[ltt-dev] LTTng specialized probes

[compudj@krystal.dyndns.org (Mathieu Desnoyers)]

Hi,

I'm currently working towards getting LTTng in shape for what is
required for mainline. I got the "TLB-less" buffers and splice()
working last week. I then did some performance testing on the flight
recorder mode and noticed an optimization that's really worth doing :

LTTng "ltt-serialize.c", which parses the format strings and formats
data into the trace buffers takes a lot of CPU time. I tried only
keeping the size calculation (first pass on the format string) and
disabling the real data write and basically got something like :

(default LTTng instrumentation, very approximate numbers)

tbench no tracing : ~1900MB/s
       Markers enabled : ~1800MB/s
       with size calculation : ~1400MB/s
       size calc + data write : ~950MB/s

I then remembered I've done ltt-serialize in such a way that it can be
easily overridden by per-format string specialized callbacks.

Therefore, it would be worthwhile to create such specialized serializers
so the common cases can be made much faster. I think it will have a very
significant impact on performance.

It's simply a matter of creating a new .c kernel module in ltt/ and to
create structures similar to :

ltt-serialize.c :

struct ltt_available_probe default_probe = {
        .name = "default",
        .format = NULL,
        .probe_func = ltt_vtrace,
        .callbacks[0] = ltt_serialize_data,
};

Give it a non-null format string (just giving the types expected by the
callback), a good name, and a callback function, which implements the
specialized serialization. Note that kernel/marker.c currently expects
the format string to match exactly the marker format string, including
the type names, which should be changed. The type verification should
only check that the %X parameters are the same (and that there are the
same amount of arguments expected).

That should not be hard, but it's not what I plan to focus on next.
Anyone is willing to work on this ?

Mathieu

-- 
Mathieu Desnoyers
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