Why would writes to pgsql_tmp bottleneck at 1mb/s?

PG Hackers,

What follows is iostat output from a TPC-H test on Solaris 10. The machine
is creating indexes on a table which is 50G in size, so it needs to use
pgsql_tmp for internal swapping:

tty md15 sd1 sd2 sd3 cpu
tin tout kps tps serv kps tps serv kps tps serv kps tps serv us sy wt id
0 84 22526 1211 1 1024 1 5 0 0 0 5634 337 1 30 8
0 61
0 242 24004 1337 1 1024 1 5 0 0 0 6007 355 1 33 8
0 59
0 85 22687 1277 1 1024 1 5 0 0 0 5656 322 1 31 8
0 62
0 85 20876 1099 1 1024 2 9 0 0 0 5185 292 1 28 7
0 64

md15 is WAL (pg_xlog).
sd3 is PGDATA.
sd1 i pgsql_tmp.

As you can see, we're getting a nice 23mb/s peak for WAL (thanks to
forcedirectio) and database writes peak at 6mb/s. However, pgsql_tmp, which
is being used heavily, hovers around 1mb/s, and never goes above 1.5mb/s.
This seems to be throttling the whole system.

Any suggestions on why this should be? Do we have a performance bug in the
pg_tmp code?

--
--Josh

Josh Berkus
Aglio Database Solutions
San Francisco