netdev - Re: pktgen question

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Date:	Mon, 08 Oct 2007 16:43:39 -0500
From:	Steve Wise <swise@...ngridcomputing.com>
To:	Ben Greear <greearb@...delatech.com>
CC:	Rick Jones <rick.jones2@...com>, hadi@...erus.ca,
	Evgeniy Polyakov <johnpol@....mipt.ru>, netdev@...r.kernel.org,
	Robert Olsson <Robert.Olsson@...a.slu.se>
Subject: Re: pktgen question



Ben Greear wrote:
> Rick Jones wrote:
>>>> Perf-wise, you could clone the skbs up front, then deliver them to 
>>>> the nic in a tight loop.  This would mitigate the added overhead 
>>>> introduced by calling skb_clone() in the loop doing transmits...
>>>
>>> That only works if you are sending a small number of skbs.  You can't 
>>> pre-clone several minutes worth of 10Gbe traffic
>>> with any normal amount of RAM.
>>
>> Does pktgen really need to allocate anything more than some smallish 
>> fraction more than the depth of the driver's transmit queue?
> 
> If you want to send sustained high rates of traffic, for more than
> just a trivial amount of time, then you either have to play the current
> trick with the skb_get(), or you have to allocate a real packet each time
> (maybe with skb_clone() or similar, but it's still more overhead than 
> the skb_get
> which only bumps a reference count.)
> 
> I see no other way, but if you can think of one, please let me know.
> 

You can keep freed skb's that were cloned on a free list, then reuse 
them once freed.  You can detect when the driver frees them by adding a 
destroy function to the skb.  So what will happen is the set of cloned 
skbs needed will eventually settled down to a constent amount and the 
amount will be based on the latency involved in transmitting a single 
skb.  And it should be bounded by the max txq depth.  Yes?  (or am I all 
wet :)

So you would pay the overhead of cloning only until you hit this steady 
state.

Whatchathink?


> Thanks,
> Ben
> 
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