Re: [MirageOS-devel] Mirage tracing

[Thomas Gazagnaire <thomas@xxxxxxxxxxxxxx>]

>> I'm currently working on improving the profiling support in Mirage.
>> Previously [1], I was just graphing stats in libreoffice and looking
>> at call traces, but I've been thinking about how to get more useful
>> data.
>> 
>> Tracing individual functions was too fine-grained, I think, and failed
>> to follow Lwt threads, so I intrumented Lwt to record when threads are
>> created and resolved, and the interactions between them. Graphing the
>> results looks like this:
>> 
>>  http://test.roscidus.com/static/block-read-mirage-x86.png
>> 
>>  ( trace file: http://test.roscidus.com/static/log-x86.sexp )

That's pretty cool! That would be even cooler to have a HTML/CSS/javascript 
output on a "debug" port for any unikernel :-)

> How should tracing work with existing mirage libraries? We want
> libraries to be able to report extra information (thread labels, when
> significant events occur, performance counters, etc), but we also want
> high performance when tracing is off.
> 
> One possibility is a mirage-tracing library whose default
> implementation does nothing. I imagine the OCaml compiler would
> optimise all calls to the library away, using cross-module inlining.
> Then you could opam pin the real version if you wanted tracing on.
> 
> Or is it better to make it dynamic, so tracing can be enabled at any
> time without recompiling? Or with a functor applied by the mirage
> tool? Any suggestions?

I think we were discussing having a special "Tracer" (or "Profiler") module 
type, similar to the "Console" one. Or to extend the "Console" one with more 
structured kinds of debug statement. Not sure what is the most practical, but 
we should certainly do something to support what you have started to do at a 
larger scale.

And yes, having a dummy Profiler where no-op are optimised away is a good idea. 
Hopefully, Pierre Chambart's patches for improving the inliner will be in OCaml 
before Mirage 3.0 :-)

Thomas

> 
> 
> -- 
> Dr Thomas Leonard        http://0install.net/
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