[Rd] Wish: a way to track progress of parallel operations

[Henrik Bengtsson]

Hello,

thanks for bringing this topic up, and it would be excellent if we
could come of with a generic solution for this in base R.  It is one
of the top frequently asked questions and requested features in
parallel processing, but also in sequential processing. We have also
seen lots of variants on how to attack the problem of reporting on
progress when running in parallel.

As the author Futureverse (a parallel framework), I've been exposed to
these requests and I thought quite a bit about how we could solve this
problem. I'll outline my opinionated view and suggestions on this
below:

* Target a solution that works the same regardless whether we run in
parallel or not, i.e. the code/API should look the same regardless of
using, say, parallel::parLapply(), parallel::mclapply(), or
base::lapply(). The solution should also work as-is in other parallel
frameworks.

* Consider who owns the control of whether progress updates should be
reported or not. I believe it's best to separate what the end-user and
the developer controls.  I argue the end-user should be able to
decided whether they want to "see" progress updates or not, and the
developer should focus on where to report on progress, but not how and
when.

* In line with the previous comment, controlling progress reporting
via an argument (e.g. `.progress`) is not powerful enough. With such
an approach, one need to make sure that that argument is exposed and
relayed throughout in all nested function calls. If a package decides
to introduce such an argument, what should the default be? If they set
`.progress = TRUE`, then all of a sudden, any code/packages that
depend on this function will all of a sudden see progress updates.
There are endless per-package versions of this on CRAN and
Bioconductor, any they rarely work in harmony.

* Consider accessibility as well as graphical user interfaces. This
means, don't assume progress is necessarily reported in the terminal.
I found it a good practice to never use the term "progress bar",
because that is too focused on how progress is reported.

* Let the end-user control how progress is reported, e.g. a progress
bar in the terminal, a progress bar in their favorite IDE/GUI,
OS-specific notifications, third-party notification services, auditory
output, etc.

The above objectives challenge you to take a step back and think about
what progress reporting is about, because the most immediate needs.
Based on these, I came up with the 'progressr' package
(https://progressr.futureverse.org/). FWIW, it was originally actually
meant to be a proof-of-concept proposal for a universal, generic
solution to this problem, but as the demands grew and the prototype
showed to be useful, I made it official.  Here is the gist:

* Motto: "The developer is responsible for providing progress updates,
but it’s only the end user who decides if, when, and how progress
should be presented. No exceptions will be allowed."

* It rely on R's condition system to signal progress. The developer
signals progress conditions. Condition handlers, which the end-user
controls, are used to report/render these progress updates. The
support for global condition handlers, introduced in R 4.0.0, makes
this much more convenient. It is useful to think of the condition
mechanism in R as a back channel for communication that operates
separately from the rest of the "communication" stream (calling
functions with arguments and returning value).

* For parallel processing, progress conditions can be relayed back to
the parent process via back channels in a "near-live" fashion, or at
the very end when the parallel task is completed. Technically,
progress conditions inherit from 'immediateCondition', which is a
special class indicating that such conditions are allowed to be
relayed immediately and out of order. It is possible to use the
existing PSOCK socket connections to send such 'immediateCondition':s.

* No assumption is made on progress updates arriving in a certain
order. They are just a stream of "progress of this and that amount"
was made.

* There is a progress handler API. Using this API, various types of
progress reporting can be implemented. This allows anyone to implement
progress handlers in contributed R packages.

See https://progressr.futureverse.org/ for more details.

> I would be happy to prepare code and documentation. If there is no time now, we can return to it after R-4.4 is released.

I strongly recommend to not rush this. This is an important, big
problem that goes beyond the 'parallel' package. I think it would be a
disfavor to introduce a '.progress' argument. As mentioned above, I
think a solution should work throughout the R ecosystem - all base-R
packages and beyond. I honestly think we could arrive at a solution
where base-R proposes a very light, yet powerful, progress API that
handles all of the above. The main task is to come up with a standard
API/protocol - then the implementation does not matter.

/Henrik

On Mon, Mar 25, 2024 at 8:41 AM Ivan Krylov via R-devel
<r-devel using r-project.org> wrote:
>
> Hello R-devel,
>
> A function to be run inside lapply() or one of its friends is trivial
> to augment with side effects to show a progress bar. When the code is
> intended to be run on a 'parallel' cluster, it generally cannot rely on
> its own side effects to report progress.
>
> I've found three approaches to progress bars for parallel processes on
> CRAN:
>
>  - Importing 'snow' (not 'parallel') internals like sendCall and
>    implementing parallel processing on top of them (doSNOW). This has
>    the downside of having to write higher-level code from scratch
>    using undocumented inferfaces.
>
>  - Splitting the workload into length(cluster)-sized chunks and
>    processing them in separate parLapply() calls between updating the
>    progress bar (pbapply). This approach trades off parallelism against
>    the precision of the progress information: the function has to wait
>    until all chunk elements have been processed before updating the
>    progress bar and submitting a new portion; dynamic load balancing
>    becomes much less efficient.
>
>  - Adding local side effects to the function and detecting them while
>    the parallel function is running in a child process (parabar). A
>    clever hack, but much harder to extend to distributed clusters.
>
> With recvData and recvOneData becoming exported in R-4.4 [*], another
> approach becomes feasible: wrap the cluster object (and all nodes) into
> another class, attach the progress callback as an attribute, and let
> recvData / recvOneData call it. This makes it possible to give wrapped
> cluster objects to unchanged code, but requires knowing the precise
> number of chunks that the workload will be split into.
>
> Could it be feasible to add an optional .progress argument after the
> ellipsis to parLapply() and its friends? We can require it to be a
> function accepting (done_chunk, total_chunks, ...). If not a new
> argument, what other interfaces could be used to get accurate progress
> information from staticClusterApply and dynamicClusterApply?
>
> I understand that the default parLapply() behaviour is not very
> amenable to progress tracking, but when running clusterMap(.scheduling
> = 'dynamic') spanning multiple hours if not whole days, having progress
> information sets the mind at ease.
>
> I would be happy to prepare code and documentation. If there is no time
> now, we can return to it after R-4.4 is released.
>
> --
> Best regards,
> Ivan
>
> [*] https://bugs.r-project.org/show_bug.cgi?id=18587
>
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