An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 87
1
0.8
Graph of Sender RX Interrupt Effect on CPU
Sender Kernel
Receiver Kernel
Sender User
Receiver User
1
0.8
Graph of Receiver RX Interrupt Effect on CPU
Sender Kernel
Receiver Kernel
Sender User
Receiver User
CPU Load (fraction)
0.6
0.4
CPU Load (fraction)
0.6
0.4
0.2
0.2
0
0 20 40 60 80 100
RXInt (usec)
(a) Sender
0
0 20 40 60 80 100
RXInt (usec)
(b) Receiver
Figure 5.4: Effect of varying RX Interrupt values on CPU load.
can be deactivated using Web100’s net.ipv4.WAD_IFQ sysctl.
However, imposing a large txqueuelen may add to extra delays in the
transport of TCP segments due to extra queuing latencies that may be experienced.
This could result in large variations of RTT and hence result in
poor ack-clocking and spurious timeouts [LK02, GL03].
The default value of 100 for txqueuelen in the Linux 2.4 kernels has been
reported to be insufficient to maintain high throughput transport [ABL + 03].
Newer Linux 2.6 kernels implement a default of 1,000.
Similarly, at the receiving end of transport, packets will be queued, once
past the device driver, in a queue called max_backlog.
5.2.3 Driver Modifications
Interrupt Coalescing
By reducing the number of context switches required to process a packet,
CPU overheads can be reduced at the expense of increased latency upon the
incoming/outgoing packets.
Intel e1000 network interface card drivers enable interrupt coalescing in