconds.
> > If you run something like a ping that sends a packet every second, it should
> > stay in the kernel.
> >
> > --Justin
> >
> >
> > On May 5, 2012, at 2:21 AM, "Danny Y. Huang" <yuh024 at cs.ucsd.edu> wrote:
> >
> > Hello, I am a graduate student. I've been trying to understand why OVS keeps
> > one flow table in kernel, and the other in the user-space. In particular,
> > why would the flow still have to go through the user-space, even though the
> > relevant rules haven already been set up in the kernel's flow table?
> >
> > To illustrate this problem, I ran a simple experiment that involves two
> > hosts as traffic source and sink, a host that ran OVS, and a host that ran
> > NOX. The controller application would install a rule for any new flows.
> >
> > First, I started OVS with an empty flow table. Then I had a packet sent from
> > the source host to the sink. Since this was a new flow,
> > ovs_flow_tbl_lookup() would not find the flow's key. As a result, the kernel
> > module sent the flow to the user-space via ovs_dp_upcall(). Once inside the
> > user-space, the insert_rule() within classifier.c was invoked, followed by
> > the installation of the rule in the user-space flow table, and subsequently
> > in the kernel's flow table.
> >
> > Here's where the confusion kicks in. I had the same packet sent from the
> > source host to the sink the second time. I expected that, since the kernel's
> > flow table already contained the relevant rule, the flow would be matched
> > entirely within the kernel, and that no user-space would be involved.
> > However, I was wrong. As the packet arrived, ovs_flow_tbl_lookup() still
> > reported that the flow-key was not found, causing ovs_dp_upcall() to be
> > invoked. While in the user-space, a classifier_lookup() was carried out and
> > the flow was found in the flow table. The rule was added to the kernel
> > module's flow table again, via the ovs_flow_tbl_insert() call, as if the
> > events in the previous paragraph had not happened at all.
> >
> > I had the same packet sent through OVS the third time. Again, an upcall was
> > made, the flow was found in the user-space's flow table, the rule was
> > inserted in the kernel module's flow table, before the kernel module
> > executed the rule's actions.
> >
> > It seemed that a flow had to repeatedly go through the user space even
> > though it has a matching rule. Why is this so? Why would the kernel module's
> > flow table fail to remember installed rules, while the one in classifier.c
> > managed to do so?
> >
> > Thank you.
> >
> > Danny Y. Huang
> > Ph.D. Candidate
> > Systems and Networking Group
> > University of California, San Diego
> > http://sysnet.ucsd.edu/~dhuang/
> >
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> >
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