Problems with default UnrollFactor in V0.11.0

[Tornhoof]

Description
I just upgraded SpanJson to v0.11 of Benchmark.NET and the new UnrollFactor logic does not work for codegen-heavy benchmarks.
Actual Behaviour

// Benchmark: ModelBenchmark.DeserializeAccessTokenWithJilSerializer: DefaultJob
// *** Execute ***
// Launch: 1 / 1
// BeforeAnythingElse
// Benchmark Process Environment Information:
// Runtime=.NET Core 2.1.1 (CoreCLR 4.6.26606.02, CoreFX 4.6.26606.05), 64bit RyuJIT
// GC=Concurrent Workstation
// Job: DefaultJob
OverheadJitting  1: 1 op, 327424.34 ns, 327.4243 us/op
WorkloadJitting  1: 1 op, 2934831805.27 ns, 2.9348 s/op
WorkloadWarmup   1: 1 op, 16384.02 ns, 16.3840 us/op
WorkloadWarmup   2: 1 op, 10496.01 ns, 10.4960 us/op
WorkloadWarmup   3: 1 op, 9728.01 ns, 9.7280 us/op
WorkloadWarmup   4: 1 op, 9216.01 ns, 9.2160 us/op
WorkloadWarmup   5: 1 op, 8960.01 ns, 8.9600 us/op
WorkloadWarmup   6: 1 op, 23040.02 ns, 23.0400 us/op
WorkloadWarmup   7: 1 op, 7936.01 ns, 7.9360 us/op
WorkloadWarmup   8: 1 op, 8704.01 ns, 8.7040 us/op
WorkloadWarmup   9: 1 op, 9472.01 ns, 9.4720 us/op
WorkloadWarmup  10: 1 op, 8448.01 ns, 8.4480 us/op
// BeforeActualRun
WorkloadActual   1: 1 op, 10240.01 ns, 10.2400 us/op
WorkloadActual   2: 1 op, 25088.03 ns, 25.0880 us/op

The workloadActual lines then repeat around 90 times with quite a large variation in timings.
Expected behaviour
I get a larger amount of ops with a more consistent timing, the 1 op benchmark is wrong on a large large scale.

// **************************
// Benchmark: ModelBenchmark.DeserializeAccessTokenWithJilSerializer: Job-GXPFMQ(UnrollFactor=2)
// *** Execute ***
// Launch: 1 / 1
// BeforeAnythingElse
// Benchmark Process Environment Information:
// Runtime=.NET Core 2.1.1 (CoreCLR 4.6.26606.02, CoreFX 4.6.26606.05), 64bit RyuJIT
// GC=Concurrent Workstation
// Job: Job-CZJWNW(UnrollFactor=2)
OverheadJitting  1: 2 op, 352000.36 ns, 176.0002 us/op
WorkloadJitting  1: 2 op, 2947211977.95 ns, 1.4736 s/op
WorkloadPilot    1: 4 op, 13568.01 ns, 3.3920 us/op
WorkloadPilot    2: 8 op, 14848.02 ns, 1.8560 us/op
WorkloadPilot    3: 16 op, 15872.02 ns, 992.0010 ns/op
WorkloadPilot    4: 32 op, 25344.03 ns, 792.0008 ns/op
WorkloadPilot    5: 64 op, 44288.05 ns, 692.0007 ns/op
WorkloadPilot    6: 128 op, 84480.09 ns, 660.0007 ns/op
WorkloadPilot    7: 256 op, 152576.16 ns, 596.0006 ns/op
WorkloadPilot    8: 512 op, 300032.31 ns, 586.0006 ns/op
WorkloadPilot    9: 1024 op, 574208.59 ns, 560.7506 ns/op
WorkloadPilot   10: 2048 op, 1207041.24 ns, 589.3756 ns/op
WorkloadPilot   11: 4096 op, 2299650.35 ns, 561.4381 ns/op
WorkloadPilot   12: 8192 op, 4912389.03 ns, 599.6569 ns/op
WorkloadPilot   13: 16384 op, 9578249.81 ns, 584.6100 ns/op
WorkloadPilot   14: 32768 op, 18317842.76 ns, 559.0162 ns/op
WorkloadPilot   15: 65536 op, 36165925.03 ns, 551.8482 ns/op
WorkloadPilot   16: 131072 op, 72334922.07 ns, 551.8717 ns/op
WorkloadPilot   17: 262144 op, 143300498.74 ns, 546.6480 ns/op
WorkloadPilot   18: 524288 op, 285817892.68 ns, 545.1544 ns/op
WorkloadPilot   19: 1048576 op, 572262474.00 ns, 545.7520 ns/op
OverheadWarmup   1: 1048576 op, 3304963.38 ns, 3.1519 ns/op
OverheadWarmup   2: 1048576 op, 3340035.42 ns, 3.1853 ns/op
OverheadWarmup   3: 1048576 op, 3255811.33 ns, 3.1050 ns/op
OverheadWarmup   4: 1048576 op, 3256067.33 ns, 3.1052 ns/op
OverheadWarmup   5: 1048576 op, 3302403.38 ns, 3.1494 ns/op
OverheadWarmup   6: 1048576 op, 3288835.37 ns, 3.1365 ns/op
OverheadActual   1: 1048576 op, 3316483.40 ns, 3.1628 ns/op
OverheadActual   2: 1048576 op, 3276291.35 ns, 3.1245 ns/op
OverheadActual   3: 1048576 op, 3233283.31 ns, 3.0835 ns/op
OverheadActual   4: 1048576 op, 3339267.42 ns, 3.1846 ns/op
OverheadActual   5: 1048576 op, 3338755.42 ns, 3.1841 ns/op
OverheadActual   6: 1048576 op, 3284739.36 ns, 3.1326 ns/op
OverheadActual   7: 1048576 op, 3252483.33 ns, 3.1018 ns/op
OverheadActual   8: 1048576 op, 3229443.31 ns, 3.0798 ns/op
OverheadActual   9: 1048576 op, 3227139.30 ns, 3.0776 ns/op
OverheadActual  10: 1048576 op, 3257347.34 ns, 3.1064 ns/op
OverheadActual  11: 1048576 op, 3340035.42 ns, 3.1853 ns/op
OverheadActual  12: 1048576 op, 3311619.39 ns, 3.1582 ns/op
OverheadActual  13: 1048576 op, 3286019.36 ns, 3.1338 ns/op
OverheadActual  14: 1048576 op, 3352835.43 ns, 3.1975 ns/op
OverheadActual  15: 1048576 op, 3325187.40 ns, 3.1711 ns/op
WorkloadWarmup   1: 1048576 op, 571615561.33 ns, 545.1351 ns/op
WorkloadWarmup   2: 1048576 op, 571237192.95 ns, 544.7742 ns/op
WorkloadWarmup   3: 1048576 op, 572946506.70 ns, 546.4044 ns/op
WorkloadWarmup   4: 1048576 op, 571277128.99 ns, 544.8123 ns/op
WorkloadWarmup   5: 1048576 op, 571070280.78 ns, 544.6151 ns/op
WorkloadWarmup   6: 1048576 op, 571714377.44 ns, 545.2293 ns/op
WorkloadWarmup   7: 1048576 op, 571312713.02 ns, 544.8463 ns/op
// BeforeActualRun
WorkloadActual   1: 1048576 op, 571627337.35 ns, 545.1463 ns/op
WorkloadActual   2: 1048576 op, 571613257.33 ns, 545.1329 ns/op

I think that's bececause the WorkloadJitting is taking that long (yeah it's codegen heavy), as soon as I change the Unrollfactor to 2 it works as expected (atleast as far as I've run the benchmarks).
The benchmark is that codegen heavy, because I initialize ~480 serializers statically in that class (the whole benchmark class is one large t4 script, see https://github.com/Tornhoof/SpanJson/blob/master/SpanJson.Benchmarks/ModelBenchmark.cs)

Repro

1. Checkout Branch BenchmarkUpdates on https://github.com/Tornhoof/SpanJson
2. cd SpanJson.Benchmarks
3. dotnet run -c Release and select 2

[AndreyAkinshin]

@Tornhoof, thank you very much for the report.

You correctly described the problem: BenchmarkDotNet ignores the Pilot stage (where we should choose the best amount of invocation) because WorkloadJitting takes too much time. We did this "optimization" for heavy macrobenchmark, but obviously it doesn't work for your case.

I think we should always run the Pilot stage if the jitting (cold start) took a reasonable amount of time (e.g., < 20 sec). It should automatically fix the invocation count and solve your problem. Currently, we compare the WorkloadJitting time with engineParameters.IterationTime which is 500 ms by default. Probably it makes sense to introduce additional TimeInterval option for the WorkloadJitting threshold value.
We should discuss it with @adamsitnik, but he will be available only next week.

As a temporary workaround, you can manually set the invocation count or increase the IterationTime value.

[Tornhoof]

Thank you for the workaround.

[adamsitnik]

@Tornhoof big thanks for the bug report and a clear repro case!

I will try to find some solution and fix it during the next weekend

[AndreyAkinshin]

@adamsitnik do you have any progress with this issue?
I have another idea about how to solve the problem without additional settings. Let's check that we do enough operations during the WorkloadWarmup stage. If it turns out that the number of operations is not enough and our WorkloadJitting heuristics produced a wrong hypothesis about the warmed operation duration, we should restart iterations from the Pilot stage.
What do you think?

[adamsitnik]

@AndreyAkinshin Great idea!

So far I have observed that this issue happens only for benchmarks that rely heavily on Reflection.ILEmit and my only idea was to check if the benchmark relies on this assembly and if it does, run it one more time. But I think that your solution is much more universal and clean!

[AndreyAkinshin]

I have observed that this issue happens only for benchmarks that rely heavily on Reflection.ILEmit

In fact, there are too many factors that can slow down the first iteration (from heavy assembly loading to custom user logic).

[adamsitnik]

In fact, there are too many factors that can slow down the first iteration (from heavy assembly loading to custom user logic).

Totally agree! Would you like to implement it?

[AndreyAkinshin]

Would you like to implement it?

Yes.