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The network: * A windows 7 PC with 2 NICs - one connected to an old style ethernet hub - the other to the internet - with internet sharing enabled * An Apple iMac connected to the hub, successfully utilizing the ICS to access the internet.

My problem: Using the Mac, copying from the internet is fast. However, if I connect to a SMB: share on the Windows 7 PC and try and copy anything > a few kb the copy operation is appallingly slow with my network card using the Windows 7 control panel showing ~.1% utilization.

The NICs are 100Mbs and show a 10x larger throughput (now ~1%) if I download large files over the internet using the Mac.

WTF?

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What happens if you kick off multiple transfers from the 7 machine to the iMac? Does your card's use go up linearly or stay at ~0.1% –  nicorellius May 18 '10 at 15:51

1 Answer 1

Just a hunch but it's quick to test. Try doing this on your Mac:

sysctl net.inet.tcp.delayed_ack

...and note what it reports. It'll be an integer, probably the value 2 or 3. After recording how it was previously set, try setting it to zero:

sudo sysctl -w net.inet.tcp.delayed_ack=0

...then unmount all SMB-mounted volumes and re-mount them so that you're establishing new TCP connections. Then try your file copy test again and see if the speed is significantly improved.

I've run into situations where Mac OS X's TCP Delayed Ack algorithm was less than optimal, so it may be good to do this test to quickly rule that out.

Part of my hypothesis is that when doing a local file transfer, Windows is the TCP endpoint, so you're dealing with Windows' TCP stack. If there's a TCP tuning interop problem between your Mac and Windows, you'd see it there. In contrast, when your Windows machine is doing Internet Connection Sharing, it's just passing your TCP traffic through, not acting as an endpoint of the connection, so the behavior will be different.

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I did this, but as luck would have it, the Mac started transferring files with about 68% utilization. So it must be a cable problem - Or it was a different PC. There was an UBuntu PC I was trying to update. Ill check the copy speeds there. Perhaps im fingering the wrong culprit. –  Chris Becke May 20 '10 at 14:24

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