[ovs-discuss] Flow eviction based on LRU

Avi Cohen (A) avi.cohen at huawei.com
Thu Jan 26 07:32:13 UTC 2017



> -----Original Message-----
> From: ovs-discuss-bounces at openvswitch.org [mailto:ovs-discuss-
> bounces at openvswitch.org] On Behalf Of Joe Stringer
> Sent: Wednesday, 25 January, 2017 8:47 PM
> To: Avi Cohen (A)
> Cc: ovs-discuss at openvswitch.org
> Subject: Re: [ovs-discuss] Flow eviction based on LRU
> 
> On 24 January 2017 at 23:55, Avi Cohen (A) <avi.cohen at huawei.com> wrote:
> > Thanks Joe,
> > 1.  so the only flows-eviction process  (from datapath cache)  is
> implemented in udpif_revalidator thread ?  deleting flows that exceed  10s
> idle time ?
> 
> Yes. Alternatively if flow dump duration is long (takes ~1s to dump all flows),
> then additional heuristics will kick in to delete low-throughput flows, see
> should_revalidate(). Furthermore, if any configuration or OpenFlow changes
> then that thread will delete related datapath flows.
> 
> > 2.  Where is the implementation of http://openvswitch.org/ovs-
> vswitchd.conf.db.5.pdf  ?
> > This intends to be  a LRU approximation
> 
> Can you be more specific where in the 58-page document you're referring
> to? It's possible the documentation is a little out of sync with the current
> implementation.
[Avi Cohen (A)] 
Joe - look at page 36 -  pasted below: - specially look at the pseudo LRU eviction algorithm 3 steps    

overflow_policy: optional string, either refuse or evict
Controls the switch’s behavior when an OpenFlow flow table modification request would add
flows in excess of flow_limit. The supported values are:
refuse Refuse to add the flow or flows. This is also the default policy when overflow_policy is
unset.
evict Delete the flow that will expire soonest. See groups for details.
groups: set of strings
When overflow_policy is evict, this controls how flows are chosen for eviction when the flow table
would otherwise exceed flow_limit flows. Its value is a set of NXM fields or sub-fields, each
of which takes one of the forms field[] or field[start..end], e.g. NXM_OF_IN_PORT[]. Please
see nicira−ext.h for a complete list of NXM field names.
When a flow must be evicted due to overflow, the flow to evict is chosen through an approximation
of the following algorithm:
1. Divide the flows in the table into groups based on the values of the specified fields or sub-
fields, so that all of the flows in a given group have the same values for those fields. If a flow
does not specify a given field, that field’s value is treated as 0.
2. Consider the flows in the largest group, that is, the group that contains the greatest number of
flows. If two or more groups all have the same largest number of flows, consider the flows in
all of those groups.
3. Among the flows under consideration, choose the flow that expires soonest for eviction.
The eviction process only considers flows that have an idle timeout or a hard timeout. That is,
eviction never deletes permanent flows. (Permanent flows do count against flow_limit.)
Open vSwitch ignores any inv alid or unknown field specifications.
When overflow_policy is not evict, this column has no effect.

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