Some investigations makes me suspect that select(2) is not working
properly, especially it doesn't wake up when there's data ready in the
socket buffer: when the rsync process is idle it's waiting on select,
when it's idle netstat shows that the receive socket queue is full (I
tried with both net.inet.tcp.recvbuf_auto set to 1 and 0) and
ktrace shows:
4102 1 rsync 1311780519.324063908 CALL
select(4,0x7f7fffff83b0,0x7f7fffff8390,0,0x7f7fffff83d0)
4102 1 rsync 1311780579.483436279 RET select 0
4102 1 rsync 1311780579.483440327 CALL
select(4,0x7f7fffff83b0,0x7f7fffff8390,0,0x7f7fffff83d0)
4102 1 rsync 1311780579.483442445 RET select 1
4102 1 rsync 1311780579.483443326 CALL read(3,0x7f7ff7a36de2,0x21a)
4102 1 rsync 1311780579.483451341 GIO fd 3 read 538 bytes
So select blocks (maybe because there's effectively nothing to read at this
time), but instead of waking up when there's data ready it wakes up
when the timeout expires. The next select call returns immediatly.
Does it ring a bell to someone ? Any recent change in this area
(either in select(2), or tcp) recently ?