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

On Tue, Feb 15, 2011 at 4:26 PM, Frank Ch. Eigler <> wrote:
> Julien Desfossez <> 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.


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