Strange CPU distributionat very high level bandwidth

[Invernizzi Fabrizio]

Hi all

i am going on with some performance tests on a 10gbe network card with FreeBSD.

I am doing this test: I send UDP traffic to be forwarded to the other port of the card on both the card ports.
Using 1492-long packets i am uppering the number  of packets per second i sent In order to see wich is the maximum bandwidth (or pps) the system can support without losses.

The limit seems to be about 1890Mbps per port (3870 Mbps total).
Looking more in deep the CPU behaviour i see this :
  - uppering the sent pps results in uppering the intterrupt time (about 90%)
  - when i am very strict to the limit, interrupt time falls to about 10% and CPU is always (85%) in system (rx/tx driver procedure)

Questions:
- Is not the AIM intended to contrast this behaviour to limit interrupts sent to CPU? (nothing changes if i disable it)
- Why does the system start loosing pkts in that condition?
- Why does the system seem to perform better when it is managing more context switches?



These are my system details:

- HP 380 G5 (XEON X5420, CPU speed: 2.50GHz, BUS speed: 1333 MHz, L2 cache size: 12 MB, L2 cache speed: 2,5 GHz) with 1 quad-core installed.

- Network card: Silicom PE10G2i-LR - Dual Port Fiber (LR) 10 Gigabit Ethernet PCI Express Server Adapter Intel(r) based (chip 82598EB).

- FreeBSD 7.2-RELEASE (64 bit)

Driver ixgbe-1.8.6

hw.intr_storm_threshold:2000000

dev.ix.0.low_latency: 128
dev.ix.0.ave_latency: 400
dev.ix.0.bulk_latency: 1200
dev.ix.1.low_latency: 128
dev.ix.1.ave_latency: 400
dev.ix.1.bulk_latency: 1200

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