[lttng-dev] HugePages shared memory support in LLTng

[Mathieu Desnoyers]

----- On Jul 23, 2019, at 9:27 PM, Jonathan Rajotte jonathan.rajotte-julien at efficios.com wrote:

> CC'ing the mailing list back.
> 
> On Tue, Jul 23, 2019 at 03:58:09PM -0400, Yiteng Guo wrote:
>> Hi Jonathan,
>> 
>> Thank you for the patch! It is really helpful.
> 
> Were you able to observe a positive impact?
> 
> This is something we might be interested in upstreaming if we have good
> feedback.
> 
>> 
>> Is there any disadvantage of per-pid buffering? I don't want to have
>> processes interfere with each other so I choose per-pid buffering.
> 
> The main downside is that each registered applications will get their own
> subbuffers resulting in a lot of memory usage depending on your session
> configuration. This can get out of hand quickly, especially on systems withs
> lots
> of cores and unknown number of instrumented applications.

I can add 2 extra cents (or actually a few more) to this answer:

There are a few reasons for using per-uid buffers over per-pid:

- Lower memory consumption for use-cases with many processes,
- Faster process launch time: no need to allocate buffers for each process.
  Useful for use-cases with short-lived processes.
- Keep a flight recorder "snapshot" available for all processes, including
  those which recently exited. Indeed, the per-pid buffers don't stay around
  for snapshot after a process exits or is killed.

There are however a few advantages for per-pid buffers:

- Isolation: if one PID generates corrupted trace data, it does not interfere
  with other PIDs buffers,
- If one PID is killed between reserve and commit, it does not make that specific
  per-cpu ring buffer unusable for the rest of the tracing session lifetime.

Hoping this information helps making the right choice for your deployment!

Thanks,

Mathieu


> 
> If you completely control the runtime, for example when doing benchmarking or
> simple analysis, feel free to use what make more sense to you as long as you
> understand the pitfalls of each mode.
> 
> Cheers
> 
> --
> Jonathan Rajotte-Julien
> EfficiOS
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com