Xen devel - Mar 2005 - some questions (xfrd, syslog, mem, preempt)

Hi list,

i have some minor non-critical questions about xen:


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?


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.


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)

   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.
   
   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)?


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"?


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?


/nils.
-- 
no sig



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> 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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Am Montag, den 21.03.2005, 21:10 +0000 schrieb Ian
Pratt:
> > 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.
How can i disable it if i do not need that?

> 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.
The same discussion was here for xend itself and the console ports, many
arguing that this assumtion restricts the possible application cases for
Xen extremly (many of us use Xen _because_ of security reasons).
Firewalling is allways an _additional_ layer of security, no application
should rely on it.

Shortly after that discussion, the option "xend-address" showed up. So
i
hoped same would happen with "xfrd-address" ;-)

> > 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.
I look forward to new releases ...
 
> > 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.
Even for servers? Ok!

 
> Nice questions, BTW. 
Thanks for the nice & really fast answers (for my other postings, too).
Superb.

/nils.



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