NAME

       makepp_sandboxes -- How to partition a makepp build

DESCRIPTION

       D: --do-build,
         --dont-build,
         --dont-read,
         --do-read,  I: --in-sandbox,
         --inside-sandbox,  O: --out-of-sandbox,  S: --sandbox,
         --sandbox-warn,
         --sandbox-warning,  V: --virtual-sandbox

       There are a couple of reasons that you might want to partition the file tree for a makepp
       build:

       1.  If you know that the majority of the tree is not affected by any changes made to
           source files since the previous build, then you can tell makepp to assume that files
           in those parts of the tree are already up-to-date, which means not even implicitly
           loading their makefiles, let alone computing and checking their dependencies.  (Note
           that explicitly loaded makefiles are still loaded, however.)

       2.  If you have multiple makepp processes accessing the same tree, then you want to raise
           an error if you detect that two concurrent processes are writing the same part of the
           tree, or that one process is reading a part of the tree that a concurrent process is
           writing.  Either way, you have a race condition in which the relative order of events
           in two concurrent processes (which cannot be guaranteed) may affect the result.

       Makepp has sandboxing facilities that address both concerns.

   Sandboxing Options
       The following makepp options may be used to set the sandboxing properties of the subtree
       given by path and all of its files and potential files:

       --dont-build path
       --do-build path
           Set or reset the "dont-build" property.  Any file with this property set is assumed to
           be up-to-date already, and no build checks will be performed.  The default is reset
           (i.e. "do-build"), except if you have a "RootMakeppfile", in which case everything
           outside of its subtree id "dont-build".

       --sandbox path
       --in-sandbox path
       --inside-sandbox path
       --out-of-sandbox path
           Set or reset the "in-sandbox" property.  An error is raised if makepp would otherwise
           write a file with this property reset.  Build checks are still performed, unless the
           "dont-build" property is also set.  The default is set (i.e. "in-sandbox"), unless
           there are any --sandbox options, in which case the default for all other files is
           reset (i.e. "out-of-sandbox").

       --sandbox-warn
       --sandbox-warning
           Downgrade violations of "in-sandbox" and "dont-read" to warnings instead of errors.
           This is useful when there are hundreds of violations, so that you can collect all of
           them in a single run and take appropriate corrective action.  Otherwise, you see only
           one violation per makepp invocation, and you don't know how many are left until
           they're all fixed.

       --dont-read path
       --do-read path
           Set or reset the "dont-read" property.  An error is raised if makepp would otherwise
           read a file with this property set.  The default is reset (i.e. "do-read").

       --virtual-sandbox
           Don't rewrite build infos of files that were not created by this makepp process.  This
           is useful when running concurrent makepp processes with overlapping sandboxes, and you
           are certain that no two processes will attempt to build the same target.  Makepp will
           then refrain from caching additional information about files that it reads, because
           there might be other concurrent readers.

       Each of these 3 properties applies to the entire subtree, including to files that do not
       yet exist.  More specific paths override less specific paths.  A specified path may be an
       individual file, even if the file does not yet exist.

       If a property is both set and reset on the exact same path, then the option that appears
       furthest to the right on the command line takes precedence.

   Sandboxing for Acceleration
       If you want to prevent makepp from wasting time processing files that you know are already
       up-to-date (in particular, files that are generated by a build tool other than makepp),
       then --dont-build is the option for you.

       By far the most common case for such an optimization is that you know that everything not
       at or below the starting directory is already up-to-date.  This can be communicated to
       makepp using "--dont-build /. --do-build .".

   Sandboxing for Concurrent Processes
       One technique that can reduce build latency is to have multiple makepp processes working
       on the same tree.  This is quite a bit more difficult to manage than using the -j option,
       but it can also be substantially more effective because:

       • With sandboxing, the processes may be running on multiple hosts, for example, via a job
         queuing system.  Increasing the -j limit eventually exhausts the CPU resources of a
         single host, and can even slow the build due to excessive process forking.

       • -j does not currently parallelize some of makepp's time-consuming tasks such as loading
         makefiles, scanning, building implicit dependencies while scanning, and checking
         dependencies.

       The biggest risk with this approach is that the build can become nondeterministic if
       processes that might be concurrent interact with one another.  This leads to build systems
       that produce incorrect results sporadically, and with no simple mechanism to determine why
       it happens.

       To address this risk, it is advisable to partition the tree among concurrent processes
       such that if any process accesses the filesystem improperly, then an error is
       deterministically raised immediately.  Normally, this is accomplished by assigning to each
       concurrent process a "sandbox" in which it is allowed to write, where the sandboxes of no
       two concurrent processes may overlap.

       In addition, each process marks the sandboxes of any other possibly concurrent processes
       as "dont-read."  If a process reads a file that another concurrent process is responsible
       for writing (and which therefore might not yet be written), then an error is raised
       immediately.

   Sandboxing for Sequential Processes
       When the build is partitioned for concurrent makepp processes, there is also usually a
       sequential relationship between various pairs of processes.  For example, there may be a
       dozen concurrent compile processes, followed by a single link process that cannot begin
       until all of the compile processes have completed.  Such sequential relationships must be
       enforced by whatever mechanism is orchestrating the various makepp processes (for example,
       the job queuing system).

       When processes have a known sequential relationship, there is normally no need to raise an
       error when they access the same part of the tree, because the result is nonetheless
       deterministic.

       However, it is generally beneficial to specify --dont-build options to the dependent
       process (the link process in our example) that notify it of the areas that have already
       been updated by the prerequisite processes (the compile jobs in our example).  In this
       manner, we avoid most of the unnecessary work of null-building targets that were just
       updated.

AUTHOR

       Anders Johnson (anders@ieee.org)