[lttng-dev] LTTng user mode performance observation

[Mathieu Desnoyers]

---- On Jun 2, 2016, at 1:29 AM, Richard Schmitt <richard.schmitt at nxp.com> wrote: 

> We are trying to use user space LTTng to perform some performance analysis of
> fast-path layer 3 forwarding. We have seen that with the tracepoints enabled,
> we experience about a 30% increase in service time. This is spread out over two
> tracepoints so each one is still contributing a good amount.

> This fast path is fast, about 300 cycles so we are asking a lot of efficiency of
> the tracepoint but for comparison, we implemented a simple ring buffer. When we
> run with this type of capture, we only experience a 5% increase in service
> time.

> I’m looking for pointers. We’d rather use LTTng. Are there ways to tune LTTng
> ust for ultra high performance requirements. Perhaps there is a way we could
> write directly to ust’s ring buffers, or perhaps preallocate the buffers?

What is the target architecture ? 

I'm currently working on proposing restartable sequence system call to the kernel to 
improve the speed of getcpu and remove atomic operations from the fast path 
on arm32, intel 32/64. This will be portable to other architectures too. 

Make sure you run a kernel with sys_membarrier available, and that you have 
a recent liburcu 0.9.x. This should take care of removing memory barriers from 
the tracing fast path. 

Thanks, 

Mathieu 

> Rich

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-- 
Mathieu Desnoyers 
EfficiOS Inc. 
http://www.efficios.com 
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