[ovs-discuss] Discussion on the logical rationality of flow-limit

[taoyunupt]

在 2021-05-06 03:26:46，"Ben Pfaff" <blp at ovn.org> 写道：

>On Fri, Apr 30, 2021 at 06:10:43PM +0800, taoyunupt wrote:
>> 
>> 
>> 
>> At 2021-04-29 06:39:11, "Ben Pfaff" <blp at ovn.org> wrote:
>> >On Wed, Apr 28, 2021 at 08:12:06PM +0800, taoyunupt wrote:
>> >> Hi，
>> >>      Recently I encountered a TCP connection performance problem, the test tool is Apache benchmark.
>> >>      The OVS  in my environment is set for  hardware offload solution.  The "Requests per second" is about 6000/s， it closed to non-offload solution.
>> >> 
>> >> 
>> >>       "flow-lmit"  has a dynamic balance in udpif_revalidator, it will modify by the OVS condition（which is pind to "duration"）.   In the revalidate function, when the number of flows is greater than twice the "flow-limit" , the delete flow operation will be triggered to delete all flows; when the number of flows is greater than the "flow-limit", the aging time will be adjusted to 0.1s, Slowly delete flow.   
>> >> 
>> >> 
>> >>      
>> >>      I found that the reason for the poor performance is that when the number of flows in the datapath increases and the processing power of OVS decreases, a large number of flow deletions are generated. 
>> >>      As we know, In the hardware offloading scenario, although there are a lot of flows, in fact, apart from the first packet, there is no need to process subsequent packets. 
>> >>      In my opinion, the dynamic balance mechanism is very necessary, but we need to increase the value of “duration”, or provide some new switches for some high-performance scenarios, such as hardware offloading.
>> >>      Do we still need to restrict the number of flows so strictly? By the way, do you have another solution to resolve this?   
>> >
>> >It's been a long time since I worked on this, but I recall two reasons
>> >for the flow limit.  First, each flow takes up memory.  Second, each
>> >flow must be revalidated periodically, meaning that it uses CPU as
>> >well.
>> >
>> >I don't, off-hand, remember the real reasons why the logic for deleting
>> >flows works as it does.  It might be in the comments or the commit
>> >messages.  But, I suspect, it is because above the flow-limit we want to
>> >try to reduce the amount of memory and CPU time dedicated to the cache
>> >and, if we arrive at twice the flow limit, we conclude that that try
>> >failed and that we must have a large number of very short flows so that
>> >caching is not very valuable anyhow.
>> >
>> >In a hardware offload scenario, we get rid of some costs (the cost of
>> >processing and forwarding packets and perhaps the memory cost in the
>> >datapath) but we still have the cost of revalidating them.  When there
>> >are many flows, we add the extra cost of balancing flows between
>> >software and the offload hardware.
>> >
>> >Because of the remaining cost and the added ones when there is hardware
>> >offload, it's not obvious to me that we can stop limiting the number of
>> >flows.  I think that experimentation and measurements would be needed.
>> >Perhaps this would be an adjustment to the dynamic algorithm, rather
>> 
>> >than a removal of it.
>> 
>> 
>> I think we can increase the init `flow_limit` in udpif_create,10000 is a small number for current server and OS, and if 'duration' is small ,we should increase faster by a lager number not `flow_limit += 1000;`.
>> I have not better idea for this situation. Do you have some suggestion? I am very glad to do this change.
>
>What kind of number are you thinking about?  I'd like to come up with a
>rationale for choosing it.  It might be even better to come up with an

>algorithm or a heuristic for choosing it.


I think we could  set the initial value to 200,000, and adjust the increase to 20,000 each time.  Can  you describe the rationale algorithm you meationed in detailed ? 
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