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xen-devel
Re: [Xen-devel] unconditionally enable the trace buffer
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
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