I have an Intel Pro 1000 MT (PWLA8490MT) NIC (em(4) driver 1.7.35) installed in a Pentium III 500 Mhz with 512 MB RAM (100 Mhz) running FreeBSD 5.4-RC3. The machine is routing traffic between multiple VLANs. Recently I did a benchmark with/without device polling enabled. Without device polling I was able to transfer roughly 180 Mbit/s. The router however was suffering when doing this benchmark. Interrupt load was peaking 100% - overall the system itself was quite unusable (_very_ high system load). With device polling enabled the interrupt kept stable around 40-50% and max transfer rate was nearly 70 Mbit/s. Not very scientific tests, but it gave me a pin point.
The card is plugged into a 32-bit PCI slot, correct? If so, 180 Mbit/s is decent. I have a gigabit LAN at home using Pro 1000 MTs (in 32-bit PCI slots) and get NFS transfers maxing out around 23 MB/s, which is ~180 Mbit/s. Gigabit performance with 32-bit cards is atrocious. It reminds me of the old 100 Mbit/s ISA cards...
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HZ set to 1000 as recommended in README for the em(4) driver. Driver is of cource compiled into kernel.
You'll need HZ set to more than 1000 for gigabit; bump it up to at least 2000. That should increase polling throughput a lot. I'm not sure about other polling parameters, however.
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