RE: [Xen-devel] some questions (xfrd, syslog, mem, preempt)

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 1) I found one line about that obscure xfrd in the "Xen Users' 
>    manual":
> 
>        As xend runs, events will be logged to /var/log/
>        /xend.log and, if the migration assistant daemon
>        (xfrd) has been started, /var/log/xfrd.log
> 
>    That's it, nothing in the FAQ. What is it? Why is it 
>    started by "xend start"? Why is it listening on 0:8002?
>    If it is necessary, how can i bind it to localhost? If
>    not, how can i prevent it from being started?

xfrd is needed for save/restore/migrate, and isn't needed otherwise.

An option could be added such that it only binds to localhost, but the
current assumption is that dom0's exist on a private VLAN.

> 2) Can i configure xend to use syslog? xend.log's time-
>    stamps have non standard format, and xend-debug.log has
>    no timestamps at all.

The log messages get written via python's logger package, which I
believe can be configured quite flexibly.
 
> 3) When i create a domain configured for "memory = 64" i 
>    observe that "xm info" reports "free_memory" decreasing
>    exactly 64MB. But "xm list" reports that the domain 
>    consumes 63MB, and /proc/meminfo even less (63136 kB). 
>    This happens allways.Is the missing RAM consumed by the 
>    Xen hypervisor? (I do not use the balloon driver, yet)

The difference between xm list and xm info is robably just rounding
differences. 
/proc/meminfo reports about Linux's internal allocation pool, so will
always be smaller, just like on native. 
 
>    Same goes for domain0: I boot with "dom0_mem=100000", 
>    that should be 100000/1024 = 97.656MB, but "xm list" 
>    reports 92MB for "Domain-0" and /proc/meminfo 94740kB, 
>    at least 5MB missing.

Dom0 gives back a few MB of memory after booting, as it wants to create
certain unused 'guard frames' in it's memory map. I guess this is
slightly confusing since dom0 will end up owning less memory than
requested, but its not a big issue, and the memory isn't wasted and is
available for other domains. 

>    Is the difference between the sum of all gross (or net?)
>    domain memory footprints plus the "free_memory" and my
>    hosts RAM the memory consumption of the Xen hypervisor/
>    microkernel (in my case it's 17MB)?

For normal domains (those not using shadow page tables), Xen only needs
about 100KB per domain.

> 4) Any recommendations for CONFIG_PREEMPT and CONFIG_PRE-
>    EMPT_BKL? Or just the same as if the kernels would run
>    native, eg "If it runs a desktop use CONFIG_PREEMPT=y"?

We run with both set to Y by default.
 
> 5) By accident i had CONFIG_HANGCHECK_TIMER=y in my dom0.
>    That flooded the kernel log with "Hangcheck: hangcheck
>    value past margin!" Is that caused by the nature of Xen
>    to pause and unpause domains all the time?

Yes, I suspect you don't want to enable this if you have multiple
domains sharing a CPU.

Nice questions, BTW. 
Ian


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