IBM REDP-4285-00 User Manual

4285ch04.fm

Draft Document for Review May 4, 2007 11:35 am

136

Linux Performance and Tuning Guidelines

LAN adapters are efficient when network applications requesting data generate requests for
large frames. Applications that request small blocks of data require the LAN adapter
communication processor to spend a larger percentage of time executing overhead code for
every byte of data transmitted. This is why most LAN adapters cannot sustain full wire speed
for all frame sizes.

Refer to Tuning IBM System x Servers for Performance, SG24-5287. section 10.3. Advance d
network features for more details.

4.7.8 Increasing the packet queues

After increasing the size of the various network buffers, it is suggested that the amount of
allowed unprocessed packets be increased, so that the kernel will wait longer before dropping
packets. To do so, edit the value in /proc/sys/net/core/netdev_max_backlog.

4.7.9 Increasing the transmit queue length

Increase the txqueuelength parameter to a value between 1000 and 20000 per interface. This
is especially useful for high-speed connections that perform large, homogeneous data
transfers. The transmit queue length can be adjusted by using the ifconfig command as
shown in Example 4-23.

Example 4-23 Setting the transmit queue length

[root@linux ipv4]# ifconfig eth1 txqueuelen 2000

4.7.10 Decreasing interrupts

Handling network packets requires the Linux kernel to handle a significant amount of
interrupts and context switches unless NAPI is being used. For Intel e1000–based network
interface cards, make sure that the network card driver was compiled with the
CFLAGS_EXTRA -DCONFIG_E1000_NAPI flag. Broadcom tg3 modules should come in
their newest version with built-in NAPI support.

If you need to recompile the Intel e1000 driver in order to enable NAPI, you can do so by
issuing the following command on your build system:

make CFLAGS_EXTRA -DCONFIG_E1000_NAPI

In addition, on multiprocessor systems, binding the interrupts of the network interface cards to
a physical CPU may yield additional performance gains. To achieve this goal you first have to
identify the IRQ by the respective network interface. The data obtained via the ifconfig
command will inform you of the interrupt number.

Example 4-24 Identifying the interrupt

[root@linux ~]# ifconfig eth1
eth1      Link encap:Ethernet  HWaddr 00:11:25:3F:19:B3
        inet addr:10.1.1.11  Bcast:10.255.255.255 Mask:255.255.0.0
        inet6 addr: fe80::211:25ff:fe3f:19b3/64 Scope:Link
        UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
        RX packets:51704214 errors:0 dropped:0 overruns:0 frame:0
        TX packets:108485306 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:1000
        RX bytes:4260691222 (3.9 GiB) TX bytes:157220928436 (146.4 GiB)
        Interrupt:169