tuning for high connection rates

Philipp Wuensche cryx-freebsd at h3q.com
Tue Dec 4 16:41:04 PST 2007


we are running a FreeBSD 7-BETA4 with SCHED_4BSD on a Intel Core2Dual
E6600 2.4GHz system for our bittorrent Opentracker.

The system handles about 20Kpps (18Mbit/s) incoming and 15kpps (22
Mbit/s) outgoing traffic serving 4000 connections/sec using TCP. The
connections are very short-living, all answered within one packet.

You can find the system stats at

We are now running into some limits at peak time, system is up to 100%
and em0 takes about 80% on one CPU while the Opentracker software only
takes 10-15% CPU. The system is still responsible and answers all the
requests, but we are worried what will happen if the tracker grows at
the current rate.

Currently we are out of ideas for tuning, so we kindly ask for ideas on
tuning the system to bring down the CPU usage from the em and the system
CPU usage. We tried tuning the em int_delay and abs_int_delay but
without success.

We have updated to the latest em driver:

em0: <Intel(R) PRO/1000 Network Connection Version - 6.7.3> port
0x4000-0x401f mem 0xe8000000-0xe801ffff irq 16 at device 0.0 on pci13
em0: Using MSI interrupt
em0: Ethernet address: 00:30:48:92:06:5f
em0: [FILTER]

The debug output of em0 looks like this:

em0: CTRL = 0x40140248 RCTL = 0x8002
em0: Packet buffer = Tx=20k Rx=12k
em0: Flow control watermarks high = 10240 low = 8740
em0: tx_int_delay = 66, tx_abs_int_delay = 66
em0: rx_int_delay = 32, rx_abs_int_delay = 66
em0: fifo workaround = 0, fifo_reset_count = 0
em0: hw tdh = 183, hw tdt = 183
em0: hw rdh = 139, hw rdt = 139
em0: Num Tx descriptors avail = 223
em0: Tx Descriptors not avail1 = 6225
em0: Tx Descriptors not avail2 = 3
em0: Std mbuf failed = 0
em0: Std mbuf cluster failed = 0
em0: Driver dropped packets = 0
em0: Driver tx dma failure in encap = 0

We did some tuning already and our current sysctl.conf looks like this:



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