I am writing a network program which can calculate accurate data packet rate (packet per second, frame per second, bps). Now i have a device called testcenter which can send accurate flow to a specific pc (protocol is UDP/IP) on Linux, i like to know the accurate pps(packets per second) with my program , i have considered the gettimeofday(&start,NULL)function before i call recvfrom() and update the counter for packets, after that call gettimeofday(&end,NULL) and get the pps rate. I hope there is better solution than this since the user/kernel barrier is traversed on system calls.
Best regards.
I think you should use clock_gettime() with CLOCK_MONOTONIC_COARSE. But it will only be accurate till the last tick .. So may be off by 10s of millisec. But its definitely faster that using it with CLOCK_MONOTONIC_RAW. You can also use gettimeofday but clock_gettime with CLOCK_MONOTONIC_RAW is slightly faster and higher resolution than gettimeofday. Also gettimeofday() gives wall clock time, which might change even for daylight saving ... I don't think you should use it to measure traffic rate.
Your observation that gettimeofday switches to kernel mode is incorrect for Linux on a few popular architectures due to the use of vsyscalls. Clearly using gettimeofday here is not a bad option. You should however consider using a monotonic clock, see man 3 clock_gettime. Note that clock_gettime is not yet converted to vsyscall for as many architectures as gettimeofday.
Beyond this option you may be able to set the SO_TIMESTAMP socket option and obtain precise timestamps via recvmsg.