[syslinux] Syslinux 4.04 gpxelinux.0 http performance problem with VMware VMs

Pasi Kärkkäinen pasik at iki.fi
Mon May 7 13:40:48 PDT 2012


Hello,

In my testing environment I have two VMs on ESXi 5.0.

VM A = dhcp/tftp/PXE/http server, running CentOS 6.2. Syslinux 4.04 with the included gpxelinux.0.
VM B = PXE boot client.

If I run CentOS 6.2 also on the VM B, I can easily transfer 50+ MB/sec over http between the VMs (wget, links).

Now, if I PXE boot gpxelinux.0 on the VM B, and start to download bigger initrd image 
over http the download speed is very slow: only 200 - 500 kB/sec.

I tried using both Intel e1000 and vmxnet3 NICs on the VM B. 
No big difference, both were slow.

I tried moving the VMs on the same host, and also to separate hosts - no difference.

Then I noticed there's a lot of TCP retransmit happening when gpxelinux.0 client 
downloads files from the http server.. dozens or even hundreds of retransmit per second.
Any ideas what could cause that? 

Then I noticed if I use ethtool to disable all offloads (lro/gro/rx/tx) on the VM A NIC 
the download speed jumps to 1700 - 2800 kB/sec. So that's WAY better speed. 
But still there's a lot tcp retransmits happening all the time..

Any ideas what's causing the bad performance and those continuous retransmits? 
Anything I should try to debug this? 

On baremetal with Intel NICs I've seen much better gpxelinux.0 download speeds over http. 

Thanks,

-- Pasi




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