crossbow discuss - Apr 2009 - maxbw minimum

The minimum is set at 1200Kbits/second. However, in testing, if I set
that for a VNIC, the domU gets no traffic at all (maybe the occassional
packet). Is the minimum too low?

If I set a maximum of 2000Kbits/second, I get this from nicstat
(expecting around 250Kbytes/s total:

    Time     Int   rKB/s   wKB/s   rPk/s   wPk/s    rAvs    wAvs   %Util Sat
04:35:38 xvm15_0   146.6    5.32   102.0   73.65  1471.3   74.00   62.21 0.00
04:35:43 xvm15_0   161.6    5.92   112.0   82.24  1477.4   73.68   68.60 0.00

What is the expected accuracy of the bandwidth limiting?

I''m using b111.

thanks
john

[CC''ing crossbow-discuss]

Although 1200 kbps is supported, a lot depends on the other side as
well. This is specially true for TCP where if you set the limit too
low and the other side runs uncontrolled (i.e. too fast), we will
be forced to drop packets and TCP typically reacts badly to that.
UDP apps have their own issues under very low bandwidth limits.

In practice, we have seen TCP do fairly well (on 1 gigE links) with
bandwidth limits around 30-40 Mbps or more (the achieved speed will
very closely mirror configured limit). UDP depends a bit more on
sender and packet size etc but at 50 Mbps limit, gets close to the
configured limit.

Cheers,
Sunay

John Levon wrote:
> The minimum is set at 1200Kbits/second. However, in testing, if I set
> that for a VNIC, the domU gets no traffic at all (maybe the occassional
> packet). Is the minimum too low?
> 
> If I set a maximum of 2000Kbits/second, I get this from nicstat
> (expecting around 250Kbytes/s total:
> 
>     Time     Int   rKB/s   wKB/s   rPk/s   wPk/s    rAvs    wAvs   %Util
Sat
> 04:35:38 xvm15_0   146.6    5.32   102.0   73.65  1471.3   74.00   62.21
0.00
> 04:35:43 xvm15_0   161.6    5.92   112.0   82.24  1477.4   73.68   68.60
0.00
> 
> What is the expected accuracy of the bandwidth limiting?
> 
> I''m using b111.
> 
> thanks
> john
> _______________________________________________
> xen-discuss mailing list
> xen-discuss-xZgeD5Kw2fzokhkdeNNY6A@public.gmane.org

On Thu, Apr 09, 2009 at 10:22:02AM -0700, Sunay Tripathi wrote:
> Although 1200 kbps is supported, a lot depends on the other side as
> well. This is specially true for TCP where if you set the limit too
> low and the other side runs uncontrolled (i.e. too fast), we will
> be forced to drop packets and TCP typically reacts badly to that.
So badly that I get basically /no/ traffic, even just during a normal
boot? Surely something is wrong if a default setup cannot operate at
all. DHCP succeeds but that''s about it.

At the very least, I''d expect some very strong warnings in the dladm
manpage that you need at least 2Mbits/s in order to support a virtual
NIC in normal operation. It would also be nice to see some words about
expected accuracy.

regards
john