Long mbuf chains

Yony Yossef yonyossef.lists at gmail.com
Wed Aug 6 13:36:29 UTC 2008

Hi All

I'm working on an Ethernet driver for FreeBSD 7.0.
Taking network performance numbers I encountered very long mbuf chains on
the sender side.

The symptom is constant, always during iperf/netperf TCP stream tests with
message sizes of 128 bytes (>200 mbufs per chain),
1024 bytes (30-60 mbufs per chain) and 2048 bytes.

My problem is that long chains require some kind of defragmentation/cutting
before it can be properly DMAd.
This is pretty a expansive operation.

Is there a way of tuning the OS for sending limited length mbuf chains? I
thought setting "net.inet.ip.maxfragsperpacket" would do it but it doesn't.

Is there a better way of handling this issue?


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