PERFORMANCE.md

# Notes on performance measures during development

#### Commit 52fcb51f - Add basic random stealing

Slight improvement, needs further measurement after removing more important bottlenecks.

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| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
old     |    1659.01 us|    967.19 us|    830.08 us|    682.69 us|    737.71 us|    747.92 us|    749.37 us|    829.75 us|   7203.73 us
new     |    1676.06 us|    981.56 us|    814.71 us|    698.72 us|    680.87 us|    737.68 us|    756.91 us|    764.71 us|   7111.22 us
change  |    101.03  %|    101.49  %|     98.15  %|    102.35  %|     92.30  %|     98.63  %|    101.01  %|     92.16  %|     98.72  %

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| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
old     |    1648.65 us|    973.33 us|    820.18 us|    678.80 us|    746.21 us|    767.63 us|    747.17 us|   1025.35 us|   7407.32 us
new     |    1655.09 us|    964.99 us|    807.57 us|    731.34 us|    747.47 us|    714.71 us|    794.35 us|    760.28 us|   7175.80 us
change  |    100.39  %|     99.14  %|     98.46  %|    107.74  %|    100.17  %|     93.11  %|    106.31  %|     74.15  %|     96.87  %

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| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
old     |    1654.26 us|    969.12 us|    832.13 us|    680.69 us|    718.70 us|    750.80 us|    744.12 us|    775.24 us|   7125.07 us
new     |    1637.04 us|    978.09 us|    799.93 us|    709.33 us|    746.42 us|    684.87 us|    822.30 us|    787.61 us|   7165.59 us
change  |     98.96  %|    100.93  %|     96.13  %|    104.21  %|    103.86  %|     91.22  %|    110.51  %|    101.60  %|    100.57  %

#### Commit 3535cbd8  - Cache Align scheduler_memory

Big improvements of about 6% in our test. This seems like a little,
but 6% from the scheduler is a lot, as the 'main work' is the tasks
itself, not the scheduler.

### Commit aa27064 - Performance with ttsa spinlocks (and 'full blocking' top level)

<img src="media/aa27064_fft_average.png" width="600"/>

### Commit d16ad3e - Performance with rw-lock and backoff

<img src="media/d16ad3e_fft_average.png" width="600"/>

### Commit 18b2d744 - Performance with lock-free deque

After much tinkering we still have performance problems with higher
thread counts in the FFT benchmark. Upward from 4/5 threads the
performance gains start to saturate (before removing the top level
locks we even saw a slight drop in performance).

Currently the FFT benchmark shows the following results (average):

<img src="media/18b2d744_fft_average.png" width="600"/>

We want to positively note that the overall trend of 'performance drops'
at the hyperthreading mark is not really bad anymore, it rather
seems similar to EMBB now (with backoff + lockfree deque + top level
reader-writers lock).

This is discouraging after many tests. To see where the overhead lies
we also implemented the unbalanced tree search benchmark,
resulting in the following, suprisingly good, results (average):

<img src="media/18b2d744_unbalanced_average.png" width="600"/>

The main difference between the two benchmarks is, that the second
one has more work and the work is relatively independent.
Additionaly, the first one uses our high level API (parallel invoke),
while the second one uses our low level API.
It is worth investigating if either or high level API or the structure
of the memory access in FFT are the problem.