Very slow network on RHEL 6 guest (Hyper-v)

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Hello!
Hyepr-v (2012 R2), 6x RHEL guests.
On all guests network speed 100 Mbit/s (Check that by iperf, lftp and speedtest.net script)
But on one machine (Web-server) i have very slow network. Link speed 100 Mbit/s, but real tranfer speed not reach 15 Mbit/s.
Network adapter is "Legacy network adapter".
What happened?

On slow server:

[root@some_host]# dmesg | grep "eth0"
eth0: Digital DS21140 Tulip rev 32 at MMIO 0xfebff000, some-mac-address, IRQ 9.
eth0: Using EEPROM-set media 100baseTx-FDX.

[root@some_host]# iperf -c 192.168.local.ip -d -t 60 -i 10

Client connecting to 192.168.local.ip, TCP port 5001

TCP window size: 19.3 KByte (default)

[ 5] local 192.168.host.ip port 53633 connected with 192.168.local.ip port 5001
[ ID] Interval Transfer Bandwidth
[ 5] 0.0-10.0 sec 12.8 MBytes 10.7 Mbits/sec
[ 5] 10.0-20.0 sec 13.8 MBytes 11.5 Mbits/sec
[ 5] 20.0-30.0 sec 19.0 MBytes 15.9 Mbits/sec
[ 5] 30.0-40.0 sec 12.4 MBytes 10.4 Mbits/sec
[ 5] 40.0-50.0 sec 12.1 MBytes 10.2 Mbits/sec
[ 5] 50.0-60.0 sec 10.6 MBytes 8.91 Mbits/sec
[ 5] 0.0-60.0 sec 80.8 MBytes 11.3 Mbits/sec

On any other server:

[root@other_host ~]# iperf -c 192.168.local.ip -d -t 60 -i 10

Client connecting to 192.168.local.ip, TCP port 5001

TCP window size: 69.6 KByte (default)

[ 3] local 192.168.other.server port 49551 connected with 192.168.local.ip port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 112 MBytes 94.4 Mbits/sec
[ 3] 10.0-20.0 sec 112 MBytes 93.7 Mbits/sec

Responses

Don't use the legacy adapter. The tulip driver is for an old 10/100 card design and is not suitable for modern performant workloads.

There should be sufficient LIS drivers to use the paravirt network interface in recent RHEL6 kernels, or you can install the latest LIS from Microsoft.

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