FreeBSD Network Polling

I've Intel PRO/1000 Gigabit Ethernet adapter installed in my server. How do I set network card (NIC) polling and disable interrupts under FreeBSD operating systems to boost network performance for 100M and 1000M network links?

Device polling is a technique that lets the operating system periodically poll devices, instead of relying on the devices to generate interrupts when they need attention. This increase server network responsiveness and performance. In particular, polling reduces the overhead for context switches which is iincurred when servicing interrupts, and gives more control on the scheduling of the CPU between various tasks (user processes, software interrupts, device handling) which ultimately reduces the chances of livelock in the system.

Make Sure Your Card Supports Network Polling

Device polling requires support in the device drivers. As of this writing FreeBSD 7.2 support only following NIC device drivers:

Driver Name Description
bge Broadcom BCM570x/5714/5721/5722/5750/5751/5752/5789 PCI Gigabit Ethernet adapter driver
dc DEC/Intel 21143 and clone 10/100 Ethernet driver
em Intel(R) PRO/1000 Gigabit Ethernet adapter driver
fwe Ethernet emulation driver for FireWire
fwip IP over FireWire driver
fxp Intel EtherExpress PRO/100 Ethernet device driver
ixgb Intel(R) PRO/10GbE Ethernet driver for the FreeBSD operating system
nfe NVIDIA nForce MCP Ethernet driver
nge National Semiconductor PCI Gigabit Ethernet adapter driver
re RealTek 8139C+/8169/816xS/811xS/8101E PCI/PCIe Ethernet adapter driver
rl RealTek 8129/8139 Fast Ethernet device driver
sf Adaptec AIC-6915 "Starfire" PCI Fast Ethernet adapter driver
sis SiS 900, SiS 7016 and NS DP83815/DP83816 Fast Ethernet device driver
ste Sundance Technologies ST201 Fast Ethernet device driver
stge Sundance/Tamarack TC9021 Gigabit Ethernet adapter driver
vge VIA Networking Technologies VT6122 PCI Gigabit Ethernet adapter driver
vr VIA Technologies Rhine I/II/III Ethernet device driver
xl 3Com Etherlink XL and Fast Etherlink XL Ethernet device driver

Refer above man pages to get information about polling and driver loading settings settings.

Compile FreeBSD Kernel

To enable polling in the driver, add the following options to the kernel configuration, and then recompile the kernel. The default kernel file name is GENERIC, which is located at the following location:

  • AMD 64 (64 bit) kernel is at /usr/src/sys/amd64/conf/
  • i386 (32 bit) kernel is at /usr/src/sys/i386/conf/

Open kernel config file:

# cd /usr/src/sys/`uname -p`/conf
Add the following for em driver (see above man pages for device specific polling settings)

options HZ=1000

It is strongly recommended to use HZ=1000 or 2000 with DEVICE_POLLING to achieve smoother behaviour. Save and close the file. Recompile FreeBSD kernel, enter:

# make buildkernel KERNCONF=GENERIC
# make installkernel KERNCONF=GENERIC

Reboot the box and boot into new kernel:

# reboot

How Do I Enable Polling?

At runtime use ifconfig command to turn polling on:

# ifconfig device polling
# ifconfig em0 polling

How Do I Disable Polling?

At runtime use ifconfig command to turn polling off:

# ifconfig device -polling
# ifconfig em0 -polling

Make Configuration Persistence

Update /etc/rc.conf as follows:

ifconfig_em0="inet netmask media 10baseT/UTP mediaopt full-duplex polling"

Save and close the file.

How Do I Verify Polling Settings?

To verify settings enter:

# ifconfig device
# ifconfig em0

To see MIB related to polling enter:

# sysctl -a kern.polling

Sample Output:

kern.polling.idlepoll_sleeping: 1
kern.polling.stalled: 0
kern.polling.suspect: 0
kern.polling.phase: 4
kern.polling.enable: 0
kern.polling.handlers: 2
kern.polling.residual_burst: 0
kern.polling.pending_polls: 0
kern.polling.lost_polls: 0
kern.polling.short_ticks: 0
kern.polling.reg_frac: 20
kern.polling.user_frac: 50
kern.polling.idle_poll: 0
kern.polling.each_burst: 5
kern.polling.burst_max: 150
kern.polling.burst: 150

Understanding MIB Variables

Quoting from the man page:

	     When polling is enabled, and provided that there is some work to
	     do, up to this percent of the CPU cycles is reserved to userland
	     tasks, the remaining fraction being available for polling pro-
	     cessing.  Default is 50.

	     Maximum number of packets grabbed from each network interface in
	     each timer tick.  This number is dynamically adjusted by the ker-
	     nel, according to the programmed user_frac, burst_max, CPU speed,
	     and system load.

	     The burst above is split into smaller chunks of this number of
	     packets, going round-robin among all interfaces registered for
	     polling.  This prevents the case that a large burst from a single
	     interface can saturate the IP interrupt queue
	     (net.inet.ip.intr_queue_maxlen).  Default is 5.

	     Upper bound for kern.polling.burst.  Note that when polling is
	     enabled, each interface can receive at most (HZ * burst_max)
	     packets per second unless there are spare CPU cycles available
	     for polling in the idle loop.  This number should be tuned to
	     match the expected load (which can be quite high with GigE
	     cards).  Default is 150 which is adequate for 100Mbit network and

	     Controls if polling is enabled in the idle loop.  There are no
	     reasons (other than power saving or bugs in the scheduler's han-
	     dling of idle priority kernel threads) to disable this.

	     Controls how often (every reg_frac / HZ seconds) the status reg-
	     isters of the device are checked for error conditions and the
	     like.  Increasing this value reduces the load on the bus, but
	     also delays the error detection.  Default is 20.

	     How many active devices have registered for polling.

	     Legacy MIB, that was used to enable or disable polling globally.
	     Currently if set to 1, polling is enabled on all capable inter-
	     faces.  If set to 0, polling is disabled on all interfaces.

How Do I Verify Interrupt Usage Dropped Or Not?

Run top command on both server and look for interrupt line in output:

# top

How Do I Test Network Throughput?

Install iperf, enter:

# portsnap fetch update
# cd /usr/ports/benchmarks/iperf
# make install clean && rehash

After polling is enabled, use iperf tool for measuring the maximum data throughput rate of a communications link or network access. On server start iperf as follows:

# iperf -s -B 

On client type the same command as follows:

# iperf -c  -d -t 60 -i 10

  • -s : run in server mode
  • -B : IP bind to IP
  • -c IP : run in client mode, connecting to IP
  • -d : Do a bidirectional test simultaneously
  • -t 60 : time in 60 seconds to transmit for.
  • -i 10 : pause 10 seconds between periodic bandwidth reports.

Test Results

After enabling polling you should see nice performance increase while large number of clients connects to server under heavy load. My testing result between two WAN links connected via 10Mbits/sec metro Ethernet network:

Test Polling Interrupt
Data Transmit 8.87 Mbits/sec 6.28 Mbits/sec
CPU load 34% 73%

You can clearly see increased network throughput while RX CPU load went down while using network polling.

Recommended readings:

  1. The official iperf project home page.
  2. man pages - iperf, polling(4) and ifconfig