Bizarre performance limitation

[John Klos <john%ziaspace.com@localhost>]

Hi, all,

I've used NetBSD-macppc extensively and still have many machines running 
it. However, the last time I had a PowerMac G4 tower running NetBSD was 
quite a long time ago (pre-NetBSD 4, I believe). At one point I had a 
tower saturating a 100 Mbps Internet connection while serving video files 
via Apache, so I know they can be quite fast.

Recently I set up a Quicksilver tower with 1.5 gigs of memory, a 1.733 GHz 
7448 accelerator, an ATTO UL3D 160 MB/sec UltraSCSI card, a 64 bit Intel 
gigabit PCI card and NetBSD 5 so that I could do tape backups over the 
network to an LTO-4 tape drive. However, it appears I cannot get more than 
7 or 8 MB/sec tops through the network.

The machine providing NFS shares is an 8 core Xserve running Mac OS X 
10.6.2 with a direct gigabit connection. I've used the same machine with 
an amd64 system running NetBSD to do tape backups before, and I've been 
able to get at minimun 30 MB/sec.

I've tried using 9000 byte frames (no difference), adjusting the size of 
the NFS transfers, using / not using hardware checksumming, and so on. The 
PowerPC system is 85% or more idle with 2% interrupt and less than 15% 
system in top.

The ethernet card is doing about 800-850 interrupts a second. With a 9000 
byte packet size and an 8k NFS transfer size, that makes for about 6.5 to 
7 MB/sec speed, which netstat bears out.

Does anyone have any clue what might be so different between the macppc 
system and an otherwise identical amd64 system which might explain the 
disparity?

Thanks,
John Klos