Grep from the end of a file to the beginning

I have a file with about 30.000.000 lines (Radius Accounting) and I need to find the last match of a given pattern. The command: tac accounting.log | grep $pattern gives what I need, but it's too slow because the OS has to first read the whole file and then send to the pipe. So, I need something fast that can read the file from the last line to the first.