Losing serial console boot output / sync_console

Losing serial console boot output / sync_console

[Andy Smith]

Hello,

About two months ago I sent this email to the Xen users lists
regarding a problem I was having losing some of my serial console
output on boot:

http://lists.xensource.com/archives/html/xen-users/2011-04/msg00340.html

Unfortunately it got no replies.

I was just wondering and hoping if any developers could answer the
following questions:

- Should I open a bug regarding the loss of serial console output on
  boot, and;

- Is sync_console as bad an idea as the warning suggests it is?

I get all my serial console output if I use sync_console, but I
haven't really put the machines under heavy load and would rather
trade off truncated boot messages as opposed to performance under
load.

Cheers,
Andy

Re: Losing serial console boot output / sync_console

[Keir Fraser]

On 02/07/2011 21:04, "Andy Smith" <andy@strugglers.net> wrote:

> Hello,
> 
> About two months ago I sent this email to the Xen users lists
> regarding a problem I was having losing some of my serial console
> output on boot:
> 
> http://lists.xensource.com/archives/html/xen-users/2011-04/msg00340.html

If your serial line is very slow (I don't know why you're limited to 9600
baud -- 115200 baud at least is usually possible) and there is a lot of boot
output then you can expect some of it to be discarded. We prefer to discard
data rather than spin-wait to send it as that can indeed cause other
time-critical services to be delayed too long.

You should still get Xen output, for example if you CTRL-a three times and
use the debug keys (try 'h' to get a list of them).

If you want to capture all Xen's boot output, the safest way is to increase
the serial transmit buffer size. Try adding serial_tx_buffer=64k to the Xen
command line.

 -- Keir

> Unfortunately it got no replies.
> 
> I was just wondering and hoping if any developers could answer the
> following questions:
> 
> - Should I open a bug regarding the loss of serial console output on
>   boot, and;
> 
> - Is sync_console as bad an idea as the warning suggests it is?
> 
> I get all my serial console output if I use sync_console, but I
> haven't really put the machines under heavy load and would rather
> trade off truncated boot messages as opposed to performance under
> load.
> 
> Cheers,
> Andy
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

Re: Losing serial console boot output / sync_console

[Andy Smith]

Hi Keir,

On Sun, Jul 03, 2011 at 12:14:32AM +0100, Keir Fraser wrote:
> On 02/07/2011 21:04, "Andy Smith" <andy@strugglers.net> wrote:
> > About two months ago I sent this email to the Xen users lists
> > regarding a problem I was having losing some of my serial console
> > output on boot:
> > 
> > http://lists.xensource.com/archives/html/xen-users/2011-04/msg00340.html
> 
> If your serial line is very slow (I don't know why you're limited to 9600
> baud -- 115200 baud at least is usually possible) and there is a lot of boot
> output then you can expect some of it to be discarded. We prefer to discard
> data rather than spin-wait to send it as that can indeed cause other
> time-critical services to be delayed too long.

Fair enough. I'm not really concerned about things being slow during
boot, more if sync_console would introduce strange latencies during
normal operation even without excessive ongoing console output.

Unfortunately I have no control over the terminal server
configuration in this instance otherwise I'd certainly be using a
higher baud rate. Having never had this problem with earlier
versions of Xen or anything else I thought I better ask.

> If you want to capture all Xen's boot output, the safest way is to increase
> the serial transmit buffer size. Try adding serial_tx_buffer=64k to the Xen
> command line.

Thanks, I'll try that.

Cheers,
Andy