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RE: Static/dynamic userspace/kernel trace


On Mon, 2010-04-19 at 15:50 -0400, Dominique Toupin wrote:
> I agree that GDB could use a better ptrace.
> 
> Regarding tracing, for me the main use case is to get the data out of
> the system very fast and with very low overhead, besides conditional
> tracing and a few minor things, most of the analysis is done on the
> host side, either after the fact or in streaming mode. Doing
> historical analysis on the target or big actions are not an option for
> me. I guess in the IT industry more cycles are available to do those
> kinds of analysis directly on the target server.

One of the advantages of pre-filtering your tracing results is that the
overhead will be a lot lower than having to process a stream of
multi-megabytes i/o to disk or off an machines. With SystemTap I am
often not doing any kind of real analysis of the trace data on the
server itself, because it makes it possible to have the trace results in
a compact enough format that the results can be directly interpreted.

Having a very low overhead pre-filter of the trace output using full
expressions based on context variables, keeping statistics through
aggregate state variables and deciding what to push through the trace
output buffer using formatted output and data kept in associative arrays
helps a lot. Since all can be done without incurring extra i/o,
context-switches or external/post filtering it makes
interpreting/analyzing the actual trace data a lot easier and lower
overhead. It might also help in your use case, since you don't have to
push multi-megabytes of trace data off a machine but can tailor the
trace buffers to only have a couple of K of targeted output.

Cheers,

Mark


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