benchmarks for openmp parallel for skip flag

[jangorecki]

Towards #7371

this code assumes you have 8+ threads

cc()
do = function(what) {
  n = 1e8
  rbindlist(lapply(c("1e8+1"=1e8+1, "1e7"=1e7, "10"=10), function(half) {
    rbindlist(lapply(c("nothing"=1L,"volatile"=2L,"volatile+shared"=3L,"atomic write"=4L,"atomic read write"=5L, "reduction"=6L, "cancellation"=7L), function(variant) {
      a2<-system.time(
        a1<-omp_flags(variant, n, half, 8)
      )
      if (what == "time") as.list(a2)
      else if (what == "iters") as.list(a1)
    }), idcol="variant")
  }), idcol="halt")
}
do("iters")
do("time")

      halt           variant       V1       V2       V3       V4       V5       V6       V7       V8
    <char>            <char>    <int>    <int>    <int>    <int>    <int>    <int>    <int>    <int>
 1:  1e8+1           nothing 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 2:  1e8+1          volatile 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 3:  1e8+1   volatile+shared 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 4:  1e8+1      atomic write 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 5:  1e8+1 atomic read write 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 6:  1e8+1         reduction 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 7:  1e8+1      cancellation 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 8:    1e7           nothing 10000000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 9:    1e7          volatile 10000000  9368188  2219459  2374732  2843583  2794889  2256903  2389781
10:    1e7   volatile+shared 10000000  9561233  2291372  2512503  3533221  3080863  2444200  2472036
11:    1e7      atomic write 10000000  9732664  2249224  2339365  2973864  2526774  2218488  2205693
12:    1e7 atomic read write 10000000 10005487  1710672  1566658  1930782  1774714  1555453  1497075
13:    1e7         reduction 10000000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
14:    1e7      cancellation 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
15:     10           nothing       10        0        0        0        0        0        0        0
16:     10          volatile       10        0        0        0        0        0        0        0
17:     10   volatile+shared       10        0        0        0        0        0        0        0
18:     10      atomic write       10        0        0        0        0        0        0        0
19:     10 atomic read write       10        0        0        0        0        0        0        0
20:     10         reduction       10 12500000 12500000 12500000 12500000 12500000 12500000 12500000
21:     10      cancellation 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
      halt           variant       V1       V2       V3       V4       V5       V6       V7       V8
    <char>            <char>    <int>    <int>    <int>    <int>    <int>    <int>    <int>    <int>
      halt           variant user.self sys.self elapsed user.child sys.child
    <char>            <char>     <num>    <num>   <num>      <num>     <num>
 1:  1e8+1           nothing     3.596    0.000   0.472      0.002     0.002
 2:  1e8+1          volatile     3.588    0.001   0.481      0.000     0.005
 3:  1e8+1   volatile+shared     3.529    0.001   0.477      0.001     0.003
 4:  1e8+1      atomic write     3.689    0.001   0.494      0.002     0.003
 5:  1e8+1 atomic read write     3.668    0.000   0.493      0.001     0.004
 6:  1e8+1         reduction     3.493    0.000   0.467      0.000     0.004
 7:  1e8+1      cancellation     4.284    0.001   0.572      0.000     0.005
 8:    1e7           nothing     3.460    0.000   0.461      0.001     0.003
 9:    1e7          volatile     2.852    0.000   0.363      0.001     0.004
10:    1e7   volatile+shared     2.654    0.000   0.335      0.002     0.003
11:    1e7      atomic write     2.608    0.000   0.330      0.002     0.002
12:    1e7 atomic read write     2.534    0.000   0.320      0.002     0.003
13:    1e7         reduction     3.279    0.001   0.446      0.000     0.005
14:    1e7      cancellation     4.361    0.001   0.581      0.000     0.004
15:     10           nothing     0.001    0.000   0.004      0.000     0.006
16:     10          volatile     0.045    0.000   0.010      0.002     0.002
17:     10   volatile+shared     0.038    0.000   0.009      0.000     0.005
18:     10      atomic write     0.000    0.000   0.004      0.000     0.005
19:     10 atomic read write     0.044    0.000   0.010      0.002     0.003
20:     10         reduction     2.520    0.000   0.382      0.000     0.004
21:     10      cancellation     4.296    0.002   0.576      0.000     0.005
      halt           variant user.self sys.self elapsed user.child sys.child
    <char>            <char>     <num>    <num>   <num>      <num>     <num>

btw. I read volatile should not be used in favor of atomic

[jangorecki]

if atomic write alone would do then seems best fit

[codecov]

Codecov Report

❌ Patch coverage is 0% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.90%. Comparing base (55b0de6) to head (e023ba3).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/omp-flags.c	0.00%	29 Missing ⚠️
R/utils.R	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7372      +/-   ##
==========================================
- Coverage   99.11%   98.90%   -0.22%     
==========================================
  Files          85       86       +1     
  Lines       16443    16479      +36     
==========================================
  Hits        16298    16298              
- Misses        145      181      +36     

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[jangorecki]

@ben-schwen it sems that new options are not much of use, maybe it is a compiler issue? I am on recent gcc, omp 201511.
And I observed warning:

omp-flags.c: In function ‘benchmark_omp_flag’:
omp-flags.c:95:17: warning: ‘cancel for’ inside ‘nowait’ for construct [-Wopenmp]
   95 |         #pragma omp cancel for
      |                 ^~~

[ben-schwen]

Interesting. With 20 threads I get this (which was my main motivation to include the reduction). I have gcc 11.4.0 and openmp 201511

      halt           variant       V1       V2       V3       V4       V5       V6       V7       V8
    <char>            <char>    <int>    <int>    <int>    <int>    <int>    <int>    <int>    <int>
 1:  1e8+1           nothing 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 2:  1e8+1          volatile 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 3:  1e8+1   volatile+shared 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 4:  1e8+1      atomic write 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 5:  1e8+1 atomic read write 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 6:  1e8+1         reduction 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 7:  1e8+1      cancellation 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 8:    1e7           nothing 10000000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
 9:    1e7          volatile 10000000  7523756  6139470 10163068  5543296  9819239 10011390 10223716
10:    1e7   volatile+shared 10000000  9721537 10062042  2737256 10091377  2760686  9725714  9918532
11:    1e7      atomic write 10000000 10143687  9958940 10128160  5549333  5473646  9742693 10022550
12:    1e7 atomic read write 10000000  9916360  9925820  2955716 10075786 10073474  9892110  2933117
13:    1e7         reduction 10000000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
14:    1e7      cancellation 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
15:     10           nothing       10        0        0 12500000        0 12500000        0        0
16:     10          volatile       10        0        0        0    83056        0        0    74028
17:     10   volatile+shared       10    46021    19186    55439        0    28853    75632        0
18:     10      atomic write       10    90927    14187        0    95527    36864        0   142229
19:     10 atomic read write       10        0    69753   129379        0        0    90041        0
20:     10         reduction       10 12500000 12500000 12500000 12500000 12500000 12500000 12500000
21:     10      cancellation 12500000 12500000 12500000 12500000 12500000 12500000 12500000 12500000
      halt           variant       V1       V2       V3       V4       V5       V6       V7       V8
    <char>            <char>    <int>    <int>    <int>    <int>    <int>    <int>    <int>    <int>
      halt           variant user.self sys.self elapsed user.child sys.child
    <char>            <char>     <num>    <num>   <num>      <num>     <num>
 1:  1e8+1           nothing     0.125    0.000   0.017      0.002     0.001
 2:  1e8+1          volatile     0.055    0.000   0.012      0.002     0.000
 3:  1e8+1   volatile+shared     0.080    0.000   0.014      0.003     0.000
 4:  1e8+1      atomic write     0.047    0.000   0.010      0.002     0.000
 5:  1e8+1 atomic read write     0.080    0.000   0.014      0.002     0.000
 6:  1e8+1         reduction     0.033    0.000   0.009      0.002     0.001
 7:  1e8+1      cancellation     0.964    0.000   0.126      0.002     0.000
 8:    1e7           nothing     0.070    0.000   0.011      0.003     0.000
 9:    1e7          volatile     0.053    0.000   0.012      0.002     0.000
10:    1e7   volatile+shared     0.057    0.000   0.011      0.002     0.000
11:    1e7      atomic write     0.015    0.004   0.007      0.003     0.000
12:    1e7 atomic read write     0.098    0.000   0.014      0.001     0.001
13:    1e7         reduction     0.062    0.000   0.011      0.002     0.000
14:    1e7      cancellation     1.012    0.000   0.123      0.002     0.001
15:     10           nothing     0.031    0.000   0.008      0.002     0.001
16:     10          volatile     0.069    0.000   0.010      0.002     0.000
17:     10   volatile+shared     0.054    0.000   0.010      0.002     0.000
18:     10      atomic write     0.001    0.000   0.003      0.001     0.002
19:     10 atomic read write     0.085    0.000   0.012      0.003     0.000
20:     10         reduction     0.031    0.000   0.008      0.001     0.002
21:     10      cancellation     1.061    0.000   0.152      0.002     0.000
      halt           variant user.self sys.self elapsed user.child sys.child
    <char>            <char>     <num>    <num>   <num>      <num>     <num>

[jangorecki]

@ben-schwen can you share also iterations made by each thread?
It is quite surprising that your code runs so much faster...

[ben-schwen]

@jangorecki I have added the iterations above. I have an 13th Gen Intel(R) Core(TM) i7-1370P CPU which can apparently clock up to 5.2 GHz which might be the reason why my times are flying

[jangorecki]

I think we can close this PR, changes according to this benchmark has been made in #7376 and #7361