let me try to translate my mail to english...
* Henning Brauer  [2007-09-07 12:49]:

quoted text
> * Sam Fourman Jr.  [2007-09-07 04:44]:

> > I guess on this Subject I need educated because I am not all together

> > sure what interrupt mitigation is and why I want it.

>

> traditionally, and interrupt cards intterupts once per packet it has 

traditionally, a network inteerface card interrupted once per packet ...
> received (or successfully sent, but let us ignore the tx path for now).

quoted text
> interrupt mitigation, coalescing, *insert name-of-the-day for int

> mitgation here* etc all describe teh same thing, a certain heuristic to

> not interrupt once per packet, but once for a bunch. this is usually

> achieved by a combination of amount of packets (i. e. "interrupt every

> 25 packets") or bytes and a timer (to make sure there is no packet

> sitting in the rx queue for too long).

and let me explain why that makes so much of a difference. rocessing

the packets is one thing, context switching between the interrupt

handler (which takes teh packet from the nic's rx ring and puts it into

the ipintrq), the soft interrupt handling (where the packet is taken

from the ipintrq and running through ip_input, then maybe ip_forward and

_output and put into the (potentially other) nic's output queue, well,

that switching is not exactly free, performance wise, to put it nicely.

so instead of interrupt, one packet into ipintrq, processing at softnet,

oh wait interrupt, peel another out of the rx ring, etc yadda yadda it

is much cheaper to process a bunch of packets at once.
the downside is that you potentially increase latency, since after

packet reception teh nic does not interupt immediately and you don't

process immediately (for some value of immediately), but only after

enough othe rpackets are there. to cope with that effect the time I

mentioned above is there, so that a apcket doesn't sit in teh rx ring

for longer than XXX msec, wether otehrsd arrive or not. 
--

Henning Brauer, hb@bsws.de, henning@openbsd.org

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