An Investigation into Transport Protocols and Data Transport ...

A.2. Network Interface Cards 285
200
Mean Latency (usec)
150
100
50
RxInt 0
RxInt 5
RxInt 40
RxInt 64
RxInt 100
0
0 200 400 600 800 1000 1200 1400 1600
UDP Packet Size (bytes)
Figure A.12: Effect of Interrupt Coalescing upon packet latency.
A.2.4
Interrupt Coalescing
Depending on the configuration set by the driver, a modern NIC can interrupt
the host for each packet sent or received or it can continue to buffer packets for
a certain period of time. This is known as interrupt coalescing [PJD04] and
the details and options are hardware and NIC driver dependent. The NIC
may generate interrupts after a fixed number of packets have been processed
or after a fixed time from the first packet transferred after the last interrupt.
In some cases, the NIC dynamically changes the interrupt coalescence times
dependent on the packet receive rate [Cor]. Separate parameters are usually
available for the transmit and receive functions of the NIC.
The effect of interrupt coalescing are most apparent upon packet latencies
as shown in Figure A.12.
It shows that as the packet size is increased,
a predictable increase in the latency for each interrupt coalesce setting is
achieved.
More importantly, higher values of interrupt coalesce result in
proportionate increases in the experienced latency.
The effect of a high value of coalescing often lowers throughput as packets
could be dropped due to the lack of buffer space. When set too low, high
CPU utilisation occurs due to the necessary context switching between kernel
processes to absorb/transmit the packet .