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

Amit Margalit AMITM at il.ibm.com
Wed Jul 10 09:42:01 EDT 2013


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?


Amit Margalit
IBM XIV - Storage Reinvented
XIV-NAS Development Team
Tel. 03-689-7774
Fax. 03-689-7230
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