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

[Justin Pettit]

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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