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meerkt
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iperf full-duplex slower than each half-duplex

Sat Sep 19, 2020 7:49 am

I'm seeing some anomalies with iperf2 --dualtest on Windows.

1) Each direction by itself gets to max speed (95Mb), but full-duplex sees one direction slower.
The slower direction is the same physical one regardless of which computer acts as the server.

2) The slower direction is faster when one computer is acting as server rather than client.
Image

3) The speed of the slower direction varies in different runs. 40-65Mb when the "correct" computer is the server, 20-35Mb otherwise.

Any ideas?
 
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Re: iperf full-duplex slower than each half-duplex

Sat Sep 19, 2020 12:40 pm

Are both ends running the same type of NIC, same drivers, and same version of Windows? Are CPUs same (or at least similar)? Have you checked CPU load on both systems while the test is running?
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meerkt
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Re: iperf full-duplex slower than each half-duplex

Sat Sep 19, 2020 12:57 pm

Different NICs (Realtek, Atheros), different CPUs, same Windows.
Will check CPU next time, though it's just 100Mb.
Last edited by meerkt on Sun Sep 20, 2020 8:35 am, edited 1 time in total.
 
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Re: iperf full-duplex slower than each half-duplex

Sat Sep 19, 2020 6:21 pm

Ahh, good point. At 100 Mb pretty much anything from the past 20 years should be able to keep up.

Do the numbers go to zero when the test is stopped? I'm wondering whether something else is causing traffic in one direction that could be slowing the test down.
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tfp
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Re: iperf full-duplex slower than each half-duplex

Sat Sep 19, 2020 6:25 pm

Could you be limited by hard disk performance for the file transfer?
 
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Re: iperf full-duplex slower than each half-duplex

Sat Sep 19, 2020 6:28 pm

tfp wrote:
Could you be limited by hard disk performance for the file transfer?

Only if it is transferring a lot of small files or the disk is badly fragmented. 100 Mb is only ~12 MB/sec, which any HDD made in the past 2+ decades can sustain without even breaking a sweat if the transfer is sequential.

In any case, I don't think iperf relies on disk (i.e. it is solely a network benchmark)?
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tfp
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Re: iperf full-duplex slower than each half-duplex

Sat Sep 19, 2020 7:18 pm

Right, shouldn't be an issue either way.

Nothing comes to mind unless there is a driver setting wrong. MTU, large packet setting, some sort of ethernet priority setting?

Are you on a cross over cable between the computers or a switch/router in between? Could be the router impacting things.
 
meerkt
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Re: iperf full-duplex slower than each half-duplex

Sun Sep 20, 2020 5:07 am

There's no activity when not testing (see traffic graph above).
Yeah, iperf doesn't use disk by default.

Will also try without the switch in the middle.

Driver settings should be the default, but they're always a semi-mystery. I suppose I could try to disable Green Ethernet and/or Energy Efficient.
 
meerkt
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Re: iperf full-duplex slower than each half-duplex

Sun Sep 20, 2020 8:33 am

More results...

Ubuntu on both sides does get ~90Mb full-duplex. And with PCs connected directly, >900Mb + >700Mb.

Ubuntu-Windows, on whichever side, shows similar behavior to Windows-Windows.

Removing the switch in the middle didn't change the general behavior, other than allowing faster speeds.
Half-duplex >900Mb either direction. Full-duplex still highly asymmetric, but a few hundreds each direction.

Ubuntu's iperf2 was 2.0.13, I think. Windows 2.0.14a. I might try iperf3.

BTW, every now and then I get to try Ubuntu. Still rough edges everywhere, still surprising.
 
Topinio
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Re: iperf full-duplex slower than each half-duplex

Sun Sep 20, 2020 11:19 am

So, it's a Windows issue with one of these installations on one of these machines, either a config issue or something's gone wrong, or a poor driver. Have you reinstalled the drivers, using the latest releases available?

Are all anti-virus and protection software disabled on both machines during testing? Including looking for any possibly unexpected ones e.g Windows Defender and the one built into newer versions of Acronis True Image? (Maybe one end has something scanning and slowing things down?)

What's CPU load look like on both machines while doing the transfer? (Maybe one driver uses a lot of CPU?)

Anything odd in netstat -s, netstat -e, or Get-NetAdapterStatistics -Name "Ethernet" | Format-List -Property "*" ?

(If it's not named that, and you don't know the name, you can find it with Get-NetAdapterHardwareInfo )

Failing that, can you post the specs ? (Ethernet adapter model, CPU, RAM, primary storage - so we can see what the machines are.)
Desktop: 750W Snow Silent, X11SAT-F, E3-1270 v5, 32GB ECC, RX 5700 XT, 500GB P1 + 250GB BX100 + 250GB BX100 + 4TB 7E8, XL2730Z + L22e-20
HTPC: X-650, DH67GD, i5-2500K, 4GB, GT 1030, 250GB MX500 + 1.5TB ST1500DL003, KD-43XH9196 + KA220HQ
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meerkt
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Re: iperf full-duplex slower than each half-duplex

Tue Sep 22, 2020 9:10 am

It happened when either computer was running Windows (and Ubuntu on the other side), not just a specific one.

No antivirus meddling (other than what Defender might do when disabled). I might suspect the non-Microsoft firewall, but didn't check in depth.

CPU seemed fine when I looked, but I didn't check every run. Also, it was faster than 100Mb full-duplex on the gigabit connection, so 100Mb is unlikely to be CPU-limited.

netstat and all, you mean error counts?

I think I should try iperf3 to rule out test tool anomalies.

Anyway, thanks. I might spend more time on this eventually but for now I'll let it be. Initially I just wanted to check WiFi speed with LAN and WAN, then got curious. :)
 
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Re: iperf full-duplex slower than each half-duplex

Tue Sep 22, 2020 11:22 am

Ahh, down the proverbial rabbit hole...
Nostalgia isn't what it used to be.
 
tfp
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Re: iperf full-duplex slower than each half-duplex

Tue Sep 22, 2020 12:34 pm

I have an old Intel D510 Atom running a Windows 2012 R2 file server and a newer Intel i-8600k Windows 10, they are connected to a router at a 1GB connection and I tested with iperf3 and a remote desktop window to the server running.

D510 Atom as iperf servier/i-8600k client:
941 Mbits/sec sender
941 Mbits/sec receiver

i-8600k as iperf servier/D510 Atom receiver
948 Mbits/sec sender
948 Mbits/sec receiver


Based on this there is no way your machines are under powered. Not that that should be a surprise. I'm still leaning towards a driver issue/configuration issue as I seem to remember running into that years ago when setting up the Atom server but I don't remember what I did to make it run better. Something like installing latest driver or maybe making background applications a priority on the server.
 
meerkt
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Re: iperf full-duplex slower than each half-duplex

Tue Sep 22, 2020 1:24 pm

Will try iperf3.

I started with the iperf2 branch because iperf3 doesn't have official Windows builds, unlike iperf2
(iperf3 isn't strictly a newer version; it's more of a new project from a different group).

(BTW, Ubuntu, at 2.5GB, doesn't come with net-tools?!)
 
tfp
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Re: iperf full-duplex slower than each half-duplex

Tue Sep 22, 2020 1:39 pm

I got my bins from here, used the 64bit windows bins.

https://iperf.fr/iperf-download.php

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