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I measured the throughput of a locally bound TCP socket connection on my computer (Intel Q9550, 64 GB RAM, Windows XP 64 bit) using iperf. I got dissatisfying results (around 1.6 Gbit/s) each time, no matter how I tweaked the TCP settings (buffer length, window size, max segment size, no delay). I got similar results when I tried netperf.

Now, I understand (from sources like these) that the average throughput of a loopback connection should be around 5 Gbit/s. What could be the reasons for such poor performance?

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Yodan Tauber
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  • You may be interested in trying the Windows 8 and Windows Server 2012 TCP Loopback Fast Path feature. I've written about it here: http://blogs.technet.com/b/wincat/archive/2012/12/05/fast-tcp-loopback-performance-and-low-latency-with-windows-server-2012-tcp-loopback-fast-path.aspx – Ed Briggs Dec 05 '12 at 18:45
  • You have to modify your application to use this feature unfortunately :( Not everybody has a source code access to the apps we use. – BaronSamedi1958 Aug 10 '17 at 16:39

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Antivirus is the first thing I would blame, and note that includes AV-only not just ones that include a firewall. If CPU usage of AV is above normal you know it is the culprit.

Otherwise, have you tried jumbo frames? What device/driver is it?

Joshua D'Alton
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