cvs commit: src/sys/dev/bge if_bge.c

[Bruce Evans]

On Sun, 24 Dec 2006, Robert Watson wrote:

>> From the perspective of optimizing these particular paths, small packet 
>> sizes 
> best reveal processing overhead up to about the TCP/socket buffer layer on 
> modern hardware (DMA, etc).  The uni/bidirectional axis is interesting 
> because it helps reveal the impact of the direct dispatch vs. netisr dispatch 
> choice for the IP layer with respect to exercising parallelism.  I didn't 
> explicitly measure CPU, but as the configurations max out the CPUs in my test 
> bed, typically any significant CPU reduction is measurable in an improvement 
> in throughput.  For example, I was easily able to measure the CPU reduction 
> in switching from using the socket reference to the file descriptor reference 
> in sosend() on small packet transmit, which was a relatively minor functional 
> change in locking and reference counting.

Be careful with micro-optimizations.  I saw a single change (adding
about 1K in unrelated code that is never executed) give a pessimization
of 15% for tx bge (from 360 kpps to 300 kpps).  Before that I was
trying harder than now to find optimizations involving avoiding copying,
and thought that I had increased the speed from 330 kpps to 360 kpps
by removing things, but I may have just increased the speed by moving
cache phenomena.  The phenomena in this case seem to be related to
instructions more than data and I suspect that they are very MD.  The
machine that has them doesn't support APIC or ACPI, so hwpmc cannot
do anything useful on it.

Bruce