Re: [lttng-dev] Profiling LTTng tracepoint latency on different arm platforms

[Michel Dagenais via lttng-dev <lttng-dev@lists.lttng.org>]

You are probably referring to the work of Mohammad Gebai:

Survey and Analysis of Kernel and Userspace Tracers on Linux: Design, Implementation, and Overhead

https://dl.acm.org/doi/abs/10.1145/3158644 

----- Le 11 Sep 23, à 11:52, Mathieu Desnoyers mathieu.desnoyers@efficios.com a écrit :

> On 9/10/23 10:18, Mousa, Anas wrote:
>> Hey Mathieu,
> 
> Hi Anas,
> 
>> 
>> We see that upon recording a tracepoint, there are multiple stages of
>> reserve-commit-write, where atomics and shared memory accesses take up a big
>> part of the
>> recording time,
>> 
>> we're wondering, is there a "light-mode" of recording a tracepoint
>> involving less logic or
>> 
>> a mode which can potentially have lower latency?
> 
> I've been working on the rseq(2) system call for a few years now, and
> this is intended to help reduce the cost of lttng-ust's ring buffer
> atomics on the tracing fast-path. The road ahead there is integration of
> rseq with lttng-ust, which did not show up on our customer feature
> requirements radar yet.
> 
> In terms of logic involved in the lttng-ust tracepoints, I hope that my
> current work on "libside" will help steer away from tracepoint providers
> based on macros and generated code, replacing this by an efficient
> bytecode interpreter. This should allow me to inline many of the calls
> that are currently needed between the tracepoint probe provider and the
> lttng-ust ring buffer. Again, this is an area where I think we can have
> great speed improvements, but it did not show up on our customer's
> feature requirement radar yet.
> 
>> Also, are there any recent docs to share regarding tracepoint latency?
> 
> There is a Polytechnique student who extensively analyzed this recently.
> Michel, do you have a pointer to his work ?
> 
> Thanks,
> 
> Mathieu
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
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