Dynamic Ticks/HZ

Joe Holden lists at rewt.org.uk
Mon Nov 5 18:28:27 UTC 2012

Joe Holden wrote:
> Luigi Rizzo wrote:
>> On Mon, Nov 05, 2012 at 04:25:36PM +0000, Joe Holden wrote:
>>> Luigi Rizzo wrote:
>>>> On Mon, Nov 05, 2012 at 08:11:41AM -0500, Ryan Stone wrote:
>>>>> On Mon, Nov 5, 2012 at 4:40 AM, Joe Holden <lists at rewt.org.uk> wrote:
>>>>>> doh, running kernel wasn't as GENERIC as I thought it was, looks like
>>>>>> device polling not only breaks dynamic ticks but also reduces rx 
>>>>>> ability
>>>>>> significantly, exactly 150,000 pps per 1000hz on igb versus 
>>>>>> 650,000 without
>>>>>> Is this a known issue? (and if device polling isn't as useful as 
>>>>>> it once
>>>>>> was, should it be removed?)
>>>>> Device polling on modern multiqueue NICs isn't very useful because 
>>>>> you're
>>>>> limited to a single thread for handling packets.  I have a patch 
>>>>> that fixes
>>>>> this that I've let fall by the wayside.
>>>> the 150,000 is result of the combination of the default value of
>>>> sysctl  kern.polling.burst_max and kern.polling.idle_poll=0
>>>> (i think this is the default value for the latter).
>>>> The 150 was sized for the peak pps on a 100Mbit/s interface,
>>>> back in 2001. You should at least be able to raise the number
>>>> and see what kind of throughput you can achieve.
>>>> This said, modern nics also have interrupt moderation so you
>>>> don't really need polling.
>>>> cheers
>>>> luigi
>>> Hi Luigi,
>>> This makes sense, am I likely to achieve better throughput (in the 
>>> forwarding path at this point) with netisr rather than polling, 
>>> especially as mentioned above the igb does indeed have multiple 
>>> queues for rx?
>> at 1Gbit/s you probably don't need multiqueue (I am actually surpised
>> you can only do 650kpps, but perhaps because you are using ipfw and
>> not just doing plain forwarding ?)
>>> On another note, is netmap usable in the forwarding context at all as 
>>> it is rather awesome
>> It depends on what you need to do. If you have a v4/v6 router you
>> won't see any advantage (at the moment; there is some work in the
>> pipeline but probably it won't be available before spring).
>> If you just need to implement a firewall to protect the internal
>> network then it is another story and you can use the ipfw on netmap
>> that I posted in august.
>> cheers
>> luigi
> Hi,
> I have a setup where box1 is connected directly to box2 (igb0), with 
> pkt-gen however box2 doesn't seem to actually forward any packets, 
> sysctl -ip 1 shows 1.48Mpps unreachables generated (matching the input), 
>  is there something specific I need to do to test forwarding?
> Thanks,
> Joe
Nevermind, didn't set destination MAC addr, apologies for the noise!

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