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Re: [ltt-dev] LTTng-UST vs SystemTap userspace tracing benchmarks
- From: Stefan Hajnoczi <stefanha at gmail dot com>
- To: "Frank Ch. Eigler" <fche at redhat dot com>
- Cc: Julien Desfossez <julien dot desfossez at polymtl dot ca>, dominique dot toupin at ericsson dot com, ltt-dev at lists dot casi dot polymtl dot ca, Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>, systemtap at sources dot redhat dot com
- Date: Tue, 15 Feb 2011 17:00:27 +0000
- Subject: Re: [ltt-dev] LTTng-UST vs SystemTap userspace tracing benchmarks
- References: <4D5AA164.1050607@polymtl.ca> <y0mvd0ltgba.fsf@fche.csb>
On Tue, Feb 15, 2011 at 4:26 PM, Frank Ch. Eigler <fche@redhat.com> wrote:
>
> Julien Desfossez <julien.desfossez@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