Xen devel - Oct 2004 - xm dmesg and performance on serial port

Hello,

I might only have configuration problem, but I couldn''t find any docs 
that could relate to the problem. The command: ''xm dmesg''
seems to stop
buffering any output after a while. I have to enable serial port to get 
the output bigger than the buffer. I don''t know how big the buffer or
if
it''s circular? Is there anyway to fix this behavior?

The second question I have since I have to enable the serial line. 
Considering the speed of the serial line is very slow. Would that affect 
the performance of XEN than just buffering the output (for ''xm
dmesg'')?
I do have to output some messages when a certain event occurs. Will 
enabling more consoles (serials, vga, tty)  slow the performance by default?

Just by reading the XEN papers, XEN has a way to register asynchronous 
events to the domains. Would it be better in terms of performance to 
send the event messages to a particular domain (e.g: Domain 0) using the 
asynchronous mechanism and have the domain kernel to handle the messages 
as XEN events.

Thanks in advance for any comments.


Kuas.


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> I might only have configuration problem, but I couldn''t find any
docs
> that could relate to the problem. The command: ''xm dmesg''
seems to stop
> buffering any output after a while. I have to enable serial port to get
> the output bigger than the buffer. I don''t know how big the buffer
or if
> it''s circular? Is there anyway to fix this behavior?
It''s not currently circular.  There used to be a -c flag to xm dmesg
(or it''s
equivalent) that cleared the buffer - does that work?  There''s code
somewhere
in Xen which could support a circular buffer with not much effort.
> The second question I have since I have to enable the serial line.
> Considering the speed of the serial line is very slow. Would that affect
> the performance of XEN than just buffering the output (for ''xm
dmesg'')?
Doing *lots* of printfs to the serial line is going to really hurt your 
performance but maybe that doesn''t matter so much for debugging? 
Occasional
printfs probably won''t matter very much.
> Just by reading the XEN papers, XEN has a way to register asynchronous
> events to the domains. Would it be better in terms of performance to
> send the event messages to a particular domain (e.g: Domain 0) using the
> asynchronous mechanism and have the domain kernel to handle the messages
> as XEN events.
We don''t use xm dmesg that much for debugging, generally just tracing
through
the code and using an attached serial line.

You might want to look at the tracebuffer and the xentrace tools.  The 
tracebuffer allows you to log events within Xen itself with low overhead into 
a circular buffer.  The xentrace command in dom0 will read this data out and 
decode it according to a format file.  There''s man pages for the
xentrace*
tools and the docs/interface.tex manual (although mostly out of date) 
explains how to put trace points into the Xen code.

HTH,
Mark
> Thanks in advance for any comments.
>
>
> Kuas.
>
>
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> Hello,
> 
> I might only have configuration problem, but I couldn''t find any
docs
> that could relate to the problem. The command: ''xm dmesg''
seems to stop
> buffering any output after a while. I have to enable serial port to get 
> the output bigger than the buffer. I don''t know how big the buffer
or if
> it''s circular? Is there anyway to fix this behavior?
It''s a circular buffer which you can change the size of
(CONSOLE_RING_SIZE in xen/drivers/char/console.c).
> The second question I have since I have to enable the serial line. 
> Considering the speed of the serial line is very slow. Would that affect 
> the performance of XEN than just buffering the output (for ''xm
dmesg'')?
> I do have to output some messages when a certain event occurs. Will 
> enabling more consoles (serials, vga, tty)  slow the performance by
default?
Yes the serial line will be the bottleneck if producing lost of
console output in DOM0. It''s much slower than placing Xen''s
output in
the ring, or writing it to VGA.
> Just by reading the XEN papers, XEN has a way to register asynchronous 
> events to the domains. Would it be better in terms of performance to 
> send the event messages to a particular domain (e.g: Domain 0) using the 
> asynchronous mechanism and have the domain kernel to handle the messages 
> as XEN events.
Batching events and sending them to DOM0 for storage will be much
faster than sending them over a serial line.

 -- Keir


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>>I might only have configuration problem, but I couldn''t find
any docs
>>that could relate to the problem. The command: ''xm
dmesg'' seems to stop
>>buffering any output after a while. I have to enable serial port to get
>>the output bigger than the buffer. I don''t know how big the
buffer or if
>>it''s circular? Is there anyway to fix this behavior?
>>    
>>
>It''s not currently circular.  There used to be a -c flag to xm
dmesg (or it''s
>equivalent) that cleared the buffer - does that work?  There''s code
somewhere
>in Xen which could support a circular buffer with not much effort.
>  
>
It is not circular or the circular mechanism might have been broken. The 
buffer just stopped after a certain limit. The code in "console.c"
does
not seems to be circular either. And the command with "xm dmesg -c" 
doesn''t seem to exist either.
>>Just by reading the XEN papers, XEN has a way to register asynchronous
>>events to the domains. Would it be better in terms of performance to
>>send the event messages to a particular domain (e.g: Domain 0) using the
>>asynchronous mechanism and have the domain kernel to handle the messages
>>as XEN events.
>>    
>>
>We don''t use xm dmesg that much for debugging, generally just
tracing through
>the code and using an attached serial line.
>
>You might want to look at the tracebuffer and the xentrace tools.  The 
>tracebuffer allows you to log events within Xen itself with low overhead
into
>a circular buffer.  The xentrace command in dom0 will read this data out and
>decode it according to a format file.  There''s man pages for the
xentrace*
>tools and the docs/interface.tex manual (although mostly out of date) 
>explains how to put trace points into the Xen code.  
>
xentrace seems to be used for debugging purpose. I am actually doing a 
security project using Xen. On a particular event that happens in the 
one of the domain, detected by a module in XEN. I''d like to report it
or
log it. I am trying to find the best way to do this. Let me know if I''m
wrong. Isn''t the "event_channel" is the communication channel
from XEN
to a domain. If I want to get XEN buffer messages (I guess it has to be 
in shared pages with the domain) to privilege domain 0, and flag the 
domain to check if there''s something in the buffer. And get a handler
in
domain 0 to consume the buffer. I think this would have much less 
performance effect. Is there any interface existed that I can use for 
this? Or I''ll just have to built it on top of event_channel.

Thanks,
Kuas.



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> It is not circular or the circular mechanism might have been broken. The
No, it''s not circular by design.
> buffer just stopped after a certain limit. The code in
"console.c" does
> not seems to be circular either. And the command with "xm dmesg
-c"
> doesn''t seem to exist either.
xm dmesg [-c|--clear] should now work if you pull the unstable repo (or wait a 
little for it to arrive in the stable repo).
> xentrace seems to be used for debugging purpose. I am actually doing a
> security project using Xen. On a particular event that happens in the
> one of the domain, detected by a module in XEN. I''d like to report
it or
> log it. I am trying to find the best way to do this.
Yes, the trace buffer is aimed at debugging timing-sensitive problems but it 
may still suit your needs, or at least provide a starting point.
> Let me know if I''m
> wrong. Isn''t the "event_channel" is the communication
channel from XEN
> to a domain.
Event channels are Xen''s equivalent to interrupts in normal hardware. 
Event
channels may be between Xen and a domain or between a pair of domains.  
Events send on these are basically like interrupts, i.e. they just mean
"look
at this!", signalling that some event has just occurred.  They
don''t actually
transfer data associated with an event.
> If I want to get XEN buffer messages (I guess it has to be
> in shared pages with the domain) to privilege domain 0, and flag the
> domain to check if there''s something in the buffer. And get a
handler in
> domain 0 to consume the buffer. I think this would have much less
> performance effect. Is there any interface existed that I can use for
> this? Or I''ll just have to built it on top of event_channel.
You''ve got the right idea.  This is basically what the xentrace / trace
buffer
stuff does, so you might want to look at xen/common/trace.c and 
tools/xentrace/xentrace.c for the details of how this stuff is done.  The 
trace buffer doesn''t actually use an event channel, it just polls 
periodically but this seems to work OK for our applications.

It would be possible to add a pseudo-device in dom0, which you could just read 
the trace data out of, with the reading process woken up when new data 
arrives (as signalled to dom0 by a suitable event channel).

HTH,
Mark



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