why public-inbox is currently implemented in Perl 5 --------------------------------------------------- While Perl has many detractors and there's a lot not to like about Perl, we use it anyways because it offers benefits not (yet) available from other languages. This document is somewhat inspired by https://sqlite.org/whyc.html Other languages and runtimes may eventually be a possibility for us, and this document can serve as our requirements list for possible replacements. As always, comments and corrections and additions welcome at . We're not Perl experts, either. Good Things ----------- * Availability Perl 5 is installed on many, if not most GNU/Linux and BSD-based servers and workstations. It is likely the most widely-installed programming environment that offers a significant amount of POSIX functionality. Users won't have to waste bandwidth or space with giant toolchains or architecture-specific binaries. Furthermore, Perl documentation is typically installed as manpages, allowing users to quickly access and learn it offline. * Scripted, always editable by the end user Users cannot lose access to the source code. Code written entirely in any scripting language automatically satisfies the GPL-2.0, making it easier to satisfy the AGPL-3.0. Use of a scripting language improves auditability for malicious changes. It also reduces storage and bandwidth requirements for distributors, as the same scripts can be shared across multiple OSes and architectures. Perl's availability and the low barrier to entry of scripting ensures it's easy for users to exercise their software freedom. * Predictable performance While Perl is neither fast or memory-efficient, its performance and memory use are predictable and does not require GC tuning by the user. public-inbox is developed for (and mostly on) old hardware. Perl was fast enough to power the web of the late 1990s, and any cheap VPS today has more than enough RAM and CPU for handling plain-text email. Low hardware requirements increases the reach of our software to more users, improving centralization resistance. * Compatibility Unlike similarly powerful scripting languages, there is no forced migration to a major new version. From 2000-2020, Perl had fewer breaking changes than Python or Ruby; we expect that trend to continue given the inertia of Perl 5. * Built for text processing Our focus is plain-text mail, and Perl has many built-ins optimized for text processing. It also has good support for UTF-8 and legacy encodings found in old mail archives. * Integration with distros and non-Perl libraries Perl modules and bindings to common libraries such as SQLite and Xapian are already distributed by many GNU/Linux distros and BSD ports. There should be no need to rely on language-specific package managers such as cpan(1), those systems increase the learning curve for users and systems administrators. * Compactness and terseness Less code generally means less bugs. We try to avoid the "line noise" stereotype of some Perl codebases, yet still manage to write less code than one would with non-scripting languages. * Performance ceiling and escape hatch With optional Inline::C, we can be "as fast as C" in some cases. Inline::C is widely-packaged by distros and it gives us an escape hatch for dealing with missing bindings or performance problems should they arise. Inline::C use (as opposed to XS) also preserves the software freedom and auditability benefits to all users. Unfortunately, most C toolchains are big; so Inline::C will always be optional for users who cannot afford the bandwidth or space. Bad Things ---------- * Slow startup time. Tokenization, parsing, and compilation of pure Perl is not cached. Inline::C does cache its results, however. We work around slow startup times in tests by preloading code, similar to how mod_perl works for CGI. * High space overhead and poor locality of small data structures, including the optree. This may not be fixable in Perl itself given compatibility requirements of the C API. These problems are exacerbated on modern 64-bit platforms, though the Linux x32 ABI offers promise. * Lack of vectored I/O support (writev, sendmmsg, etc. syscalls) and "newer" POSIX functions in general. APIs end up being slurpy, favoring large buffers and memory copies for concatenation rather than rope (aka "cord") structures. * While mmap(2) is available via PerlIO::mmap, string ops (m//, substr(), index(), etc.) still require memory copies into userspace, negating a benefit of zero-copy. * The XS/C API make it difficult to improve internals while preserving compatibility. * Lack of optional type checking. This may be a blessing in disguise, though, as it encourages us to simplify our data models and lowers cognitive overhead. * SMP support is mostly limited to fork(), since many libraries (including much of the standard library) are not thread-safe. Even with threads.pm, sharing data between interpreters within the same process is inefficient due to the lack of lock-free and wait-free data structures from projects such as Userspace RCU. * Process spawning speed degrades as memory use increases. We work around this optionally via Inline::C and vfork(2), since Perl lacks an approximation of posix_spawn(3). We also use `undef' and `delete' ops to free large buffers as soon as we're done using them to save memory. Red herrings to ignore when evaluating other runtimes ----------------------------------------------------- These don't discount a language or runtime from being being used, they're just not interesting. * Lightweight threading While lightweight threading implementations are convenient, they tend to be significantly heavier than a pure event-loop systems (or multi-threaded event-loop systems) Lightweight threading implementations have stack overhead and growth typically measured in kilobytes. The userspace state overhead of event-based systems is an order of magnitude less, and a sunk cost regardless of concurrency model.