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Subjectquestions on NAPI processing latency and dropped network packets
Hi all,

I've got an issue that's popped up with a deployed system running
2.6.10. I'm looking for some help figuring out why incoming network
packets aren't being processed fast enough.

After a recent userspace app change, we've started seeing packets being
dropped by the ethernet hardware (e1000, NAPI is enabled). The
error/dropped/fifo counts are going up in ethtool:

rx_packets: 32180834
rx_bytes: 5480756958
rx_errors: 862506
rx_dropped: 771345
rx_length_errors: 0
rx_over_errors: 0
rx_crc_errors: 0
rx_frame_errors: 0
rx_fifo_errors: 91161
rx_missed_errors: 91161

This link is receiving roughly 13K packets/sec, and we're dropping
roughly 51 packets/sec due to fifo errors.

Increasing the rx descriptor ring size from 256 up to around 3000 or so
seems to make the problem stop, but it seems to me that this is just a
workaround for the latency in processing the incoming packets.

So, I'm looking for some suggestions on how to fix this or to figure out
where the latency is coming from.

Some additional information:


1) Interrupts are being processed on both cpus:

root@base0-0-0-13-0-11-1:/root> cat /proc/interrupts
CPU0 CPU1
30: 1703756 4530785 U3-MPIC Level eth0




2) "top" shows a fair amount of time processing softirqs, but very
little time in ksoftirqd (or is that a sampling artifact?).


Tasks: 79 total, 1 running, 78 sleeping, 0 stopped, 0 zombie
Cpu0: 23.6% us, 30.9% sy, 0.0% ni, 36.9% id, 0.0% wa, 0.3% hi, 8.3% si
Cpu1: 30.4% us, 24.1% sy, 0.0% ni, 5.9% id, 0.0% wa, 0.7% hi, 38.9% si
Mem: 4007812k total, 2199148k used, 1808664k free, 0k buffers
Swap: 0k total, 0k used, 0k free, 219844k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
5375 root 15 0 2682m 1.8g 6640 S 99.9 46.7 31:17.68
SigtranServices
7696 root 17 0 6952 3212 1192 S 7.3 0.1 0:15.75
schedmon.ppc210
7859 root 16 0 2688 1228 964 R 0.7 0.0 0:00.04 top
2956 root 8 -8 18940 7436 5776 S 0.3 0.2 0:01.35 blademtc
1 root 16 0 1660 620 532 S 0.0 0.0 0:30.62 init
2 root RT 0 0 0 0 S 0.0 0.0 0:00.01 migration/0
3 root 15 0 0 0 0 S 0.0 0.0 0:00.55 ksoftirqd/0
4 root RT 0 0 0 0 S 0.0 0.0 0:00.01 migration/1
5 root 15 0 0 0 0 S 0.0 0.0 0:00.43 ksoftirqd/1


3) /proc/sys/net/core/netdev_max_backlog is set to the default of 300


So...anyone have any ideas/suggestions?

Thanks,

Chris


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