[ltt-dev] LTTng-UST vs SystemTap userspace tracing benchmarks

[Stefan Hajnoczi]

On Tue, Feb 15, 2011 at 4:26 PM, Frank Ch. Eigler <fche at redhat.com> wrote:
>
> Julien Desfossez <julien.desfossez at polymtl.ca> writes:
>
>> LTTng-UST vs SystemTap userspace tracing benchmarks
>
> Thank you.
>
>> [...]  For flight recorder tracing, UST is 289 times faster than
>> SystemTap on an 8-core system with a LTTng kernel and 279 times with
>> a vanilla+utrace kernel.
>
> This is not that surprising, considering how the two tools work.  UST
> does its work in userspace, and is therefore focused on an individual
> process's activities.  Systemtap does its work in kernelspace, and can
> therefore focus on many different processes and the kernel at the same
> time.  This entails some ring transitions.
>
> (One may imagine a future version of systemtap where scripts that
> happen to independently probe single processes are executed with a
> pure userspace backend, but this is not in our immediate roadmap.)

What is the fundamental mechanism that UST and SystemTap use for tracing?

e.g. Here's a guess:
UST: a conditional function call within the same process
SystemTap: a software interrupt on x86

I don't know the implementations details but would be interested in
understanding this.

Stefan