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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> Hello,
> 
> 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 a circular buffer which you can change the size of
(CONSOLE_RING_SIZE in xen/drivers/char/console.c).

> 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')? 
> I do have to output some messages when a certain event occurs. Will 
> enabling more consoles (serials, vga, tty)  slow the performance by default?

Yes the serial line will be the bottleneck if producing lost of
console output in DOM0. It's much slower than placing Xen's output in
the ring, or writing it to VGA.

> 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.

Batching events and sending them to DOM0 for storage will be much
faster than sending them over a serial line.

 -- Keir


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