strange ping response times...

[Luigi Rizzo]

I noticed this first on a 10G interface, but now there seems
to be a similar issue on the loopback.

Apparently a ping -f has a much lower RTT than one with non-zero
delay between transmissions. Part of the story could be that
the flood version invokes a non-blocking select.
On the other hand, pinging on the loopback should make
the response available right away, so what could be the reason
for the additional 3..10us in the ping response time ?

The following are numbers on an i7-2600k at 3400 MHz + turboboost,
running stable/9 amd64. Note how the min ping time significantly
increases moving from flood to 10ms to 1s.
On an Intel 10G interface i am seeing a min of 14-16us with
a ping flood, and up to 33-35us with the standard 1s interval
(using -q probably trims another 2..5us)

    > sudo ping -c 1000 -q -f 127.0.0.1
	round-trip min/avg/max/stddev = 0.002/0.003/0.012/0.001 ms
    > sudo ping -c 1000 -q -f 127.0.0.1
	round-trip min/avg/max/stddev = 0.002/0.002/0.013/0.001 ms
    > sudo ping -c 1000 -q -f 127.0.0.1
	round-trip min/avg/max/stddev = 0.002/0.002/0.013/0.001 ms
    > sudo ping -c 10000 -q -f 127.0.0.1
	round-trip min/avg/max/stddev = 0.002/0.002/0.011/0.001 ms

    > sudo ping -c 1000 -q -i 0.01 127.0.0.1
	round-trip min/avg/max/stddev = 0.005/0.012/0.017/0.001 ms
    > sudo ping -c 1000 -q -i 0.01 127.0.0.1
	round-trip min/avg/max/stddev = 0.004/0.012/0.016/0.001 ms
    > sudo ping -c 200 -q -i 0.01 127.0.0.1
	round-trip min/avg/max/stddev = 0.007/0.012/0.017/0.002 ms
    > sudo ping -c 200 -q -i 0.01 127.0.0.1
	round-trip min/avg/max/stddev = 0.005/0.012/0.018/0.002 ms
    > sudo ping -c 200 -q -i 0.01 127.0.0.1
	round-trip min/avg/max/stddev = 0.009/0.012/0.020/0.002 ms
    > sudo ping -c 200 -q -i 0.01 127.0.0.1
	round-trip min/avg/max/stddev = 0.006/0.012/0.016/0.001 ms

    > sudo ping -c 200 -q -i 0.1 127.0.0.1
	round-trip min/avg/max/stddev = 0.007/0.014/0.020/0.001 ms
    > sudo ping -c 200 -q -i 0.1 127.0.0.1
	round-trip min/avg/max/stddev = 0.006/0.014/0.019/0.002 ms
    > sudo ping -c 200 -q -i 0.1 127.0.0.1
	round-trip min/avg/max/stddev = 0.007/0.014/0.021/0.001 ms
    > sudo ping -c 200 -q -i 0.1 127.0.0.1
	round-trip min/avg/max/stddev = 0.007/0.014/0.020/0.001 ms
    > sudo ping -c 200 -q -i 0.1 127.0.0.1
	round-trip min/avg/max/stddev = 0.006/0.014/0.021/0.002 ms
    > sudo ping -c 200 -q -i 0.1 127.0.0.1
	round-trip min/avg/max/stddev = 0.010/0.014/0.022/0.001 ms

    > sudo ping -c 20 -q -i 1  127.0.0.1
	round-trip min/avg/max/stddev = 0.013/0.018/0.022/0.002 ms
    > sudo ping -c 20 -q -i 1 127.0.0.1
	round-trip min/avg/max/stddev = 0.012/0.018/0.021/0.002 ms
    > sudo ping -c 20 -q -i 1 127.0.0.1
	round-trip min/avg/max/stddev = 0.009/0.017/0.018/0.002 ms
    > sudo ping -c 20 -q -i 1 127.0.0.1
	round-trip min/avg/max/stddev = 0.011/0.017/0.021/0.002 ms
    > sudo ping -c 20 -q -i 1 127.0.0.1
	round-trip min/avg/max/stddev = 0.010/0.017/0.020/0.002 ms
    > sudo ping -c 20 -q -i 1 127.0.0.1
	round-trip min/avg/max/stddev = 0.009/0.017/0.028/0.004 ms

cheers
luigi