pg_basebackup state?

I need help determining the state of the pg_basebackup command. This
works command works on other hosts and I make a connection as recorded
in the postmaster log:

2015-07-02 12:10:53 EDT,0,authentication LOG: replication connection
authorized: user=rep

the process is running:
> ps -ef | grep pg_base
pg 32600 11558 0 12:10 pts/1 00:00:00 pg_basebackup -D
basebackup/ --xlog --format=t -P -U rep
pg 32601 1473 0 12:10 ? 00:00:00 postgres: wal sender
process rep [local] sending backup "pg_basebackup base backup"

but no data is written. Looks like wal sender is hurt. max_wal_senders
= 3, I wonder if this is the reason for the stall. How to eval if this
is set to low? Thanks.