Re: [Xen-devel] xm dmesg and performance on serial port

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

> I might only have configuration problem, but I couldn't find any docs
> that could relate to the problem. The command: 'xm dmesg' seems to stop
> buffering any output after a while. I have to enable serial port to get
> the output bigger than the buffer. I don't know how big the buffer or if
> it's circular? Is there anyway to fix this behavior?

It's not currently circular.  There used to be a -c flag to xm dmesg (or it's 
equivalent) that cleared the buffer - does that work?  There's code somewhere 
in Xen which could support a circular buffer with not much effort.

> The second question I have since I have to enable the serial line.
> Considering the speed of the serial line is very slow. Would that affect
> the performance of XEN than just buffering the output (for 'xm dmesg')?

Doing *lots* of printfs to the serial line is going to really hurt your 
performance but maybe that doesn't matter so much for debugging?  Occasional 
printfs probably won't matter very much.

> Just by reading the XEN papers, XEN has a way to register asynchronous
> events to the domains. Would it be better in terms of performance to
> send the event messages to a particular domain (e.g: Domain 0) using the
> asynchronous mechanism and have the domain kernel to handle the messages
> as XEN events.

We don't use xm dmesg that much for debugging, generally just tracing through 
the code and using an attached serial line.

You might want to look at the tracebuffer and the xentrace tools.  The 
tracebuffer allows you to log events within Xen itself with low overhead into 
a circular buffer.  The xentrace command in dom0 will read this data out and 
decode it according to a format file.  There's man pages for the xentrace* 
tools and the docs/interface.tex manual (although mostly out of date) 
explains how to put trace points into the Xen code.

HTH,
Mark

> Thanks in advance for any comments.
>
>
> Kuas.
>
>
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