tree 62898733e9e9123896788aaa020486c2abdf5ff4
parent 3e6388c561e0b1aa3f2fbb2a996f2b81004db6bb
author Roman Lebedev <lebedev.ri@gmail.com> 1550654044 +0000
committer Roman Lebedev <lebedev.ri@gmail.com> 1550654044 +0000

[llvm-exegesis] Opcode stabilization / reclusterization (PR40715)

Summary:
Given an instruction `Opcode`, we can make benchmarks (measurements) of the
instruction characteristics/performance. Then, to facilitate further analysis
we group the benchmarks with *similar* characteristics into clusters.
Now, this is all not entirely deterministic. Some instructions have variable
characteristics, depending on their arguments. And thus, if we do several
benchmarks of the same instruction `Opcode`, we may end up with *different*
performance characteristics measurements. And when we then do clustering,
these several benchmarks of the same instruction `Opcode` may end up being
clustered into *different* clusters. This is not great for further analysis.

We shall find every `Opcode` with benchmarks not in just one cluster, and move
*all* the benchmarks of said `Opcode` into one new unstable cluster per `Opcode`.

I have solved this by making `ClusterId` a bit field, adding a `IsUnstable` bit,
and introducing `-analysis-display-unstable-clusters` switch to toggle between
displaying stable-only clusters and unstable-only clusters.

The reclusterization is deterministically stable, produces identical reports
between runs. (Or at least that is what i'm seeing, maybe it isn't)

Timings/comparisons:
old (current trunk/head) {F8303582}
```
$ perf stat -r 25 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-old.html
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-old.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-old.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-old.html' (25 runs):

           6624.73 msec task-clock                #    0.999 CPUs utilized            ( +-  0.53% )
               172      context-switches          #   25.965 M/sec                    ( +- 29.89% )
                 0      cpu-migrations            #    0.042 M/sec                    ( +- 56.54% )
             31073      page-faults               # 4690.754 M/sec                    ( +-  0.08% )
       26538711696      cycles                    # 4006230.292 GHz                   ( +-  0.53% )  (83.31%)
        2017496807      stalled-cycles-frontend   #    7.60% frontend cycles idle     ( +-  0.93% )  (83.32%)
       13403650062      stalled-cycles-backend    #   50.51% backend cycles idle      ( +-  0.33% )  (33.37%)
       19770706799      instructions              #    0.74  insn per cycle
                                                  #    0.68  stalled cycles per insn  ( +-  0.04% )  (50.04%)
        4419821812      branches                  # 667207369.714 M/sec               ( +-  0.03% )  (66.69%)
         121741669      branch-misses             #    2.75% of all branches          ( +-  0.28% )  (83.34%)

            6.6283 +- 0.0358 seconds time elapsed  ( +-  0.54% )
```

patch, with reclustering but without filtering (i.e. outputting all the stable *and* unstable clusters) {F8303586}
```
$ perf stat -r 25 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-new-all.html
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new-all.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new-all.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-new-all.html' (25 runs):

           6475.29 msec task-clock                #    0.999 CPUs utilized            ( +-  0.31% )
               213      context-switches          #   32.952 M/sec                    ( +- 23.81% )
                 1      cpu-migrations            #    0.130 M/sec                    ( +- 43.84% )
             31287      page-faults               # 4832.057 M/sec                    ( +-  0.08% )
       25939086577      cycles                    # 4006160.279 GHz                   ( +-  0.31% )  (83.31%)
        1958812858      stalled-cycles-frontend   #    7.55% frontend cycles idle     ( +-  0.68% )  (83.32%)
       13218961512      stalled-cycles-backend    #   50.96% backend cycles idle      ( +-  0.29% )  (33.37%)
       19752995402      instructions              #    0.76  insn per cycle
                                                  #    0.67  stalled cycles per insn  ( +-  0.04% )  (50.04%)
        4417079244      branches                  # 682195472.305 M/sec               ( +-  0.03% )  (66.70%)
         121510065      branch-misses             #    2.75% of all branches          ( +-  0.19% )  (83.34%)

            6.4832 +- 0.0229 seconds time elapsed  ( +-  0.35% )
```
Funnily, *this* measurement shows that said reclustering actually improved performance.

patch, with reclustering, only the stable clusters {F8303594}
```
$ perf stat -r 25 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-new-stable.html
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new-stable.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new-stable.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-new-stable.html' (25 runs):

           6387.71 msec task-clock                #    0.999 CPUs utilized            ( +-  0.13% )
               133      context-switches          #   20.792 M/sec                    ( +- 23.39% )
                 0      cpu-migrations            #    0.063 M/sec                    ( +- 61.24% )
             31318      page-faults               # 4903.256 M/sec                    ( +-  0.08% )
       25591984967      cycles                    # 4006786.266 GHz                   ( +-  0.13% )  (83.31%)
        1881234904      stalled-cycles-frontend   #    7.35% frontend cycles idle     ( +-  0.25% )  (83.33%)
       13209749965      stalled-cycles-backend    #   51.62% backend cycles idle      ( +-  0.16% )  (33.36%)
       19767554347      instructions              #    0.77  insn per cycle
                                                  #    0.67  stalled cycles per insn  ( +-  0.04% )  (50.03%)
        4417480305      branches                  # 691618858.046 M/sec               ( +-  0.03% )  (66.68%)
         118676358      branch-misses             #    2.69% of all branches          ( +-  0.07% )  (83.33%)

            6.3954 +- 0.0118 seconds time elapsed  ( +-  0.18% )
```
Performance improved even further?! Makes sense i guess, less clusters to print.

patch, with reclustering, only the unstable clusters {F8303601}
```
$ perf stat -r 25 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-new-unstable.html -analysis-display-unstable-clusters
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new-unstable.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 43970 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-new-unstable.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=0.5 -benchmarks-file=/home/lebedevri/PileDriver-Sched/benchmarks-inverse_throughput.yaml -analysis-inconsistencies-output-file=/tmp/clusters-new-unstable.html -analysis-display-unstable-clusters' (25 runs):

           6124.96 msec task-clock                #    1.000 CPUs utilized            ( +-  0.20% )
               194      context-switches          #   31.709 M/sec                    ( +- 20.46% )
                 0      cpu-migrations            #    0.039 M/sec                    ( +- 49.77% )
             31413      page-faults               # 5129.261 M/sec                    ( +-  0.06% )
       24536794267      cycles                    # 4006425.858 GHz                   ( +-  0.19% )  (83.31%)
        1676085087      stalled-cycles-frontend   #    6.83% frontend cycles idle     ( +-  0.46% )  (83.32%)
       13035595603      stalled-cycles-backend    #   53.13% backend cycles idle      ( +-  0.16% )  (33.36%)
       18260877653      instructions              #    0.74  insn per cycle
                                                  #    0.71  stalled cycles per insn  ( +-  0.05% )  (50.03%)
        4112411983      branches                  # 671484364.603 M/sec               ( +-  0.03% )  (66.68%)
         114066929      branch-misses             #    2.77% of all branches          ( +-  0.11% )  (83.32%)

            6.1278 +- 0.0121 seconds time elapsed  ( +-  0.20% )
```
This tells us that the actual `-analysis-inconsistencies-output-file=` outputting only takes ~0.4 sec for 43970 benchmark points (3 whole sweeps)
(Also, wow this is fast, it used to take several minutes originally)

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=40715 | PR40715 ]].

Reviewers: courbet, gchatelet

Reviewed By: courbet

Subscribers: tschuett, jdoerfert, llvm-commits, RKSimon

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58355

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354441 91177308-0d34-0410-b5e6-96231b3b80d8