[lttng-dev] How much shared memory is used by LTTng (UST) ?

[Mathieu Desnoyers]

* Amit Margalit (AMITM at il.ibm.com) wrote:
> Hi,
> 
> Chasing down an issue with adding some context (see 
> https://bugs.lttng.org/issues/579), I've encountered something that makes 
> me think the failures I am seeing are related to amount of shared memory 
> that LTTng uses...
> 
> In short - when the subbuffers have a large enough size and number, adding 
> things to context on a multi-core system can cause libringbuffer/shm.c in 
> lttng-ust to complain about "No space left on device", leading to 
> everything looking OK, except that the trace directory stays empty.
> 
> I am working on a 6-core (12 with HT) system that has 24GB of RAM. This is 
> one configuration, but we also have dual-CPU systems (i.e. 12 cores / 24 
> with HT) with 96GB.
> 
> I am using 16 subbuffers of 256KB. In this setup, I have no problem adding 
> procname, vpid and vtid to the context.
> 
> When I tried the same on the 12-core dual-CPU system, it failed as 
> explained above. I tried reducing the subbuffer sizes to 64KB, and the 
> problem disappeared.
> 
> So - how can I tell how much shared memory LTTng wants, and - if you know 
> - how can I configure my system to allow more than the default?

First thing, try using per-uid buffers rather than the 2.2 default
(per-pid buffers).

If you do that, then you will need:
  nb_traced_users * subbuf_size * nr_subbuf * nb_cpus   as shm space.

If you stay with per-pid buffers, then you need
  nb_processes * subbuf_size * nr_subbuf * nb_cpus   as shm space.

On my system here, I notice that each shm creates files within /run/shm
(this location may differ on your system). It is a tmpfs filesystem. You
might want to increase its size. For instance, here, it is 1.6GB.

Thanks,

Mathieu

> 
> Thanks,
> 
> Amit Margalit
> IBM XIV - Storage Reinvented
> XIV-NAS Development Team
> Tel. 03-689-7774
> Fax. 03-689-7230
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com