[lttng-dev] subbuffers count and size

[Vijay Anand]

Hi Sebastien,

I have written a simple program with a loop 1 million times adding a trace
everytime.
Please find the simple program at
https://github.com/vjanandr/sampleC/blob/master/lttng/benchmark/lttng_benchmark.c

I first configured the channel and then ran the benchmark program.
More logs @
https://gist.github.com/vjanandr/1db6a6a9d93e6b3f2ac30a05aadcf06b

Here is the observation below:

a. default 4 subbuffers with 128K each takes 1894063 microseconds
b. 16 subbuffers with 1024K each takes 1882392 microseconds
b. 16 subbuffer with 2M each takes 1884999 microseconds

on an average the programs takes about 1.8 seconds for each run.

Is this expected ?, if I have to improve the performance should I configure
anything different ?

Just that I understand this better, I have been trying to understand this
from looking at the code. Could you please point me to the code that might
be impacting this ?

Regards,
Vijay

On Mon, Jun 27, 2016 at 8:54 PM, Sebastien Boisvert <sboisvert at gydle.com>
wrote:

> Hi Vijay,
>
>
> On 06/27/2016 06:12 AM, Vijay Anand wrote:
> > Hi,
> >
> > I have been trying to understand the impact of performance while logging
> user space program based on the subbuffer count and size.
> >
>
> The sub-buffers count and size is well-documented here:
>
>     http://lttng.org/docs/#doc-channel-subbuf-size-vs-subbuf-count
>
>
>
> > I have a simple program logging 1 million traces
>
> Do you mean that you recorded 1 million UST events ?
>
> Or do you mean that you traced your app 1 million times and you generated
> 1 million traces ?
>
>
> > and I dont seem to see any appreciable performance between the below two
> configurations.
> >
> > a. default 4 subbufers 128KB each
> > b. 16 subbufers with 1024kB each.
>
> When you are enabling your UST events (before starting the LTTng session),
> are you enabling your UST events in your custom channel (the one with
> configuration a. or configuration b.) ?
>
>
> If not, it could explain the lack of difference in performance between
> your 2 configurations.
>
>
> Example:
>
>     lttng create
>
>     lttng enable-channel -u --subbuf-size 4M channel7
>     lttng enable-event -u -c channel7
> gydle_om:Allocator_constructor_default
>
>     lttng start
>
>     run-your-app
>
>     lttng stop
>     lttng view > trace.txt
>
>
>
> >
> > Moreover I have been looking at the lttng-ust code to understand the
> performance impacts from the implementation and unable to comprehend what
> is documented at
> >
> > http://lttng.org/docs/#doc-channel-subbuf-size-vs-subbuf-count
> >
> > I see that channel_backend_init, _shm_object_table_alloc_shm seems to
> allocate one big shared memory chunk which again is subdivided into
> subbuffers with each subbuffer referenced using the subbufer index.
> >
> > Further more lib_ring_buffer_write seems to find the subbuffer index and
> write "len" number of bytes into the subbuffer index.
> >
> > Could anyone please enlighten me what is the overhead involved while
> switching from one subbufer to another as documented.
> >
>
> The documentation indicates that the tracer's CPU overhead is caused by 2
> things:
>
> 1) marking the current sub-buffer as consumable, and
> 2) switching to an empty sub-buffer.
>
>
>
>
> > Regards,
> > Vijay
> >
> >
> > _______________________________________________
> > lttng-dev mailing list
> > lttng-dev at lists.lttng.org
> > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> >
>
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