Re: unconditionally enable the trace buffer

[Rob Gardner <rob.gardner@hp.com>]

Ian Pratt wrote:

>>>>* ability to turn on/off via hypercall
>>>>        
>>>>
>>>Not currently implemented, but would not be difficult to add.
>>>      
>>>
>>Just as an aside I'm not sure this matters: From what Rob's 
>>told me, having the (inline) trace() calls in there produces 
>>the same overhead whether tracing is active or not.  I guess 
>>it makes sense; once you've incurred the overhead of having 
>>the function there and evaluating the "is tracing on" 
>>conditional, you might as well have stored a few values also ;-)
>>    
>>
>We could cache miss on reading the tracebuffer producer counter and
>tracebuffer base address, so its not totally a done deal.
>
>I don't have a big worry about performance, but I'd feel more
>comfortable to see a more realistic assesment of overhead. Comparing
>results from ttcp in a domU with 128k sock buf and the MTU set to 552
>bytes should do it.
>  
>

I just completed some benchmarks using ttcp as Ian suggests. I was 
surprised by the results, but essentially, Ian's intuition n this case 
may be right on the money. I tested three cases:
  1. trace buffers turned off, that is not compiled in
  2. trace buffers compiled in and turned on
  3. trace buffers compiled in but disabled
Cases 1 and 3 show no performance difference at all, but case 2 does 
show a non-trivial degradation.

Details of the benchmarking: ttcp using 128k buffers to send data to a 
domU from another machine using a gigabit network. 2gb were transferred 
in each run, and 10 runs were performed for each case. Dom0 and domU 
together used up just about all of a cpu, and experienced 2000-2500 
context switches per second to handle 40-50,000 I/O's per second. Please 
note that the additional trace events for XenMon were enabled in this 
system (case 2) so each of those 50,000 I/O's each second generates a 
trace record!

I conclude the following from this:
 - there is no penalty for simply having the trace buffer code compiled 
into xen
 - there is a penalty for enabling tracing
 - therefore, the ability to turn tracing on/off via a hypercall is 
definitely important

Rob