[Xen-devel] poor domU VBD performance.

[Peter Bier <peter_bier@xxxxxx>]

I have installed XEN and linux 2.6.10 on three different machines. The slowest 
of them was my computer at home running and Athlon XP 1600+ ( 1.4 GHZ ) and 256 
MB RAM. 

My Problem is reduced file-system performance in domU guests. These guest run
faster when I use loopbacked files on Dom0 than the do when I use real 
partitions
and poulate them with a linux system. 

I found out that dom0 does file-system IO and raw IO ( using dd as a tool to 
test
throughput from the disk ) is about exactly the same as when using a standard 
linux kernel without XEN. But the raw IO from DomU to an unused disk ( a second
disk in the system ) is limited to fourty percent of the speed I get within 
Dom0.
This effect transforms to about the same ratio when doing real file-system IO.

I found this sympthom in all of the systems I installed. An early paper about 
XEN describes that the penalty when using VDBs is close to zero and neglectable.
I think this conflicts with the results I got and I believe this reflects that 
something in my configuration is wrong ( at least I hope so ). 

I have the drivers for my chipset linked into the kernel and hdparm tells me 
that
DMA is enabled for the used disks ( using hdparm under Dom0 ). 

What worries me is that the results within Dom0 are completely satisfactory, 
while those in DomU are not. Do I have to change the kernel config for DomU ? Or
is there any special option I have to set in the kernel configuration for Dom0 
or
even for xen?

I have compiled version 2.0.5 - the newest available, to my knowledge.

Any hints ??  



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