[lttng-dev] Changed scheduling when using lttng

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Tue Apr 30 10:43:24 EDT 2013

* Mats Liljegren (liljegren.mats2 at gmail.com) wrote:
> I've done some code diving now, and I feel stupid...
> I thought I could get global buffers in kernel traces using option
> --buffers-global, as the help text seemed to indicate this:
> $ lttng enable-channel --help
> usage: lttng enable-channel NAME[,NAME2,...] [-u|-k] [OPTIONS]
> Options:
>   -h, --help               Show this help
>       --list-options       Simple listing of options
>   -s, --session NAME       Apply to session name
>   -k, --kernel             Apply to the kernel tracer
>   -u, --userspace          Apply to the user-space tracer
> Channel options:
>       --discard            Discard event when buffers are full (default)
>       --overwrite          Flight recorder mode
>       --subbuf-size SIZE   Subbuffer size in bytes
>                                (default: 4096, kernel default: 262144)
>                                Needs to be a power of 2 for
>                                kernel and ust tracers
>       --num-subbuf NUM     Number of subbufers
>                                (default: 4)
>                                Needs to be a power of 2 for
>                                kernel and ust tracers
>       --switch-timer USEC  Switch timer interval in usec (default: 0)
>       --read-timer USEC    Read timer interval in usec (UST default:
> 0, kernel default: 200000)
>       --output TYPE        Channel output type (Values: mmap, splice)
>       --buffers-uid        Use per UID buffer (-u only)
>       --buffers-pid        Use per PID buffer (-u only)
>       --buffers-global     Use shared buffer for the whole system (-k only)

This option would be clearer as "--buffers-per-cpu" maybe ?

> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>   -C, --tracefile-size SIZE
>                            Maximum size of of each tracefile within a
> stream (in bytes).
>   -W, --tracefile-count COUNT
>                            Used in conjunction with -C option, this
> will limit the number
>                            of files created to the specified count.
> I look in src/bin/lttng/commands/enable_channels.c, the long option
> array will point to opt_buffer_global. The only use for this file
> private variable is for a single sanity check. I cannot see any other
> use of it. Have I missed something here?
> But in this file, function enable_channel(), I see:
>                 dom.buf_type = LTTNG_BUFFER_GLOBAL;
> This is executed for the kernel case. But my buffers are still
> allocated per-cpu. What is going on? Where did this configuration go?
> What does it do?
> If I take this from the other end, I'd like to use the line:
> found in
>     lttng-ring-buffer-metadata-client.h
> How do I make this happen?

lttng-sessiond never creates buffers shared between CPUs for now (except
for metadata). The "--buffers-global" option is misleading and should
possibly be renamed.



> Thanks
>    Mats

Mathieu Desnoyers
EfficiOS Inc.

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