Re: [Xen-users] Re: Performance Issues: I/O Wait

["Steve Senator (Senator Ent)" <sts@xxxxxxxxxxx>]

Xen can exacerbate Linux SMP issues. Do you have hyperthreading turned  
on in your CPU's? If so, at least for testing, try turning it off.
Also, beyond turning of the TX offloading in both the dom0 and domU,  
is there any chance that there's another device attached to that  
bridge which would cause network delays? In particular, is there a  
device that may incorrectly see the domU IP as coming from the dom0  
due to an ARP conflict? I see that you've specified a fixed MAC  
address. Is there any chance that that same MAC address is used by the  
dom0? Perhaps the initrd is the one from dom0 and its got the MAC  
address set in the initrd to be the same as the one in the dom0?
Try tcpdumping from both domains and see if you see any  
retransmissions, or perhaps even a smoking gun like a system ARPing  
for itself when it should know better.
It's also possible that there's a transmission size problem. There  
have been reported problems of dom0<->domU traffic not honoring the  
MTU of the bridge or virtual device, which then forces retransmission  
when the receiving side cannot handle the larger buffer.
If NFS, try changing from TCP to UDP or modifying the rsize and wsize  
buffering to fit within the MTU of your (virtual) ethernet devices.
Hope this helps,
-Steve Senator



Quoting Nick Couchman <Nick.Couchman@xxxxxxxxx>:

> Hi, again...haven't had any responses to this, yet.
> > > > Nick Couchman 10/18/07 11:05 AM >>>
> Hey, everyone,
> I'm having some issues with a Xen DomU right related to performance.  
>  ... The culprit seems to be  high I/O wait times related to the  
> network interface.
> The host machine is a Dell PowerEdge 1950 with 2 x Dual-Core Xeon   
> processors (Xeon 5150 @ 2.66GHz, 1333 FSB).  ...  Building these  
> Linux distributions on the physical system takes  70-80 minutes  
> (real time) - on the DomU system it takes 130-140  minutes.
> ...
> vif=[ 'mac=00:16:3e:75:0d:be,bridge=xenbr108', ]


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