# @turbo on sets of operations

**URL:** https://discourse.julialang.org/t/turbo-on-sets-of-operations/66068
**Category:** Performance
**Created:** [August 9, 2021, 8:41am UTC](https://discourse.julialang.org/t/turbo-on-sets-of-operations/66068 "2021-08-09T08:41:26Z")
**Posts on this page:** 1
**Showing post:** 15

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### Author: ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [August 10, 2021, 2:34pm UTC](https://discourse.julialang.org/t/turbo-on-sets-of-operations/66068/15 "2021-08-10T14:34:52Z")

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The problem might be that the computer (where, I don’t know) is caching values (the result, intermediate values, I don’t know). Then you are not really measuring the time the function really takes to run. That is why restarting the function at every sample with a new set of values is safer.

I am not sure when exactly this kind of problem may arise in benchmarking. But it can arise even if you benchmark independent runs of compiled binaries in a computer, one after the other. A lot of work of putting things into memory, etc, may be saved by the OS.

This is very common, actually:

```julia
julia> @btime sin(5.0) # this is wrong
  1.537 ns (0 allocations: 0 bytes)
-0.9589242746631385

julia> x = 5.0

julia> @btime sin($x) # this is correct, I think
  7.776 ns (0 allocations: 0 bytes)
-0.9589242746631385

julia> @btime sin(x) setup=(x=rand()) evals=1 # this will vary the value of the input
  31.000 ns (0 allocations: 0 bytes)
0.6563352918810222

```

Also, here, there is the fact that computing the `sin` of one number has a different cost than taking the sin of another number, thus one needs to know exactly what one want’s to benchmark, considering the input that the function will take.

edit:

Actually:

```julia
julia> x = 5.0
5.0

julia> @btime sin($x)
  7.777 ns (0 allocations: 0 bytes)
-0.9589242746631385

julia> @btime sin($x) evals=1
  38.000 ns (0 allocations: 0 bytes)
-0.9589242746631385

```

I don’t know. Maybe we just should file an issue. Probably `evals=1` should be default.

I share the concerns… I started a new thread here: [How to benchmark properly? Should defaults change?](https://discourse.julialang.org/t/how-to-benchmark-properly-should-defaults-change/66138).

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