[LLVMdev] RFC:LNT Improvements

[Tobias Grosser]

On 30/04/2014 16:20, Yi Kong wrote:
> Hi Tobias, Renato,
>
> Thanks for your attention to my RFC.

> On 30 April 2014 07:50, Tobias Grosser <tobias at grosser.es> wrote:
>  >> - Show and graph total compile time
>  >>    There is no obvious way to scale up the compile time of
>  >> individual benchmarks, so total time is the best thing we can do to
>  >> minimize error.
>  >>    LNT: [PATCH 1/3] Add Total to run view and graph plot
>  >
>  > I did not see the effect of these changes in your images and also
>  > honestly do not fully understand what you are doing. What is the
>  > total compile time? Don't we already show the compile time in run
>  > view? How is the total time different to this compile time?
>
> It is hard to spot minor improvements or regressions over a large number
> of tests from independent machine noise. So I added a "total time"
> analysis to the run report and able to graph its trend, hoping that
> noise will cancel out and will help us to easily spot. (Screenshot
> attached)

I understand the picture, but I still don't get how to compute "total 
time". Is this a well known term?

When looking at the plots of our existing -O3 testers, I also look for 
some kind of less noisy line. The first thing coming to my mind would 
just be the median of the set of run samples. Are you doing something 
similar? Or are you computing a value across different runs?

> On 30 April 2014 07:50, Tobias Grosser <tobias at grosser.es> wrote:
>  > I am a little sceptical on this. Machines should generally not be
>  > noisy. However, if for some reason there is noise on the machine, the
>  > noise is as likely to appear during this pre-noise-test than during
>  > the actual benchmark runs, maybe during both, but maybe also only
>  > during the benchmark. So I am afraid we might often run in the
>  > situation where this test says OK but the later test is still
>  > suffering noise.
>
> I agree that measuring before each run may not be very useful. The main
> purpose of it is for adaptive problem scaling.

I see. If it is OK with you, I would propose to first get your LNT 
improvements in, before we move to adaptive problem scaling.

> On 30 April 2014 07:50, Tobias Grosser <tobias at grosser.es> wrote:
>  > In general, I see such changes as a second step. First, we want to
>  > have a system in place that allows us to reliably detect if a
>  > benchmark is noisy or not, second we want to increase the number of
>  > benchmarks that are not noisy and where we can use the results.
> Ok.

Obviously, as you already looked into this deeper, feel free to suggest 
different priorities if necessary.

Tobias