An Investigation into Transport Protocols and Data Transport ...

A.2. Network Interface Cards 281
70
70
60
60
Latency (usec)
50
40
30
Latency (usec)
50
40
30
20
20
10
Minimum
Least Squares Fit: y=0.0041x+41.456 (R^2=0.671)
0
0 200 400 600 800 1000 1200 1400 1600
UDP Packet size (bytes)
(a) Supermicro P4DP8-G2 Dual Xeon
10
Minimum
Least Squares Fit: y=0.0035x+32.225 (R^2=0.378)
0
0 200 400 600 800 1000 1200 1400 1600
UDP Packet size (bytes)
(b) HP RX2600 Dual Itanium
Figure A.9: Ping-pong latency of 10Gb Intel NIC Cards (PCI-X at 133Mhz).
form of interrupt coalescing is occurring whereby the number of software
interrupts by the driver (to notify the kernel of data availability in the NIC) is
decreased by not triggering a software interrupt for every hardware interrupt
or by interrupting once in each period of time.
This has the benefits of lowering CPU utilisation as the number of context
switches required is reduced at the cost of higher latency of data receipt.
The very low gradient of 0.0146µsec/byte observed for the Intel makes good
agreement against theory (0.0111µsec/byte).
Figure A.9 shows the latency performance of the Intel PRO/10GbE LR
Server Adaptor on two different high performance server PCs. The Xeonbased
system (same as that used in the 1Gb/sec NIC tests) very good
agreement between the measured values of 0.0041µsec/byte and the theoretical
slope of 0.00393µsec/byte. The Itanium-based system also shows good
correlation between the measured results of 0.0035µsec/byte and theory of
0.00299µsec/byte.
Note in both cases for the 10Gb/sec NIC cards, the least squares fit of
the measured results are not as good as that of the 1Gb/sec NIC tests which
would account for discrepancies in the measured gradients.