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RE: [Fwd: Re: [Xen-devel] Zaptel PCI IRQ problem]

To: "François Delawarde" <fdelawarde@xxxxxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Fwd: Re: [Xen-devel] Zaptel PCI IRQ problem]
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Mon, 21 May 2007 16:37:09 +0200
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Thread-topic: [Fwd: Re: [Xen-devel] Zaptel PCI IRQ problem]
> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> François Delawarde
> Sent: 21 May 2007 15:20
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Fwd: Re: [Xen-devel] Zaptel PCI IRQ problem]
> Hi,
> Sorry to insist, but I would really like to be able to use Xen and my 
> zaptel hardware all together in Dom0. I was just wondering if the 3.1 
> release could contain some changes compared to 3.0.4 related to IRQ 
> handling or scheduling that could workout my problem.

As far as I can see, there's no improvement in 3.1 over 3.0.4 as to how 
interrupts are handled or how the scheduling works [I'm not really sure how 
that could practically be improved wihtout loosing performance elsewhere - this 
is a case of "you can make it right for some people some of the time, but not 
all people all of the time"]. 

This can possibly be solved by restricting which other domains run on the same 
CPU as Dom0. However, there will certainly be some load on Dom0 because of 
qemu-dm being there, but unless you're running disk or network benchmarks on 
your DomU, you should have reasonable performance in Dom0 without much effort. 

If you share the Dom0 CPU with DomU's, then you have little chance to get it to 

All this is assuming I understand correctly that the real problem here is that 
the latency between the interrupt and the actual code executing in user-mode is 
the key to the problem. Making sure Dom0 runs on it's own CPU will make sure 
that there's very little overhead compared to native. 

> Thanks,
> François.
> ----
> I actually first asked to asterisk mailing lists, and a few 
> persons told 
> me that it was Xen's fault, as it was not yet 'mature' enough 
> to have a 
> good IRQ handling under load.
> Note that I made tests the last few days as I wasn't sure if 
> it was Xen 
> or not, and the exact same system works perfectly with a normal Linux 
> kernel (same config file except for Xen stuff that are 
> removed). A Dom-0 
> kernel without any VMs running comports itself the way I described 
> (bad), and I tried both schedulers (sedf and credit) without success.
> It doesn't appear to be a load problem as the load is about the same 
> with the non-Xen kernel I tried, but with IRQ handling in 
> load period. 
> I'm talking about a machine that is certainly not 
> over-loaded, but that 
> once in a while suffers some iowait for disk access. Under 
> Xen kernel, 
> if I kill everything I can and only leave Asterisk with at most one 
> simultaneous conversation, it works quite nice.
> I'm using the debian (I think they actually come from fedora) patches 
> for 2.6.18, and just want to know if this issue is known or has/will 
> been/be resolved somehow in future versions, if there is anyway I can 
> deal with it with some kernel configuration, or if I should 
> wait a few 
> months/years more to be able to use Xen in my specific setting.
> Thanks,
> François.
> Ian Pratt wrote:
> >> I'm currently trying to run an Asterisk server in a Xen 
> kernel under
> >> Dom0 (debian kernel 2.6.18 with xen hypervisor 3.0.4). I 
> had read of
> >> some possible timing issues with ztdummy (using rtc) under 
> DomU, but I
> >> have a zaptel compatible PCI card (TDM400P), and I experience big
> >> problems with IRQ misses every time there is a bit of load 
> on the server
> >> (for example, when an HVM DomU is running). The card is supposed to
> >> report 1000 interruptions per second, but it doesn't, and 
> consequences
> >> are horrible crackling sound in communications. Running the utility
> >> zttest to check for the stability of those interrupts 
> under a small bit
> >> of load, i get:
> >>     
> >
> > I believe folk have had success running asterisk in a domU 
> and assigning the PCI device directly to the guest. It's best 
> to set the affinity masks for other guests and dom0 such that 
> the domU with asterisk in it has a dedicated physical CPU core. 
> >
> > We ran asterisk on an older version of Xen without any 
> problems, and nothing has changed that should effect xen's 
> ability to do this. [you could try using the sedf scheduler 
> if you still have problems with 'credit']
> >
> > Ian
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >
> >   
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