Doc/slub: document slabinfo-gnuplot.sh script

	slub_debug=FZ,dentry

Extended slabinfo mode and plotting

-----------------------------------

The slabinfo tool has a special 'extended' ('-X') mode that includes:

 - Slabcache Totals

 - Slabs sorted by size (up to -N <num> slabs, default 1)

 - Slabs sorted by loss (up to -N <num> slabs, default 1)

Additionally, in this mode slabinfo does not dynamically scale sizes (G/M/K)

and reports everything in bytes (this functionality is also available to

other slabinfo modes via '-B' option) which makes reporting more precise and

accurate. Moreover, in some sense the `-X' mode also simplifies the analysis

of slabs' behaviour, because its output can be plotted using the

slabinfo-gnuplot.sh script. So it pushes the analysis from looking through

the numbers (tons of numbers) to something easier -- visual analysis.

To generate plots:

a) collect slabinfo extended records, for example:

  while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done

b) pass stats file(-s) to slabinfo-gnuplot.sh script:

  slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]

The slabinfo-gnuplot.sh script will pre-processes the collected records

and generates 3 png files (and 3 pre-processing cache files) per STATS

file:

 - Slabcache Totals: FOO_STATS-totals.png

 - Slabs sorted by size: FOO_STATS-slabs-by-size.png

 - Slabs sorted by loss: FOO_STATS-slabs-by-loss.png

Another use case, when slabinfo-gnuplot can be useful, is when you need

to compare slabs' behaviour "prior to" and "after" some code modification.

To help you out there, slabinfo-gnuplot.sh script can 'merge' the

`Slabcache Totals` sections from different measurements. To visually

compare N plots:

a) Collect as many STATS1, STATS2, .. STATSN files as you need

  while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done

b) Pre-process those STATS files

  slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN

c) Execute slabinfo-gnuplot.sh in '-t' mode, passing all of the

generated pre-processed *-totals

  slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals

This will produce a single plot (png file).

Plots, expectedly, can be large so some fluctuations or small spikes

can go unnoticed. To deal with that, `slabinfo-gnuplot.sh' has two

options to 'zoom-in'/'zoom-out':

 a) -s %d,%d  overwrites the default image width and heigh

 b) -r %d,%d  specifies a range of samples to use (for example,

              in `slabinfo -X >> FOO_STATS; sleep 1;' case, using

              a "-r 40,60" range will plot only samples collected

              between 40th and 60th seconds).

Christoph Lameter, May 30, 2007

Sergey Senozhatsky, October 23, 2015