On Thu, Aug 26, 2004 at 09:35:17AM -0700, Smith, Craig wrote:

Hello,
I'm having a problem with UDP packet loss (small packets, simulated
compressed VoIP calls). The packets are being dropped in netif_rx,
and if I increase the netdev_max_backlog to something huge I get
no packet loss, but the delay is pretty bad resulting in terrible
MOS scores.

I'm trying to decide if my Linux kernel simply can't keep up
with the traffic or if there's still something I can tweak to
improve the performance (or maybe I need to try getting a 2.6
kernel to work).

It's very unlikely 2.6 will make things better.

Below, I've included a few more details about
my platform and test setup. The test runs fine on the same
board with our proprietary OS, so I know it's "physically"
possible, I'm just evaluating Linux's performance as well.

Kernel: 2.4.26 (kernel.org) w/ mpc860sar (from sourceforge.net)
CPU: 855T @ 48MHz

This is very low-end CPU :)

MEM: 16M SDRAM, 4M flash
LAN: Broadcom BCM5327 10/100 8 port switch
WAN: Conexant Bt8370 T1 (ATM using mpc860sar code)

Test: 30 compressed voice calls downstream (LAN<-WAN)
Packet size: ~64bytes IP/UDP + ATM overhead
In a 30 second test, the test sends ~45000 packets (30 different
streams), and about 5000 are dropped (unless I increase the
netdev_max_backlog which just delays the inevitable dropping).

I believe the T1/ATM interface is working fine, I found the source
of the dropped packets in /proc/net/softnet_stat so that made me
wonder if the kernel was simply not keeping up.

Should I expect to see this kind of loss with a 2.4 kernel?
Is it just a matter of tweaking some more networking related parameters?
Should I be using another distribution of kernel source?

I think you have to work on network driver, make it use NAPI
(http://www.usenix.org/publications/library/proceedings/als01/jamal.html)
and this _may_ help. Using very slow processor without NAPI-enabled
net driver is the main cause of big packet loss on RX path.

--
Eugene

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