[Xen-devel] domU console buffer behaviour

[Ferenc Wagner <wferi@xxxxxxx>]

Hi,

After the 1 MB domU console buffer fills up, what is the expected
behaviour on further output?  It clearly does not behave as a ring
(under Xen 3.2.1, dom0 2.6.18, domU 2.6.26) as the following shows.

On domU with its console disconnected (no xm console running in dom0):

# i=0; while :; do printf '%15d\n' $i >/dev/console; i=$((i+1)); echo $i; done
0
1
[...]
72317
72318
^Cbash: printf: write error: Interrupted system call

Then on dom0 capturing the output of xm console gives a file of
1048576 (exactly 1 MiB) long that goes like:

            158  -
            159   |
[...]             +- 65408 = 65536-128 lines
          65564   |
          65565  -
          72174  -
          72175   |
[...]             |
          72284   |
          72285   +- 128 lines
          72302   | -
          72303   |  |
[...]             |  + 16 lines
          72316   |  |
          72317  -  -

(Tested with perl -ne 'print $_ if $_ != $p+1; $p=$_')

That is, the first 158 lines were dropped and the last 128 lines were
overwritten.  As formatting is such that each line is 16 bytes, it
means that the last 2 kiB was sort of following the guest console
output, but in a broken way: lines between 72285--72302 were lost.
Only the last 16 lines (256 bytes) are really fresh.

All of this feels rather fishy.  Can anybody provide some insight,
please?  Is this expected?  Is my config broken?  Or is this a bug?

Why I'm after this: last night all our pvops domUs froze hard with no
sign of anything going wrong.  On two separate dom0s, within a couple
of hours.  The two surviving domUs are those which generate almost no
console output.  (I guess I should keep those buffers emptied, but I
found no best practices on that so left the thing unconfigured.)
-- 
Thanks,
Feri.

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