Chapter 28 Asynchronous and Parallel Proof Processing

Enrico Tassi

This chapter explains how proofs can be asynchronously processed by Coq. This feature improves the reactivity of the system when used in interactive mode via CoqIDE. In addition to that, it allows Coq to take advantage of parallel hardware when used as a batch compiler by decoupling the checking of statements and definitions from the construction and checking of proofs objects.

This feature is designed to help dealing with huge libraries of theorems characterized by long proofs. In the current state, it may not be beneficial on small sets of short files.

This feature has some technical limitations that may make it unsuitable for some use cases.

For example, in interactive mode, some errors coming from the kernel of Coq are signaled late. The type of errors belonging to this category are universe inconsistencies.

Last, at the time of writing, only opaque proofs (ending with Qed or Admitted) can be processed asynchronously.

28.1 Proof annotations

To process a proof asynchronously Coq needs to know the precise statement of the theorem without looking at the proof. This requires some annotations if the theorem is proved inside a Section (see Section 2.4).

When a section ends, Coq looks at the proof object to decide which section variables are actually used and hence have to be quantified in the statement of the theorem. To avoid making the construction of proofs mandatory when ending a section, one can start each proof with the Proof using command (Section 7.1.5) that declares which section variables the theorem uses.

The presence of Proof using is needed to process proofs asynchronously in interactive mode.

It is not strictly mandatory in batch mode if it is not the first time the file is compiled and if the file itself did not change. When the proof does not begin with Proof using, the system records in an auxiliary file, produced along with the .vo file, the list of section variables used.

28.2 Proof blocks and error resilience

Coq 8.6 introduces a mechanism for error resiliency: in interactive mode Coq is able to completely check a document containing errors instead of bailing out at the first failure.

Two kind of errors are supported: errors occurring in vernacular commands and errors occurring in proofs.

To properly recover from a failing tactic, Coq needs to recognize the structure of the proof in order to confine the error to a sub proof. Proof block detection is performed by looking at the syntax of the proof script (i.e. also looking at indentation). Coq comes with four kind of proof blocks, and an ML API to add new ones.

curly: blocks are delimited by { and }, see 7
par: blocks are atomic, i.e. just one tactic introduced by the par: goal selector
indent: blocks end with a tactic indented less than the previous one
bullet: blocks are delimited by two equal bullet signs at the same indentation level

28.2.1 Caveats

When a vernacular command fails the subsequent error messages may be bogus, i.e. caused by the first error. Error resiliency for vernacular commands can be switched off passing -async-proofs-command-error-resilience off to CoqIDE.

An incorrect proof block detection can result into an incorrect error recovery and hence in bogus errors. Proof block detection cannot be precise for bullets or any other non well parenthesized proof structure. Error resiliency can be turned off or selectively activated for any set of block kind passing to CoqIDE one of the following options: -async-proofs-tactic-error-resilience off, -async-proofs-tactic-error-resilience all, -async-proofs-tactic-error-resilience blocktype₁,..., blocktype_n. Valid proof block types are: “curly”, “par”, “indent”, “bullet”.

Automatic suggestion of proof annotations

The command Set Suggest Proof Using makes Coq suggest, when a Qed command is processed, a correct proof annotation. It is up to the user to modify the proof script accordingly.

28.3 Interactive mode

At the time of writing the only user interface supporting asynchronous proof processing is CoqIDE.

When CoqIDE is started, two Coq processes are created. The master one follows the user, giving feedback as soon as possible by skipping proofs, which are delegated to the worker process. The worker process, whose state can be seen by clicking on the button in the lower right corner of the main CoqIDE window, asynchronously processes the proofs. If a proof contains an error, it is reported in red in the label of the very same button, that can also be used to see the list of errors and jump to the corresponding line.

If a proof is processed asynchronously the corresponding Qed command is colored using a lighter color that usual. This signals that the proof has been delegated to a worker process (or will be processed lazily if the -async-proofs lazy option is used). Once finished, the worker process will provide the proof object, but this will not be automatically checked by the kernel of the main process. To force the kernel to check all the proof objects, one has to click the button with the gears. Only then are all the universe constraints checked.

Caveats

The number of worker processes can be increased by passing CoqIDE the -async-proofs-j n flag. Note that the memory consumption increases too, since each worker requires the same amount of memory as the master process. Also note that increasing the number of workers may reduce the reactivity of the master process to user commands.

To disable this feature, one can pass the -async-proofs off flag to CoqIDE.

Proofs that are known to take little time to process are not delegated to a worker process. The threshold can be configure with -async-proofs-delegation-threshold. Default is 0.03 seconds.

28.4 Batch mode

When Coq is used as a batch compiler by running coqc or coqtop -compile, it produces a .vo file for each .v file. A .vo file contains, among other things, theorems statements and proofs. Hence to produce a .vo Coq need to process all the proofs of the .v file.

The asynchronous processing of proofs can decouple the generation of a compiled file (like the .vo one) that can be loaded by Require from the generation and checking of the proof objects. The -quick flag can be passed to coqc or coqtop to produce, quickly, .vio files. Alternatively, when using a Makefile produced by coq_makefile, the quick target can be used to compile all files using the -quick flag.

A .vio file can be loaded using Require exactly as a .vo file but proofs will not be available (the Print command produces an error). Moreover, some universe constraints might be missing, so universes inconsistencies might go unnoticed. A .vio file does not contain proof objects, but proof tasks, i.e. what a worker process can transform into a proof object.

Compiling a set of files with the -quick flag allows one to work, interactively, on any file without waiting for all the proofs to be checked.

When working interactively, one can fully check all the .v files by running coqc as usual.

Alternatively one can turn each .vio into the corresponding .vo. All .vio files can be processed in parallel, hence this alternative might be faster. The command coqtop -schedule-vio2vo 2 a b c can be used to obtain a good scheduling for 2 workers to produce a.vo, b.vo, and c.vo. When using a Makefile produced by coq_makefile, the vio2vo target can be used for that purpose. Variable J should be set to the number of workers, e.g. make vio2vo J=2. The only caveat is that, while the .vo files obtained from .vio files are complete (they contain all proof terms and universe constraints), the satisfiability of all universe constraints has not been checked globally (they are checked to be consistent for every single proof). Constraints will be checked when these .vo files are (recursively) loaded with Require.

There is an extra, possibly even faster, alternative: just check the proof tasks stored in .vio files without producing the .vo files. This is possibly faster because all the proof tasks are independent, hence one can further partition the job to be done between workers. The coqtop -schedule-vio-checking 6 a b c command can be used to obtain a good scheduling for 6 workers to check all the proof tasks of a.vio, b.vio, and c.vio. Auxiliary files are used to predict how long a proof task will take, assuming it will take the same amount of time it took last time. When using a Makefile produced by coq_makefile, the checkproofs target can be used to check all .vio files. Variable J should be set to the number of workers, e.g. make checkproofs J=6. As when converting .vio files to .vo files, universe constraints are not checked to be globally consistent. Hence this compilation mode is only useful for quick regression testing and on developments not making heavy use of the Type hierarchy.

28.5 Limiting the number of parallel workers

Many Coq processes may run on the same computer, and each of them may start many additional worker processes. The coqworkmgr utility lets one limit the number of workers, globally.

The utility accepts the -j argument to specify the maximum number of workers (defaults to 2). coqworkmgr automatically starts in the background and prints an environment variable assignment like COQWORKMGR_SOCKET=localhost:45634. The user must set this variable in all the shells from which Coq processes will be started. If one uses just one terminal running the bash shell, then export ‘coqworkmgr -j 4‘ will do the job.

After that, all Coq processes, e.g. coqide and coqc, will honor the limit, globally.