RE: [Xen-users] xen blocked state

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Otter
> Sent: 19 May 2006 17:44
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] xen blocked state
> 
> I always assumed blocking was the same blocking that CPU 
> scheduling uses. Meaning multiple programs want to access the 
> resources and they can't all have them at once, and no one 
> program can have 100% access to resources as other programs 
> would never get CPU time. So blocking is used, such that 
> resources are distributed evenly among the competing 
> programs. For Xen I always assumed it was similar but Xen 
> treats virtual machines as processes ( in dom0 ), and 
> schedules them much like an OS would schedule programs. I am 
> sure there is much more involved in Xen blocking. But that is 
> how I always interpret the block state of virtual machines.

It's worth noting that on a single CPU, if you're running xm list (or
anything else to get information on what domains are doing what), you
will perform that operation in a combination of Dom0 and the actual
HyperVisor. That means that Dom0 _MUST_ be running at the same time as
you're doing xm list. Whether some other domain is also running or not
depends on:
1. If there are other domains that are actually using CPU at the given
time.
2. If there is any other CPU that could do this. On a single CPU system,
that wouldn't be the case, but on a dual/quad/octet system you could
conceivably have 1, 3 or 7 other domains running at the same time. 

On my dual core Athlon64, running Memtest-86 shows this:
Name                              ID Mem(MiB) VCPUs State  Time(s)
Domain-0                           0      251     2 r----- 16666.5
MemTest86                          4     1024     1 r----- 76822.6

Obviously, Dom0 is running because I'm performing xm list on it. MemTest
is ALWAYS running if nothing else is there to run... 

And yes, Blocked would be ANY waiting condition, whether it's a
semaphore to control some lock on a resource, or waiting for keyboard,
hard-disk, timer, network, serial port or whatever type of event it may
be. ANYTHING that causes the system to "wait" for something - except for
spinlocks, because they are not able to allow something else to run -
that is basically what blocked mean "I give up my right to the CPU, let
someone else use it" is what it means. 

--
Mats


> Steffen Heil wrote:
> > Hi
> >
> >   
> >> Just curious, what does "blocked" really mean?  Is  r "running" a 
> >> state reserved for Dom0 and not DomU.  I am using SLES10, 
> and I have 
> >> noticed for quite awhile that when my DomUs are running they are 
> >> usually in the blocked state and never in the running 
> state, and my 
> >> Dom0 is always in the running state.  The Xen user guide, 
> explains b 
> >> as blocked but doesn't really give any other detail.  Is 
> there some 
> >> other doc that explains this more?
> >>     
> >
> > Your dom0 is always in "running" state?
> > How do you know?
> >
> > Using xentop - thus running a software in dom0 - using the cpu for 
> > dom0 at the very moment you get the dom list?
> > For sure, dom0 must be running. So, no running is not a 
> state reserved 
> > for dom0, domUs couldn't work at all then.
> > But the dom0 IS runnign at the very moment, xentop 
> retrieves the list. 
> >
> > blocked means the domains are waiting for something. Processing is 
> > blocked due to the lack of something to do.
> > Blocked may be waiting for keystrokes of the user, movement of the 
> > mouse, network packets to arrive maybe even hard drive access 
> > (xen-devs, that's right?).
> >
> > Regards,
> >   Steffen
> >
> >
> >   
> > _______________________________________________
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> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
> >
> >   
> 
> 
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