Re: [Xen-users] vif interfaces drop packets

["Torsten Vielhak" <Torsten@xxxxxxxxxx>]

>> If I have time, I will compile new XEN kernels with greater buffers and
>> see what will happen.
>
> Please do, particularly if you can afford the reboot. :)

The problem is that the data structure in which the ring buffer is
organized has to fit into one memory page (4096 bytes). So we are limited
to a buffer size of 340 entries which means 340 packets (~30% bigger). The
result are almost the same; it gets a little bit better: loss rates <0.1%
But I think it's really not a bug but a feature. You have to ensure that
the virtual machines are not locked up with too old packets in a long
queue during high network load (this would lead to an unreachable virtual
server which cannot answer recent packets). So this is really a traffic
shaping routine (very very basic); it would be better e.g. to delay tcp
packets if the buffer gets crowded since tcp stacks would react on the
delay and adjust their sending speed; but this is not possible because the
host machine does not "see" tcp traffic but only bridges the frames. This
leads to another problem: if your guests are in a network with high
broadcast load the buffers get filled with theses broadcasts too.
So after all, IMHO it's a general design problem of virtual machines which
have to emulate the interrupts of real hardware.

cheers,
   Torsten

PS: What type of applications do you run on the productive xen machines?
What is your overall experience (beside the network issue ;-)) ?

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