Replacing FD, so the new combo would be FD_SOCK and FD_ALL. The advantage of FD_ALL is that we detect many concurrent failures much faster than FD.

E.g if we have A,B,C,D,E and B and C fail, and FD.timeout=1000 and FD.max_tries=3, FD_ALL.timeout=3000 and FD_ALL.interval=1000, then:

FD will take timeout * max_tries ms to detect the death of B and the same for C, so a total of 6 seconds.
FD_ALL will take timeout ms to detect both failures, so a total of 3 seconds.

FD_ALL is also much simpler in its implementation, therefore simpler to verify for correctness