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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Sat, Apr 08, 2006 at 08:56:19AM +0100, Keir Fraser wrote:
> 
> 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).
> 

Has anyone already tried this? I'd like to know if dedicating own
hyperthread for dom0 helps to fix these network performance problems..


> 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
> 
> 

-- Pasi
       
                                   ^
                                .     .
                                 Linux
                              /    -    \
                             Choice.of.the
                           .Next.Generation.

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