[lttng-dev] Channel buffer allocation

[Mathieu Desnoyers]

----- On Nov 13, 2017, at 1:40 PM, Nalin Pai <nalinpai at gmail.com> wrote: 

> Hi,
> In the per-pid buffer scheme, when are the ring buffers allocated for each pid?
> Is it when a channel is created or is it an event provided by that process is
> enabled for a specific channel.

In per-pid buffers, the buffers are allocated for each registered process whenever 
each of those process is registered (typically at program launch). Buffer allocation 
of per-pid buffers brings a significant overhead on systems with short-lived processes. 
Using "per-uid" buffers is recommended instead in those situations. 

> I am trying to understand whether there is an overhead to creating many
> channels. Will a process incur the allocation of buffers for a channel that is
> not interested in any of the events that the process is providing?

Yes, a process will have to allocate buffers if it's registered to lttng-sessiond, even if 
it does not contain any event enabled for that channel. This is needed to handle 
dynamically loaded libraries for instance, which may add additional events into a 
process after its initial constructor execution. 

Thanks, 

Mathieu 

> -Nalin

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