netboot is surprisingly slow for DS10

Hi,

netboot from internal NIC of DS10 is surprisingly slow. It takes about
150s to load kernel from NFS server, i.e., ~0.7Mbps!

NIC is configured to auto negotiation mode, but the situation does not
improve if it is fixed to 100BaseTX-FDX or 10BaseT-FDX.

Adding -DLIBSA_SLOW_TWIDDLE to CPPFLAGS in
sys/arch/alpha/stand/netboot/Makefile does not help.

The server is NetBSD/amd64 box, connected to 10Gbps-capable switching
hub via ixg(4). I also tried wm(4) and 1Gbps switching hub, but in vain.

Any ideas?

Thanks,
rin
----
ewa0: link up : Negotiated  100BaseTX: full duplex
(boot ewa0.0.0.9.0 -flags A)

Trying BOOTP boot.

Broadcasting BOOTP Request...
Received BOOTP Packet File Name is: /netboot
local inet address: 192.168.10.136
remote inet address: 192.168.10.128
TFTP Read File Name: /netboot
netmask = 255.255.255.0
.Server is on same subnet as client.
.
bootstrap code read in
base = 2ae000, image_start = 0, image_bytes = eec8
initializing HWRPB at 2000
initializing page table at 3ffec000
initializing machine state
setting affinity to the primary CPU
jumping to bootstrap code

NetBSD/alpha 9.99.73 Network Bootstrap, Revision 1.9 (Mon Oct  5 09:26:09 UTC 2020)

VMS PAL rev: 0x1004700010154
OSF PAL rev: 0x1003f0002014f
Switch to OSF PAL code succeeded.

Boot flags: A
boot: ethernet address: xx:xx:xx:xx:xx:xx
boot: client addr: 192.168.10.136
boot: client name: ds10_boot
boot: subnet mask: 255.255.255.0
boot: net gateway: 192.168.10.1
boot: server addr: 192.168.10.128
boot: server path: /exports/ds10
12771424+239920 [645000+431515]=0xd6fe10

Entering netbsd at 0xfffffc0000a01350...
[   1.0000000] consinit: not using prom console
...