Re: [Xen-devel] Slow guest network I/O when CPU is pegged - Looking for acknowledgement from developers

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 7 Apr 2006, at 19:25, Matt Ayres wrote:

> Ok, so we all know that guest network I/O is slow when the system 
> CPU's are being utilized extensively whether it be from dom0 or from 
> other guests.  Lots of people have written about this and I can post 
> concrete tests if required.
> I'm just looking for one of the Xen developers to acknowledge that 
> they have been able to replicate the problem and it is indeed being 
> worked on or will be sometime in the near future.  No one has 
> acknowledged any of the previous threads on either list so I want to 
> make sure it is an outstanding issue that is not being overlooked.
It depends on the setup but poor scheduling is the main reason for poor 
network performance, usually. SEDF seems to have some problems with 
real-time domains (like domain0 with its default scheduling parameters) 
and gives them all the CPU they want -- this is obviously going to be 
bad if a client domain is scheduled on the same CPU. Since UDP has no 
flow control, dom0 can keep itself busy generating or forwarding UDP 
packets to the domU that get dropped continually in netback driver. 
DomU will hardly ever get scheduled. Even in the case of TCP, any drops 
will be interpreted as congestion and transmit rate will be cut.
Basically I think the SEDF scheduler needs cleaning up: probably by 
removing the mass of confusing conditionally compiled options and then 
focusing on the remaining code that is actually compiled in. Another 
option is to try specifying the BVT scheduler and see if that works 
better. Or try setting dom0 to have non-real-time guarantees. Or give 
dom0 its own hyperthread on an Intel system (strongly recommended if 
it's possible).
Apart from that, if you really are genuinely loading up CPUs with 
CPU-intensive workloads, and expecting them also to be able to process 
a significant amount of network traffic then something has to give. You 
can only run CPUs at 100%.
  -- Keir


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