[ltt-dev] LTTng specialized probes

[jiayingz@google.com (Jiaying Zhang)]

On Mon, Oct 6, 2008 at 7:11 AM, Mathieu Desnoyers <
compudj at krystal.dyndns.org> wrote:

> 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


Thanks a lot for sharing these numbers! Looks like we should
use special probe functions for high-frequency tracing events.
Also, do you know why enabling markers adds so much overhead?

Jiaying


>
> 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
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
>
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