[ovs-discuss] If ovs doesn't masking process in flow_tbl_lookup, What are the disadvantages ?

Heung Sik Choi hschoi at os.korea.ac.kr
Fri Jun 9 05:39:25 UTC 2017


Thanks for the kind reply !!

Have a nice day !!

2017-06-09 14:37 GMT+09:00 Ben Pfaff <blp at ovn.org>:

> With microflow cache only, there's one cache miss per microflow.
> With an effective megaflow cache, the miss rate is much lower.
>
> The paper gives examples for your other question.
>
> On Fri, Jun 09, 2017 at 01:47:14PM +0900, Heung Sik Choi wrote:
> > Thanks to reply.
> >
> > Regarding your second question, which flow rules are you asking about?
> >
> > I just want to know that when ovs had only an in-kernel microflow cache,
> > How many in-kernel cache miss there are, and  also how much improving
> >  there are when using megaflow.
> >
> > And I have a last question. short-lived flows were main problem when
> there
> > was micro flow cache. I don't know the circumstance where short lived
> > connection happen often. can you let me know about it?
> >
> >
> > Thanks!!
> >
> >
> > 2017-06-09 13:29 GMT+09:00 Ben Pfaff <blp at ovn.org>:
> >
> > > When OVS had only an in-kernel microflow cache, there were at least two
> > > reasons for performance problems with many short-lived flows.  The
> first
> > > was the cost of sending packets to userspace.  The second was the cost
> > > of translating the packets through the entire OpenFlow pipeline.  The
> > > megaflow cache solves both problems: it eliminates both the
> > > kernel-to-user-to-kernel transition and (if the megaflow cache is
> > > effective) the additional OpenFlow translations.
> > >
> > > Regarding your second question, which flow rules are you asking about?
> > >
> > > On Fri, Jun 09, 2017 at 12:22:09PM +0900, Heung Sik Choi wrote:
> > > > What does "if only the microflow cache works" mean?
> > > >
> > > > sorry to confuse you. I'm not good at English.
> > > >
> > > > In the paper, the authors say that at start of ovs implementation,
> there
> > > > has been microflow cache(EMC), but megaflow wasn't implemented.
> > > >
> > > > At that time, they say  there was a problem caused by short lived
> > > > connections.
> > > >
> > > > I want to know the problem detail. I guess that the problem made ovs
> get
> > > > flow rules from userlevel, and that made performance degradation.
> > > >
> > > > also I want to know that when not using megaflow, are there big
> > > difference
> > > > in aspect of number of flow rules.If there are the big difference,
> can
> > > you
> > > > tell me the difference in numbers?
> > > >
> > > >
> > > >
> > > > 2017-06-09 0:43 GMT+09:00 Ben Pfaff <blp at ovn.org>:
> > > >
> > > > > On Thu, Jun 08, 2017 at 04:33:54PM +0900, Heung Sik Choi wrote:
> > > > > > 1. If only microflow cache works and there are many short lived
> > > > > > connections, does it make many tuples in table, and does it
> suffers
> > > > > serious
> > > > > > performance degradation by the many tuples(very many tuple makes
> > > context
> > > > > > switching to Userlevel)?
> > > > >
> > > > > What does "if only the microflow cache works" mean?
> > > > >
> > > > > > 2. I know that OVS uses hash for table index. if only microflow
> > > cache and
> > > > > > there are small amount of entries in the table, is it possible to
> > > find
> > > > > > entry in O(1) time?
> > > > >
> > > > > Yes.
> > > > >
> > > > > > 3. In paper, microflow and megaflow use the word 'cache'.
> However,
> > > when I
> > > > > > open the OVS code, it looks like they are in main memory area.
> > > doesn't
> > > > > the
> > > > > > 'cache' mean hardware cache?
> > > > >
> > > > > No.
> > > > >
> > >
>
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