It can be tricky to evaluate the effect of invidual commits or pull requests on the speed of the OCaml compiler. In this blog post, I (Florian Angeletti) report my experience on measuring such impact with some degree of statistical significance.

The OCaml typechecker is an important piece of the OCaml compiler pipeline which accounts for a significant portion of time spent on compiling an OCaml program (see the appendices).

The code of the typechecker is also quite optimised, sometimes to the detriment of the readability of the code. Recently, Jacques Garrigue and Takafumi Saikawa have worked on a series of pull requests to improve the readability of the typechecker (#10337, #10474, #10541). Unfortunately, those improvements are also expected to increase the typechecking time of OCaml programs because they add abstraction barriers, and remove some optimisations that were breaking the abstraction barriers.

The effect is particularly pronounced on #10337. Due to the improvement of the readability of the typechecker, this pull request has been merged after some quick tests to check that the compilation time increase was not too dire.

However, the discussion on this pull request highlighted the fact that it was difficult to measure OCaml compilation time on a scale large enough to enable good statistical analysis and that it would be useful.

Consequently, I decided to try my hand at a statistical analysis of OCaml compilation time, using this pull request #10337 as a case study. Beyond this specific PR, I think that it is interesting to write down a process and a handful of tools for measuring OCaml compilation time on the opam ecosystem.

Before doing any kind of analysis, the first step is to find an easy way to collect the data of interest. Fortunately, the OCaml compiler can emit timing information with flag -dtimings. However, this information is emitted on stdout, whereas my ideal sampling process would be to just pick an opam package, launch a build process and recover the timing information for each file. This doesn’t work if the data is sent to the stdout, and never see again. This first step is thus to create a version of the OCaml compiler that can output the timing information of the compilation to a specific directory. With this change (#10575), installing an opam package with

OCAMLPARAM=",_,timings=1,dump-dir= /tmp/pkgnname" opam install pkgname

outputs all profiling information to /tmp/pkgname.

This makes it possible to collect large number of data points on compilation times by using opam, and the canonical installation process of each package without the need of much glue code.

For this case study, I am using 5 core packages containers, dune, tyxml, coq and base. Once their dependencies are added, I end up with

  • ocamlfind
  • num
  • zarith
  • seq
  • containers
  • coq
  • dune
  • re
  • ocamlbuild
  • uchar
  • topkg
  • uutf
  • tyxml
  • sexplib0
  • base

Then it is a matter of repeatedly installing those packages, and measuring the compilation times before and after #10337.

In order to get more reliable statistics on each file, each package was compiled 250 times leading to 1,6 millions of data points (available here) after slightly more than a week-end of computation.

In order to try to reduce the noise induced by the operating system scheduler, the compilation process is run with OPAMJOBS=1. Similarly, the compilation process was isolated as much as possible from the other process using the cset Linux utility to reserve one full physical core to the opam processes.

The code for collecting samples, analyzing them, and plotting the graphs below is available at

Comparing averages, files by files

With the data at hand, we can compute the average compilation by files, and by stage of the OCaml compiler pipeline. In our case, we are mostly interested in the typechecking stage, and global compilation time, since #10337 should only alter the time spent on typechecking. It is therefore useful to split the compilation time into typechecking + other=total. Then for each files in the 15 packages above, we can can compute the average time for each of those stages and the relative change of average compilation time: time after/time before.

Rendering those relative changes for the typechecking time, file by file (with the corresponding 90% confidence interval) yields

Relative change in average typechecking time by files

To avoid noise, I have removed files for which the average typechecking time was inferior to one microsecond on the reference version of the compiler.

In the graph above, there are few remarkable points:

  • As expected, the average typechecking time increased for almost all files
  • A significant portion of points are stuck to the line “after/before=1”. This means that for those files there was no changes at all of the typechecking times.
  • The standard deviation time varies wildly across packages. The typechecking of some dune files tend to have a very high variances. However outside of those files, the standard deviation seems moderate, and the mean estimator seem to have converged.
  • For a handful a files for which the typechecking time more than doubled. However the relative typechecking time does seem to be confined in the [1,1.2] range for a majority of files.

Since the data is quite noisy, it is useful before trying to interpret it to check that we are not looking only at noise. Fortunately, we have the data on the time spent outside of the typechecking stage available, and those times should be mostly noise. We have thus a baseline, that looks like

Relative change in average non-typechecking time by files

This cloud of points look indeed much noisier. More importantly, it seems centred around the line after/before=1. This means that our hypothesis that the compilation time outside of the typechecking stage has not been altered is not visibly invalidated by our data points. An other interesting point is that the high variance points seems to be shared between the typechecking and other graphs.

We can even check on the graphs for the average total compilation (file by file)

Relative change in average total time by files

that those points still have a high variance here. However, outside of this cluster of points, we have a quite more compact distribution of points for the total compilation time: it seems that we have a quite consistent increase of the total compilation time of around 3%.

And this is reflected in the averages:

Typechecking average Other average Total average
1.06641 1.01756 1.03307

There is thus an increase of around 6.6% of typechecking time which translates to an increase of 3.3% of total time. However, the non-typechecking time also increased by 1.7% in average. The average is thus either tainted by some structural bias or the relative variance (mean/ratio) is still enough for the distribution of the ratio to be ill-behaved (literature seems to indicate that a relative variance < 10% is required for the distribution of ratio to be Gaussian-like). Anyway, we probably cannot count on a precision of more than 1.7%. Even with this caveat, we still have a visible effect on the total compilation time.

We might better served by comparing the geometric average. Indeed, we are comparing ratio of time, with possibly a heavy-tailed noise. By using the geometric average (which compute the exponential of the arithmetic mean of the logarithms of our ratio), we can check that rare events don’t have an undue influence on the average. In our case the geometric means looks like

Typechecking geometric average Other geometric average Total geometric average
1.05963 1.01513 1.03215

All geometric averages have decreased compared to the arithmetic means, which is a sign that the compilation time distribution is skewed towards high compilation times. However, the changes are small and do not alter our previous interpretation.

We can somewhat refine those observations by looking at the medians (which are even less affected by the heavy-tailness of distributions)

Typechecking median Other median Total media
1.03834 1.00852 1.02507

Here, the non-typechecking times seems far less affected by the structural bias (with an increase of 0.9%) whereas the increase of typechecking time and total compilation time are reduced but still here at 3.8% and 2.5% respectively.

Comparing averages, quantiles

We can refine our analysis by looking at the quantiles of those relative changes of compilation time

Quantiles of the relative change of average typechecking time by files
% mean quantiles
1% 0.875097
10% 1
25% 1.001
50% 1.03834
75% 1.08826
90% 1.162
99% 1.51411
99.9% 2.76834

Here we see that the typechecking time of around 25% of files is simply not affected at all by the changes. And for half of the files, the compilation time is inferior to 9%. Contrarily, there is 1% of files for which the typechecking time increases by more than 50% (with outliers around 200%-400% increase).

However, looking at the total compilation does seems to reduce the overall impact of the change

Quantiles of the relative change in average total time by files
% total quantiles
1% 0.945555
10% 1
25% 1.00707
50% 1.02507
75% 1.05
90% 1.07895
99% 1.17846
99.9% 1.4379

Indeed, we still have 25% of files not impacted, but for 65% of files the relative increase of compilation time is less than 8%. (and the outliers stalls at a 50% increase)

We can also have a quick look at the quantiles for the non-typechecking time

Quantiles of the relative change in average non-typechecking time by files
% other quantiles
1% 0.855129
10% 0.956174
25% 0.995239
50% 1.00852
75% 1.03743
90% 1.08618
99% 1.25541
99.9% 1.67784

but here the only curiosity if that the curve is more symmetric and we have 25% of files for which the non-typechecking compilation time decrease randomly.

Noise models and minima

One issue with our previous analysis is the high variance which is observable in the non-typechecking average times across files. A possibility to mitigate this issue is to change our noise model. Using an average, we implicitly assumed that the compilation time was mostly:

observable_compilation_time = theoretical_computation_time + noise

where noise is a random variable with at least a finite variance and a mean of 0. Indeed, with this symmetry hypothesis the expectation of the observable computation time aligns with the theoretical compilation time:

E[observable_computation_time] = E[theoretical_computation_time] + E[noise] = theoretical_computation_time

and the variance Var[observable_computation_time] is exactly the variance of the noise Var[noise]. Then our finite variance hypothesis ensure that the empirical average hypothesis converges relatively well towards the theoretical expectation.

However, we can imagine another noise model with a multiplicative noise (due to CPU scheduling for instance),

observable_compilation_time = scheduling_noise * theoretical_computation_time + noise

with both scheduling_noise>1 and noise>1. With this model, the expectation of the observable compilation time does not match up with the theoretical computation time:

E[observable_computation_time] - theoretical_computation_time =
  (E[scheduling_noise]-1) * theoretical_computation_time + E[noise]

Thus, in this model, the average observable computation time is a structurally biased estimator for the theoretical computation time. This bias might be compensated by the fact that we are only looking to ratio. Nevertheless, this model also induces a second source of variance

Var[observable_computation_time] = theoretical_computation_time^2 Var[scheduling_noise] + Var[noise]

(/assuming the two noises are not correlated), and this variance increases with the theoretical computation time. This relative standard deviation might be problematic when computing ratio.

If this second noise model is closer to reality, using the empirical average estimators might be not ideal. However, the positivity of the noise opens another avenue for estimators: we can consider the minima of a series of independent realisations. Then, we have

min(observable_compilation_time) = min(scheduling_noise * theoretical_computation_time) + min(noise) = theoretical_computation_time

if the min(scheduling_noise)=1 and min(noise)=0.

This model has another advantage: by assuming that the (essential) support of the noise distribution has finite lower bound, we know that the empirical minima will converge towards a three-parameter Weibull distribution with a strictly positive support. (To be completely explicit, we also need to assume some regularity of the distribution around this lower bound too).

This means that the distribution ratio of the empirical minima will not exhibits the infinite moments of the ratio of two Gaussians. Without this issue, our estimator should have less variance.

However, we cannot use Gaussian confidence intervals for the empirical minima. Moreover, estimating the confidence interval for the Weibull distribution is more complex. Since we are mostly interested in corroborating our previous result, we are bypassing the computation of those confidence intervals.

Comparing minima

We can then restart out analysis using the minimal compilation time file-by-file. Starting with the minimal typechecking time, we get

Relative change in minimal typechecking time by files

There are notable differences with the average version: - a very significant part of our points takes the same time to typecheck before and after #10337 - there is a discretization effects going on: data points tend to fall on exactly the same value of the ratio

Beyond those changes, there is still a visible general increase of typechecking time.

The same differences are visible for the non typechecking compilation time Relative change in minimal non-typechecking time by files

and the total compilation time

Relative change in minimal total time by files

but overall the minimal total compilation and non-typechecking time mirrors what we had seen with the average. The distribution of the non-typechecking times is maybe more evenly centred around a ratio of 1.

We can have a look at the averages and median (across files) to have more global point of view

Typechecking Other Total
Average 1.06907 1.01031 1.02901
Geometric average 1.05998 1.00672 1.0276
Median 1 1 1

A striking change is that the median for the typechecking and total compilation time is equal to one: more than half of files are not affected by the changes when looking at the minimal compilation time. This might be an issue with the granularity of time measurement, or it could be a genuine fact.

More usefully, we still have an increase of average typechecking time between 6% and 6.9% depending on the averaging methods, which translates to a total compilation time increase between 2.7% and 3.3%. And this time, the increase of unrelated compilation time is between 0.7% to 0.9%. This seems to confirms that do have a real increase of average compilation time and 3% increase time is a reasonable number.

Comparing minima, quantiles

With the discretization, the quantiles of the compilation time are quite interesting and uncharacteristic.

For instance the typechecking quantiles,

Quantiles of the relative change in minimal typechecking time by files
% min quantiles
1% 1
10% 1
25% 1
50% 1
75% 1.07692
90% 1.2
99% 2
99.9% 2

are stuck to 1 between the first and 50th centile. In other words the minimal typechecking time of more than 50% of the files in our experiment is unchanged. For 40% of the files, the increase is less than 20%. And the most extreme files see only an increase of 100% of the typechecking time. On the higher quantiles, the presence of multiple jumps is the consequence of the discretization of ratio that was already visible on the raw data.

When looking at the time spent outside of typechecking,

Quantiles of the relative change in minimal non-typechecking time by files
% min_other quantiles
1% 0.8
10% 0.947368
25% 1
50% 1
75% 1
90% 1.11111
99% 1.33333
99.9% 1.6

we observe that the non-typechecking relation compilation time for more than 80% of file is unaffected by the change (or somehow accelerated for 10% of files).

The quantiles for the total compilation time, Quantiles of the relative change in minimal total time by files

% min_total quantiles
1% 0.92
10% 1
25% 1
50% 1
75% 1.04545
90% 1.1
99% 1.22727
99.9% 1.4

mostly reflects the trends set by the typechecking time: 55% of files are unaffected. For 90% of file the increase is less than 10%, and the maximal impact on the compilation time peaks at a 40% relative increase.


To sum up, with the available data at hands, it seems sensible to conclude that #10337 resulted in an average increase of compilation time of the order of 3%, while the average relative increase of typechecking time is around 6%. Moreover, for the most impacted files (at the ninth decile), the relative increase in compilation time ranges between 10% to 40%.


Compilation profile

Since we have data for both typechecking time and non-typechecking times for a few thousand files, it is interesting to check how much time is spent on typechecking. We can start by looking at the data points files by files:

Relative time spent in typechecking by files

We have here a relatively uniform cloud of points between 20-60% of time spent in typechecking compared to total compilation time. This is is reflected on the average and median

Arithmetic average Median
38.8827% 39.7336%

Both value are quite comparable, the distribution doesn’t seem significantly skewed.

However, we have a clear cluster of files for which typechecking accounts for 90% of the total compilation time. Interestingly, this cluster of points corresponds to the dune cluster of files with a very variance that we had identified earlier. This explains why those files have essentially the same profile when looking at the total and typechecking compilation time: in their case, typechecking accounts for most of the work done during compilation.

This relatively uniform distribution is visible both on the quantiles (with an affine part of the quantiles)

Quantiles of the relative time spent in typechecking by files
% profile quantiles
1% 0.111556
10% 0.16431
25% 0.283249
50% 0.397336
75% 0.487892
90% 0.573355
99% 0.749689
99.9% 0.913336

and on the histogram of the relative time spent in typechecking

Histogram of the relative time spent in typechecking by files


Histogram versions for the quantile diagrams are also available. Due to the mixture of continuous and discrete distributions they are not that easy to read. Note that those histograms have equiprobable bins (in other words, constant area) rather than constant width bins.

Average compilation time histograms

Histogram of the relative change of average typechecking time by files Histogram of the relative change in average non-typechecking time by files Histogram of the relative change in average total time by files

An interesting take-away for those histograms is that the typechecking and total compilation time distribution are clearly skewed to the right: with very few exceptions, compilation increases. Contrarily the non-typechecking time distribution is much more symmetric. Since the change here is due to noise, there is no more reason for the compilation time to increase or decrease.

Minimal compilation time histograms

There is no much change when looking at the histogram for the minimal compilation time for a file

Histogram of the relative change in minimal typechecking time by files Histogram of the relative change in minimal non-typechecking time by files Histogram of the relative change in minimal total time by files

The most notable difference is that the non-typechecking histogram is completely dominated by the dirac distribution centred at x=1.