Re: [PATCH 00/10] Stitch LBR call stack

[Liang, Kan]

On 10/7/2019 2:24 PM, Ingo Molnar wrote:
> * kan.liang@xxxxxxxxxxxxxxx <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
> > Performance impact:
> > The processing time may increase with the LBR stitching approach
> > enabled. The impact depends on the number of samples with stitched LBRs.
> >
> > For sqlite's tcltest,
> > perf record --call-graph lbr -- make tcltest
> > perf report --stitch-lbr
> >
> > There are 4.11% samples has stitched LBRs.
> > Total number of samples:                        2833728
> > The number of samples with stitched LBRs        116478
> >
> > The processing time of perf report increases 6.8%
> > Without --stitch-lbr:                           55906106 usec
> > With --stitch-lbr:                              59728701 usec
> >
> > For a simple test case tchain_edit with 43 depth of call stacks.
> > perf record --call-graph lbr -- ./tchain_edit
> > perf report --stitch-lbr
> >
> > There are 99.9% samples has stitched LBRs.
> > Total number of samples:                        10915
> > The number of samples with stitched LBRs        10905
> >
> > The processing time of perf report increases 67.4%
> > Without --stitch-lbr:                           11970508 usec
> > With --stitch-lbr:                              20036055 usec
>
> That cost seems pretty high, while the feature sounds useful - is there
> any way to speed this up?

For each LBR entry, perf tool will calculate and generate an appended 
node for callchain_cursor.
The stitched LBR entries are from previous sample. It looks like we 
don't need to do the calculation again for them. That should speed up 
the whole process. I will do more test for it.

Thanks,
Kan