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Re: [Xen-devel] dom0 serial input overruns

To: Ferenc Wagner <wferi@xxxxxxx>
Subject: Re: [Xen-devel] dom0 serial input overruns
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Sun, 20 Mar 2011 16:43:21 -0400
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On Sun, Mar 20, 2011 at 12:02:08PM +0100, Ferenc Wagner wrote:
> (no dice on xen-users, let's try xen-devel...)
> Hi,
> I'm running a HA Xen cluster, where the dom0s are crosslinked via a null
> modem serial cable for heartbeat redundancy.  This works most of the
> time, but the serial connection is very unreliable, dropping characters

If you use the hypervisor serial connection "console=com1 com1=11152..."
and re-route the console output in Linux kernel to it (console=hvc0)
does this problem disappear?

> all the time, with lot of messages like "ttyS0: 2 input overrun(s)" in
> dmesg.  No such problem when running the same kernel on bare metal.  The
> link is running at 9600 baud, so the system should easily cope, but it
> looks like the serial interrupt isn't serviced timely enough under Xen.
> I'm running Xen 4.0.1 now with kernel 2.6.32 (stock Debian squeeze), but
> the problem isn't specific to this setup, Xen 3.2 with kernel 2.6.18 had
> much the same issue.  Raising dom0's scheduling weight didn't help much
> (or at all), pinning all domUs to CPU1-3 and vcpu0 of dom0 to CPU0
> actually made the problem worse.
> $ cat /proc/interrupts 
>             CPU0       CPU1       CPU2       CPU3       
>    1:          9          0          0          0  xen-pirq-ioapic-edge  i8042
>    4:   31712520          0          0          0  xen-pirq-ioapic-edge  
> serial
> [...]
> Is there some known solution to this problem?  It feels like overly big
> dom0 interrupt latency... maybe caused by the single-threaded hypervisor?
> Comments more than welcome!
> -- 
> Thanks,
> Feri.
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