[ovs-dev] [ovs-discuss] [ACLv2 07/19] ofproto: Make current packet counts more accurate.

Ben Pfaff blp at nicira.com
Fri Sep 4 22:26:43 UTC 2009


Jesse Gross <jesse at nicira.com> writes:

> Ben Pfaff wrote:
>> Jesse Gross <jesse at nicira.com> writes:
>>
>>   
>>> When the stats for a currently active flow are requested this
>>> ensures that the packets not handled by the kernel are counted
>>> immediately.  Before, these packets would only be counted once
>>> the kernel flow expired and the counts were combined.
>>>     
>>
>> Did you observe that this makes a difference in practice?  I'm
>> not sure that a subrule ever has a nonzero packet_count or
>> byte_count at a point where query_stats() can see it.  There are
>> only a couple of places that update a rule's 'packet_count' or
>> 'byte_count', and in each case it looks like
>> rule_post_uninstall() is called afterward, which will update the
>> parent rule's counters and zero out the subrule's counters.
>>
>>   
>
> Yes, I saw this was a problem while testing.  I gather stats using a
> few different methods for various types of counters and some of them
> were lagging 1 packet behind the others until the flow expired, at
> which point they caught up.
>
> What happens is that a packet gets kicked up to userspace and gets
> processed in handle_odp_msg.  This calls rule_execute, which updates
> the subrule's stats.  Until the flow expires the stats for that packet
> exist only in the subrule - not in the datapath because it didn't
> handle the packet and not in the super rule because it hasn't expired
> yet.  So without this change, those packets will be missing if you
> query before flow expiration.

Thanks for the explanation.

We seriously need some tests for this kind of thing.  I'm not
very confident about our stats-keeping in corner cases.




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