Bruce Evans wrote:
On Sat, 23 Dec 2006, Robert Watson wrote:
On Sat, 23 Dec 2006, John Polstra wrote:
That said, dropping and regrabbing the driver lock in the rxeof
routine of any driver is bad. It may be safe to do, but it incurs
horrible performance penalties. It essentially allows the
time-critical, high priority RX path to be constantly preempted by
the lower priority if_start or if_ioctl paths. Even without this
preemption and priority inversion, you're doing an excessive number
of expensive lock ops in the fast path.
It's not very time-critical or high priority for bge or any other device
that has a reasonably large rx ring. With a ring size of 512 and an rx
interrupt occuring not too near the end (say at half way), you have 256
packet times to finish processing the interrupt. For normal 1518 byte
packets at 1Gbps, 256 packet times is about 3 mS. bge's rx ring size
is actually larger than 512 for most hardware.
Speed testing full size packets doesn't tax the hardware or the OS, nor
does it represent real world scenarios. Testing minimum sized packets
isn't terribly real-world either, but it represents the worst-case for
the hardware and the OS and thus is a good standard for extrapolating
performance potential. And, 1Gb isn't terribly interesting these days
either, 10Gb is.
Scott
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