Fix firstBucketSize for CPU histogram

[glemsom]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Adjust firstBucketSize for CPU histograms to support less than 10m buckets.

Which issue(s) this PR fixes:

Fixes #6415

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

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[voelzmo]

Hey @glemsom thanks for the PR!
There has been an approach to change this some time ago, and I think this comment by @mwielgus still applies: this is definitely something we want to roll out with care.

A change in CheckpointVersion means that everyone using checkpoints as a history provider will lose their entire history, so wdyt @raywainman, @kwiesmueller, @adrianmoisey , should we be making this conditionally enabled behind a feature flag?

I'm kind of on the edge here: for CPU, I don't consider history samples to be very useful, the confidence for scaling up (which is the most critical concern) increases pretty quickly, such that this shouldn't cause big problems. For memory, we only record one peak per half-life period, so this is definitely a bigger impact.

[voelzmo]

/ok-to-test

[adrianmoisey]

A change in CheckpointVersion means that everyone using checkpoints as a history provider will lose their entire history, so wdyt @raywainman, @kwiesmueller, @adrianmoisey , should we be making this conditionally enabled behind a feature flag?

🤔
Yeah, I agree that a change like this needs to be handled with care.

I'm wondering if there's a way to give users a clean upgrade path.

[raywainman]

Right, the buckets won't cleanly carryover from v3 to v4 here and cause users to silently lose history which could cause VPA to go a bit wonky.

Putting this behind a flag seems like a good first step... If we wanted to change this to the default then we'd need to have some sort of "migration" code.

[adrianmoisey]

Putting this behind a flag seems like a good first step... If we wanted to change this to the default then we'd need to have some sort of "migration" code.

What's slightly annoying is that the VerticalPodAutoscalerCheckpoint object itself doesn't seem to be able to handle multiple versions at once. ie: there is no list or similar data structure containing each version.

So I guess we would need a live migration. If the VPA was set to use v4, and a v3 checkpoint existed, it would need to read that checkpoint, migrate and write to v4.

May be another option is maintaining two code paths for a few releases. New checkpoints are on v4, but the recommender supports v3 and v4.
I haven't looked at the code yet to see how viable this is.

[glemsom]

Right, the buckets won't cleanly carryover from v3 to v4 here and cause users to silently lose history which could cause VPA to go a bit wonky.

Putting this behind a flag seems like a good first step... If we wanted to change this to the default then we'd need to have some sort of "migration" code.

I will have a look at putting this behind a flag, and having the 10m as the default - if this is an acceptable way to go for now?

[adrianmoisey]

I will have a look at putting this behind a flag, and having the 10m as the default - if this is an acceptable way to go for now?

From my side, I'd prefer a plan on what to do after the feature flag has been added.
How do we migrate to the new value and eventually remove the feature flag?

[raywainman]

Wonder if you could do this "live" migration when reading the v3 weights here:

autoscaler/vertical-pod-autoscaler/pkg/recommender/util/histogram.go

Line 249 in 37b3da4

func (h *histogram) LoadFromCheckpoint(checkpoint *vpa_types.HistogramCheckpoint) error {

Then subsequent checkpointing will be done using v4 and all should be good from there.

[voelzmo]

Wonder if you could do this "live" migration when reading the v3 weights here

We could iterate over the old checkpoint and re-add all samples of buckets[i] to the new checkpoint with the sample value of start(bucket[i+1]) – we don't know exactly which value the samples were originally added with.

Looking at what this change would do to the bucket boundaries, there would be some movement, leading to pretty much everything getting new recommendations (because the bucket boundaries have changed). See the example prints below, with start bucket values in millicores.

Before this change:

bucket 1: 10.0000
bucket 2: 20.5000
bucket 3: 31.5250
bucket 4: 43.1013
bucket 5: 55.2563
bucket 6: 68.0191
bucket 7: 81.4201
bucket 8: 95.4911
bucket 9: 110.2656
bucket 10: 125.7789
bucket 11: 142.0679
bucket 12: 159.1713
bucket 13: 177.1298
bucket 14: 195.9863
bucket 15: 215.7856
bucket 16: 236.5749
bucket 17: 258.4037
bucket 18: 281.3238
bucket 19: 305.3900
bucket 20: 330.6595
bucket 21: 357.1925
bucket 22: 385.0521
bucket 23: 414.3048
bucket 24: 445.0200
bucket 25: 477.2710
bucket 26: 511.1345
bucket 27: 546.6913
bucket 28: 584.0258
bucket 29: 623.2271
bucket 30: 664.3885
bucket 31: 707.6079
bucket 32: 752.9883
bucket 33: 800.6377
bucket 34: 850.6696
bucket 35: 903.2031
bucket 36: 958.3632
bucket 37: 1016.2814
bucket 38: 1077.0955
bucket 39: 1140.9502
bucket 40: 1207.9977
bucket 41: 1278.3976
bucket 42: 1352.3175
bucket 43: 1429.9334
bucket 44: 1511.4301
bucket 45: 1597.0016
bucket 46: 1686.8516
bucket 47: 1781.1942
bucket 48: 1880.2539
bucket 49: 1984.2666
bucket 50: 2093.4800
bucket 51: 2208.1540
bucket 52: 2328.5617
bucket 53: 2454.9897
bucket 54: 2587.7392
bucket 55: 2727.1262
bucket 56: 2873.4825
bucket 57: 3027.1566
bucket 58: 3188.5144
bucket 59: 3357.9402
bucket 60: 3535.8372
bucket 61: 3722.6290
bucket 62: 3918.7605
bucket 63: 4124.6985
bucket 64: 4340.9334
bucket 65: 4567.9801
bucket 66: 4806.3791
bucket 67: 5056.6981
bucket 68: 5319.5330
bucket 69: 5595.5096
bucket 70: 5885.2851
bucket 71: 6189.5494
bucket 72: 6509.0268
bucket 73: 6844.4782
bucket 74: 7196.7021
bucket 75: 7566.5372
bucket 76: 7954.8640
bucket 77: 8362.6072
bucket 78: 8790.7376
bucket 79: 9240.2745
bucket 80: 9712.2882
bucket 81: 10207.9026
bucket 82: 10728.2978
bucket 83: 11274.7126
bucket 84: 11848.4483
bucket 85: 12450.8707
bucket 86: 13083.4142
bucket 87: 13747.5849
bucket 88: 14444.9642
bucket 89: 15177.2124
bucket 90: 15946.0730
bucket 91: 16753.3767
bucket 92: 17601.0455
bucket 93: 18491.0978
bucket 94: 19425.6527
bucket 95: 20406.9353
bucket 96: 21437.2821
bucket 97: 22519.1462
bucket 98: 23655.1035
bucket 99: 24847.8586
bucket 100: 26100.2516
bucket 101: 27415.2641
bucket 102: 28796.0274
bucket 103: 30245.8287
bucket 104: 31768.1202
bucket 105: 33366.5262
bucket 106: 35044.8525
bucket 107: 36807.0951
bucket 108: 38657.4499
bucket 109: 40600.3223
bucket 110: 42640.3385
bucket 111: 44782.3554
bucket 112: 47031.4732
bucket 113: 49393.0468
bucket 114: 51872.6992
bucket 115: 54476.3341
bucket 116: 57210.1508
bucket 117: 60080.6584
bucket 118: 63094.6913
bucket 119: 66259.4258
bucket 120: 69582.3971
bucket 121: 73071.5170
bucket 122: 76735.0928
bucket 123: 80581.8475
bucket 124: 84620.9399
bucket 125: 88861.9868
bucket 126: 93315.0862
bucket 127: 97990.8405
bucket 128: 102900.3825
bucket 129: 108055.4017
bucket 130: 113468.1717
bucket 131: 119151.5803
bucket 132: 125119.1593
bucket 133: 131385.1173
bucket 134: 137964.3732
bucket 135: 144872.5918
bucket 136: 152126.2214
bucket 137: 159742.5325
bucket 138: 167739.6591
bucket 139: 176136.6421
bucket 140: 184953.4742
bucket 141: 194211.1479
bucket 142: 203931.7053
bucket 143: 214138.2905
bucket 144: 224855.2051
bucket 145: 236107.9653
bucket 146: 247923.3636
bucket 147: 260329.5318
bucket 148: 273356.0084
bucket 149: 287033.8088
bucket 150: 301395.4992
bucket 151: 316475.2742
bucket 152: 332309.0379
bucket 153: 348934.4898
bucket 154: 366391.2143
bucket 155: 384720.7750
bucket 156: 403966.8137
bucket 157: 424175.1544
bucket 158: 445393.9121
bucket 159: 467673.6077
bucket 160: 491067.2881
bucket 161: 515630.6525
bucket 162: 541422.1852
bucket 163: 568503.2944
bucket 164: 596938.4591
bucket 165: 626795.3821
bucket 166: 658145.1512
bucket 167: 691062.4088
bucket 168: 725625.5292
bucket 169: 761916.8057
bucket 170: 800022.6459
bucket 171: 840033.7782
bucket 172: 882045.4671
bucket 173: 926157.7405
bucket 174: 972475.6275
bucket 175: 1021109.4089

After this change

bucket 1: 1.0000
bucket 2: 2.0500
bucket 3: 3.1525
bucket 4: 4.3101
bucket 5: 5.5256
bucket 6: 6.8019
bucket 7: 8.1420
bucket 8: 9.5491
bucket 9: 11.0266
bucket 10: 12.5779
bucket 11: 14.2068
bucket 12: 15.9171
bucket 13: 17.7130
bucket 14: 19.5986
bucket 15: 21.5786
bucket 16: 23.6575
bucket 17: 25.8404
bucket 18: 28.1324
bucket 19: 30.5390
bucket 20: 33.0660
bucket 21: 35.7193
bucket 22: 38.5052
bucket 23: 41.4305
bucket 24: 44.5020
bucket 25: 47.7271
bucket 26: 51.1135
bucket 27: 54.6691
bucket 28: 58.4026
bucket 29: 62.3227
bucket 30: 66.4388
bucket 31: 70.7608
bucket 32: 75.2988
bucket 33: 80.0638
bucket 34: 85.0670
bucket 35: 90.3203
bucket 36: 95.8363
bucket 37: 101.6281
bucket 38: 107.7095
bucket 39: 114.0950
bucket 40: 120.7998
bucket 41: 127.8398
bucket 42: 135.2318
bucket 43: 142.9933
bucket 44: 151.1430
bucket 45: 159.7002
bucket 46: 168.6852
bucket 47: 178.1194
bucket 48: 188.0254
bucket 49: 198.4267
bucket 50: 209.3480
bucket 51: 220.8154
bucket 52: 232.8562
bucket 53: 245.4990
bucket 54: 258.7739
bucket 55: 272.7126
bucket 56: 287.3482
bucket 57: 302.7157
bucket 58: 318.8514
bucket 59: 335.7940
bucket 60: 353.5837
bucket 61: 372.2629
bucket 62: 391.8760
bucket 63: 412.4699
bucket 64: 434.0933
bucket 65: 456.7980
bucket 66: 480.6379
bucket 67: 505.6698
bucket 68: 531.9533
bucket 69: 559.5510
bucket 70: 588.5285
bucket 71: 618.9549
bucket 72: 650.9027
bucket 73: 684.4478
bucket 74: 719.6702
bucket 75: 756.6537
bucket 76: 795.4864
bucket 77: 836.2607
bucket 78: 879.0738
bucket 79: 924.0274
bucket 80: 971.2288
bucket 81: 1020.7903
bucket 82: 1072.8298
bucket 83: 1127.4713
bucket 84: 1184.8448
bucket 85: 1245.0871
bucket 86: 1308.3414
bucket 87: 1374.7585
bucket 88: 1444.4964
bucket 89: 1517.7212
bucket 90: 1594.6073
bucket 91: 1675.3377
bucket 92: 1760.1045
bucket 93: 1849.1098
bucket 94: 1942.5653
bucket 95: 2040.6935
bucket 96: 2143.7282
bucket 97: 2251.9146
bucket 98: 2365.5103
bucket 99: 2484.7859
bucket 100: 2610.0252
bucket 101: 2741.5264
bucket 102: 2879.6027
bucket 103: 3024.5829
bucket 104: 3176.8120
bucket 105: 3336.6526
bucket 106: 3504.4852
bucket 107: 3680.7095
bucket 108: 3865.7450
bucket 109: 4060.0322
bucket 110: 4264.0338
bucket 111: 4478.2355
bucket 112: 4703.1473
bucket 113: 4939.3047
bucket 114: 5187.2699
bucket 115: 5447.6334
bucket 116: 5721.0151
bucket 117: 6008.0658
bucket 118: 6309.4691
bucket 119: 6625.9426
bucket 120: 6958.2397
bucket 121: 7307.1517
bucket 122: 7673.5093
bucket 123: 8058.1847
bucket 124: 8462.0940
bucket 125: 8886.1987
bucket 126: 9331.5086
bucket 127: 9799.0841
bucket 128: 10290.0383
bucket 129: 10805.5402
bucket 130: 11346.8172
bucket 131: 11915.1580
bucket 132: 12511.9159
bucket 133: 13138.5117
bucket 134: 13796.4373
bucket 135: 14487.2592
bucket 136: 15212.6221
bucket 137: 15974.2532
bucket 138: 16773.9659
bucket 139: 17613.6642
bucket 140: 18495.3474
bucket 141: 19421.1148
bucket 142: 20393.1705
bucket 143: 21413.8291
bucket 144: 22485.5205
bucket 145: 23610.7965
bucket 146: 24792.3364
bucket 147: 26032.9532
bucket 148: 27335.6008
bucket 149: 28703.3809
bucket 150: 30139.5499
bucket 151: 31647.5274
bucket 152: 33230.9038
bucket 153: 34893.4490
bucket 154: 36639.1214
bucket 155: 38472.0775
bucket 156: 40396.6814
bucket 157: 42417.5154
bucket 158: 44539.3912
bucket 159: 46767.3608
bucket 160: 49106.7288
bucket 161: 51563.0653
bucket 162: 54142.2185
bucket 163: 56850.3294
bucket 164: 59693.8459
bucket 165: 62679.5382
bucket 166: 65814.5151
bucket 167: 69106.2409
bucket 168: 72562.5529
bucket 169: 76191.6806
bucket 170: 80002.2646
bucket 171: 84003.3778
bucket 172: 88204.5467
bucket 173: 92615.7741
bucket 174: 97247.5628
bucket 175: 102110.9409
bucket 176: 107217.4879
bucket 177: 112579.3623
bucket 178: 118209.3304
bucket 179: 124120.7970
bucket 180: 130327.8368
bucket 181: 136845.2287
bucket 182: 143688.4901
bucket 183: 150873.9146
bucket 184: 158418.6103
bucket 185: 166340.5408
bucket 186: 174658.5679
bucket 187: 183392.4963
bucket 188: 192563.1211
bucket 189: 202192.2771
bucket 190: 212302.8910
bucket 191: 222919.0356
bucket 192: 234065.9873
bucket 193: 245770.2867
bucket 194: 258059.8010
bucket 195: 270963.7911
bucket 196: 284512.9806
bucket 197: 298739.6297
bucket 198: 313677.6112
bucket 199: 329362.4917
bucket 200: 345831.6163
bucket 201: 363124.1971
bucket 202: 381281.4070
bucket 203: 400346.4773
bucket 204: 420364.8012
bucket 205: 441384.0412
bucket 206: 463454.2433
bucket 207: 486627.9555
bucket 208: 510960.3533
bucket 209: 536509.3709
bucket 210: 563335.8395
bucket 211: 591503.6314
bucket 212: 621079.8130
bucket 213: 652134.8037
bucket 214: 684742.5438
bucket 215: 718980.6710
bucket 216: 754930.7046
bucket 217: 792678.2398
bucket 218: 832313.1518
bucket 219: 873929.8094
bucket 220: 917627.2999
bucket 221: 963509.6649
bucket 222: 1011686.1481

In the end, we'd probably rather do this and indicate to users that they can expect a massive eviction instead of hiding this behind a feature flag forever?