[BuildStream] Weekly performance update

[Tristan Daniël Maat <tristan maat codethink co uk>]

Heya,

Since we don't want this to stop whenever James isn't around, some
others will try their hands at this for the next couple of weeks. It's
my turn this week!

This is a little late because the pipeline failed in response to
casd. I've also taken the liberty to make our memory unit explicit
(MiB).

For a little commentary; You'll notice disappointingly small
performance improvements for large numbers of builders, considering
that we landed casd - I suspect our benchmark doesn't tickle the cache
expiry case too much. For real projects I still believe that this
should be a fair improvement :)

Without further ado, onto the performance update.

---

Ipython notebook with the results: 
https://mybinder.org/v2/gh/james-ennis/bst-benchmarks-notebooks/master?filepath=all_results.ipynb

Don't forget to run "Kernel" -> "Restart and Run all" to actually
generate the graphs.

Note: If the link has expired, you'll need to go to
https://mybinder.org/ and enter:
https://github.com/james-ennis/bst-benchmarks-notebooks into the url
field, with branch "master" and file "all_results.ipynb". Then "launch".

---

All results were obtained on the same hardware (a Codethink dev
machine configured as a GitLab runner - not being used for anything
else), and the results files are being appended to by a CI job. Specs
as follows:

* Linux (Debian stretch)
* x86_64
* 16G RAM
* 500Gb SSD
* Intel i7-3770
* 8 cores @ 3.40 GHz

---

Here are the times and max-rss (peak memory usage) results for a `bst
show` of base-files/base-files.bst in the Debian-like project (branch:
jennis/add_all_files [1]) for the last 5 weeks:

+----------+-------------------------+--------------------------------+
|          |          Show           |        Show once built         |
+----------+----------+--------------+-----------------+--------------+
|  Dates   | Time (s) | Memory (MiB) |    Time (s)     | Memory (MiB) |
+----------+----------+--------------+-----------------+--------------+
| 22/07 -> |      6.51|           201| 6.39|           225|
| 28/07    |          |              | |              |
+----------+----------+--------------+-----------------+--------------+
| 29/07 -> |      6.58|           201| 6.33|           225|
| 04/08    |          |              | |              |
+----------+----------+--------------+-----------------+--------------+
| 05/08 -> |      6.57|           201| 6.79|           226|
| 11/08    |          |              | |              |
+----------+----------+--------------+-----------------+--------------+
| 12/08 -> |      6.58|           201| 6.77|           226|
| 18/08    |          |              | |              |
+----------+----------+--------------+-----------------+--------------+
| 19/08 -> |      6.29|           199| 6.55|           229|
| 25/08    |          |              | |              |
+----------+----------+--------------+-----------------+--------------+

---

Here are the times and max-rss results for a `bst build` (4, 8 and 12
builders) of base-files/base-files.bst in the Debian-like project
(branch: jennis/add_all_files [1]).

+----------+---------------------------+---------------------------+---------------------------+
|          |        4 Builders         |        8 Builders         
|        12 Builders        |
+----------+------------+--------------+------------+--------------+-------------+-------------+
|  Dates   |  Time (s)  | Memory (MiB) |  Time (s)  | Memory (MiB) |   
Time(s)   | Memory(MiB) |
+----------+------------+--------------+------------+--------------+-------------+-------------+
| 22/07 -> |      129.61|           223| 131.63|           223|       
128.31|          223|
| 28/07    |            |              | |              |             
|             |
+----------+------------+--------------+------------+--------------+-------------+-------------+
| 29/07 -> |      130.06|           223| 133.03|           224|       
129.37|          224|
| 04/08    |            |              | |              |             
|             |
+----------+------------+--------------+------------+--------------+-------------+-------------+
| 05/08 -> |      129.05|           223| 134.05|           224|       
129.83|          224|
| 11/08    |            |              | |              |             
|             |
+----------+------------+--------------+------------+--------------+-------------+-------------+
| 12/08 -> |      134.24|           223| 135.14|           224|       
132.45|          224|
| 18/08    |            |              | |              |             
|             |
+----------+------------+--------------+------------+--------------+-------------+-------------+
| 19/08 -> |      131.38|           226| 133.26|           226|       
134.09|          226|
| 25/08    |            |              | |              |             
|             |
+----------+------------+--------------+------------+--------------+-------------+-------------+

---

Please see [2] if you're unable to open the weekly notebook locally.

Thanks,
Tristan Maat

[1] https://gitlab.com/jennis/debian-stretch-bst/tree/jennis/add_all_files
[2] 
https://mail.gnome.org/archives/buildstream-list/2019-February/msg00046.html