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VM Traffic over port 80 slows to a crawl; other ports unaffected

I have 2 VMs running CentOS 5.2 x86_64 under VMware ESXi 3.5. I followed the installation guide for best practices, installing the VMwareTools, making kernel tweaks for time drift compensation, etc.

Everything works great, except that throughput from my webserver over port 80 never gets above T1 speed (around 1.8Mb/s). All other protocols (FTP, SFTP, etc) run at "full speed" (maxes out our 10M Internet port).

If I change Apache or lighttpd to listen on ANY OTHER port other than 80, it operates at normal speed - consistently around 7-8Mb/s. But if I change the port back to 80, it slows back down to atoun 1.5-1.8Mb/s.

I have tried EVERYTHING--OS reinstall, trying 32-bit vs. 64-bit guest OSs, compiling VMwareTools vs. installing the RPM, googled all day and all night, and I can't find the solution.

I thought about low level things like frame size, buffer size, etc, but concluded that it couldn't be that since any port other than port 80 works fine.

I know it's not an issue of ISP throttling because we are in a Class A datacenter with a 10M bursting to 100M pipe, and I can test to another box in our cabinet and reproduce the issues.

It's as if something "upstream" is throttling the bandwidth--but the only thing upstream is the ESX hypervisor itself.

- No firewall

- Disk-to-disk copy of a 100m file happens instantaneously

- SFTP of the same 100m file over the Internet goes at about 8Mb/s

- HTTP transfer of the same 100m file over the Internet goes at 7Mb/s over any port other than port 80; slows to 1.5Mb/s over port 80


Any advice is greatly appreciated.

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