Merge pull request #128 from pypy/2024-blog-tracing

tracing blog post

posts/2022/02/nlp-icelandic-pypy.rst

@@ -1,7 +1,7 @@
 .. title: Natural Language Processing for Icelandic with PyPy: A Case Study
 .. slug: nlp-icelandic-case-study
 .. date: 2022-02-06 15:00:00 UTC
-.. tags: casestudy
+.. tags: casestudy, guestpost
 .. category: 
 .. link: 
 .. description: 

posts/2025/01/state-of-tracing.md

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+<!--
+.. title: Musings on Tracing in PyPy
+.. slug: musings-tracing
+.. date: 2025-01-05 17:01:09 UTC
+.. tags:
+.. category:
+.. link:
+.. description:
+.. type: text
+.. author: CF Bolz-Tereick
+-->
+
+Last summer, [Shriram Krishnamurthi](https://cs.brown.edu/~sk/) [asked on
+Twitter](https://twitter.com/ShriramKMurthi/status/1818009884484583459):
+
+> "I'm curious what the current state of tracing JITs is. They used to be all the
+> rage for a while, then I though I heard they weren't so effective, then I
+> haven't heard of them at all. Is the latter because they are ubiquitous, or
+> because they proved to not work so well?"
+
+I replied with my personal (pretty subjective) opinions about the
+question in a lengthy Twitter thread (which also spawned an even lengthier
+discussion). I wanted to turn what I wrote there into a blog post to make it
+more widely available (Twitter is no longer easily consumable without an
+account), and also because I'm mostly not using Twitter anymore. The blog post
+i still somewhat terse, I've written a small background section and tried to at
+least add links to further information. Please ask in the comments if something
+is particularly unclear.
+
+
+## Background
+
+I'll explain a few of the central terms of the rest of the post. *JIT compilers*
+are compilers that do their work at runtime, interleaved (or concurrent with)
+the execution of the program. There are (at least) two common general styles of
+JIT compiler architectures. The most common one is that of a method-based JIT,
+which will compile one method or function at a time. Then there are tracing JIT
+compilers, which generate code by tracing the execution of the user's program.
+They often focus on loops as stheir main unit of compilation.
+
+Then there is the distinction between a "regular" JIT compiler and that of a
+*meta-JIT*. A regular JIT is built to compile one specific source language to
+machine code. A meta-JIT is a framework for building JIT compilers for a
+variety of different languages, re-using as much machinery as possible between
+the different implementation.
+
+## Personal and Project Context
+
+Some personal context: my perspective is informed by nearly [two
+decades](https://mail.python.org/archives/list/[email protected]/thread/TZM37YJ733G445R6JGTV26333RQEPLRX/)
+of work on PyPy. PyPy's implementation language, [RPython](https://rpython.readthedocs.io/), has support for a
+meta-JIT, which allows it to reuse its JIT infrastructure for the various
+Python versions that we support (currently we do releases of PyPy2.7 and
+PyPy3.10 together). Our meta-JIT infrastructure has been used for some
+experimental different languages like:
+
+- PyPy's [regular expression engine](https://pypy.org/posts/2010/11/pypy-14-ouroboros-in-practice-5437628000869417542.html#more-highlights)
+- [RPySom](https://github.com/SOM-st/PySOM), a tiny Smalltalk
+- [Ruby](https://github.com/topazproject/topaz)
+- [PHP](https://github.com/hippyvm/hippyvm)
+- [Prolog](https://dl.acm.org/doi/10.1145/1836089.1836102),
+- [Racket](https://dl.acm.org/doi/10.1145/2784731.2784740),
+- a [database (SQLite)](https://drops.dagstuhl.de/storage/00lipics/lipics-vol056-ecoop2016/LIPIcs.ECOOP.2016.4/LIPIcs.ECOOP.2016.4.pdf)
+- [Lox](https://www.youtube.com/watch?v=fZj3uljJl_k), the language of [Crafting Interpreters](https://craftinginterpreters.com/)
+- an [ARM and RISC-V emulator](https://docs.pydrofoil.org/en/latest/)
+- and many more
+
+Those implementations had various degrees of maturity and many of them are
+research software and aren't maintained any more.
+
+PyPy gives itself the goal to try to be extremely compatible with all the
+quirks of the Python language. We don't change the Python language to make
+things easier to compile and we support the introspection and debugging
+features of Python. We try very hard to have no opinions on language design.
+The CPython core developers come up with the semantics, we somehow deal with
+them.
+
+## Meta-tracing
+
+PyPy started using a [tracing
+JIT](https://en.wikipedia.org/wiki/Tracing_just-in-time_compilation) approach
+*not* because we thought method-based just-in-time compilers are bad.
+Historically we [had
+tried](https://foss.heptapod.net/pypy/extradoc/-/blob/branch/extradoc/eu-report/D08.2_JIT_Compiler_Architecture-2007-05-01.pdf)
+to implement a method-based meta-JIT that was using partial evaluation (we wrote
+three or four method-based prototypes that all weren't as good as we hoped).
+After all those [experiments
+failed](https://pypy.org/posts/2008/10/sprint-discussions-jit-generator-3301578822967655604.html)
+we switched to the [tracing
+approach](https://dl.acm.org/doi/10.1145/1565824.1565827), and only at this
+point did our meta-JIT start producing interesting performance.
+
+In the meta-JIT context tracing has good properties, because tracing has
+relatively understandable behavior and its easy(ish) to tweak how things work
+[with extra annotations in the interpreter
+source](https://dl.acm.org/doi/10.1145/2069172.2069181).
+
+Another reason why meta-tracing often works well for PyPy is that it can often
+slice through the complicated layers of Python quite effectively and remove a
+lot of overhead. Python is often described as simple, but I think that's
+actually a misconception. On the implementation level it's a very big and
+complicated language, and it is also continuously getting new features every
+year (the language is quite a bit more complicated than Javascript, for
+example[^help]).
+
+[^help]: (A side point: people who haven't worked on Python tend to
+    underestimate its complexity and pace of development. A pet peeve of mine
+    is C++ compiler devs/static analysis/Javascript people/other well-meaning
+    communities coming with statements like "why don't you just..."  🤷‍♀️)
+
+
+### Truffle
+
+Later [Truffle](https://dl.acm.org/doi/abs/10.1145/2509578.2509581) came along
+and made a method-based meta-JIT using partial evaluation work. However Truffle
+(and [Graal](https://www.oracle.com/java/graalvm/)) has had significantly more people working on it and much more
+money invested. In addition, it at first required a quite specific style of
+[AST-based interpreters](https://dl.acm.org/doi/10.1145/2384577.2384587) (in
+the last few years they have also added support for bytecode-based
+interpreters).
+
+It's still my impression that getting similar results with Truffle is [more
+work for language
+implementers](https://stefan-marr.de/downloads/oopsla15-marr-ducasse-meta-tracing-vs-partial-evaluation.pdf)
+than with RPython, and the [warmup](https://arxiv.org/pdf/1602.00602) of
+Truffle can often pretty bad. But Truffle is definitely an existence proof that
+meta-JITs don't *have* to be based on tracing.
+
+
+## Tracing, the good
+
+Let's now actually get to he heart of Shriram's question and discuss some of
+the advantages of tracing that go beyond the ease of using tracing for a
+meta-JIT.
+
+Tracing allows for doing very aggressive [partial
+inlining](https://www.cs.fsu.edu/~xyuan/INTERACT-15/papers/paper11.pdf),
+Following just the hot path through lots of layers of abstraction is obviously
+often really useful for generating fast code.
+
+It's definitely possible to achieve the same effect in a method-based context
+with [path splitting](https://dl.acm.org/doi/pdf/10.1145/117954.117955). But it
+requires a lot more implementation work and is not trivial, because the path
+[execution counts](https://dl.acm.org/doi/10.1145/504282.504295) of inlined
+functions can often be very call-site dependent. Tracing, on the other hand,
+gives you call-site dependent path splitting "for free".
+
+(The aggressive partial inlining and path splitting is even more important in
+the meta-tracing context of PyPy, where some of inlined layers are a part of
+the language runtime, and where rare corner cases that are never executed in
+practice are everywhere.)
+
+Another advantage of tracing is that it makes a number of optimizations
+really easy to implement, because there are (to first approximation) no control
+flow merges. This makes all the optimizations that we do (super-)[local
+optimizations](https://en.wikipedia.org/wiki/Optimizing_compiler#Local_vs._global_scope),
+that operate on a single (very long) basic block. This allows the JIT to do the
+optimizations in exactly one forwards and one backwards pass. An example is our
+[allocation removal](https://dl.acm.org/doi/10.1145/1929501.1929508)/partial
+escape analysis pass, which is [quite
+simple](https://pypy.org/posts/2022/10/toy-optimizer-allocation-removal.html),
+whereas the [version for general control
+flow](https://ssw.jku.at/Teaching/PhDTheses/Stadler/Thesis_Stadler_14.pdf) has
+a lot more complexity, particularly in its handling of loops.
+
+This ease of implementation of optimizations allowed us to implement some
+pretty decent optimizations. Our allocation removal, the way PyPy's JIT can
+reason about the heap, about dictionary accesses, about properties of functions
+of the runtime, about the range and [known bits of integer
+variables](https://pypy.org/posts/2024/08/toy-knownbits.html) is all quite
+solid.
+
+
+## Tracing, the bad
+
+Tracing also comes with a significant number of downsides. Probably the biggest
+one is that it tends to have big performance cliffs (PyPy certainly has them,
+and other tracing JITs such as TraceMonkey had them too). In my experience the
+'good' cases of tracing are really good, but if something goes wrong you are
+annoyed and performance can become a lot slower. With a simple method JIT the
+performance is often much more "even".
+
+Another set of downsides is that tracing has a number of corner cases and
+"weird" behaviour in certain situations. Questions such as:
+- When do you stop inlining?
+- What happens when you [trace recursion](https://mail.python.org/archives/list/[email protected]/thread/GQQ7ABUFHGEAHWN7RQZM6D54CDROQINR/)?
+- What happens if your traces are [consistently too long, even without inling](https://pypy.org/posts/2021/09/jit-auto-generated-code.html)?
+- and so on...
+
+Some of these problems can be solved by adding heuristics to the tracing JIT,
+but doing so loses a lot of the simplicity of tracing again.
+
+There are also some classes of programs that tend to generally perform quite
+poorly when they are executed by a tracing JIT, bytecode interpreters in
+particularly, and other extremely unpredictably branchy code. This is because
+the core assumption of the tracing jit "loops take similar control flow paths"
+is just really wrong in the case of interpreters.
+
+
+## Discussion
+
+The Twitter thread spawned quite a bit of discussion, please look at the
+original thread for all of the comments. Here are three that I wanted to
+highlight:
+
+> "This is a really great summary. Meta-tracing is probably the one biggest
+> success story. I think it has to do with how big and branchy the bytecode
+> implementations are for typical dynamic languages; the trace captures latent
+> type feedback naturally.
+>
+> There is an upper limit, tho."
+
+[Ben Titzer](https://twitter.com/TitzerBL/status/1818385622203298265)
+
+I agree with this completely. The complexity of Python bytecodes is a big
+factor for why meta tracing works well for us. But also in Python there are
+many builtin types (collection types, types that form the [meta-object
+protocol](https://en.wikipedia.org/wiki/Metaobject#Metaobject_protocol) of
+Python, standard library modules implemented in C/RPython) and tracing
+operations on them is very important too, for good performance.
+
+
+----
+
+> "I think Mozilla had a blog post talking more about the difficulty with
+> TraceMonkey, could only find this one:
+> https://blog.mozilla.org/nnethercote/category/jagermonkey/"
+
+[Stefan Marr](https://twitter.com/smarr/status/1818600052752797990)
+
+> "imo doing tracing for JS is really hard mode, because the browser is so
+> incredibly warmup-sensitive. IIRC tracemonkey used a really low loop trip count
+> (single-digit?) to decide when to start tracing (pypy uses >1000). the JS
+> interpreters of the time were also quite slow."
+
+[me](https://twitter.com/cfbolz/status/1818609594219811245)
+
+In the meantime there were some more reminiscences about tracing in Javascript
+by [Shu-Yu Guo in a panel
+discussion](https://www.youtube.com/live/_VF3pISRYRc?t=24797s) and by [Jason
+Orendorff on Mastodon](https://kfogel.org/notice/AngH0uqyJl231yLLOa).
+
+----
+
+> "There are a number of corner cases you have to deal with in a tracing JIT. It's
+> unfortunately not as simple and easy as the initial papers would have you
+> believe. One example is how would you deal with a loop inside a loop? Is your
+> tracing now recursive?
+
+> There's been some research work on trace stitching to deal with trace explosion
+> but it does add complexity. With the increase in complexity, I think most
+> industrial VM developers would rather pick tried-and-true method-based JITs
+> that are well understood."
+
+[Maxime Chevalier](https://twitter.com/Love2Code/status/1818292516753383644)
+
+
+## Conclusion
+
+Given access to enough developers and in the context of "normal" jitting (ie
+not meta-jitting) it's very unclear to me that you should use tracing. It makes
+more sense to rather spend effort on a solid control-flow-graph-based baseline
+and then try to get some of the good properties of tracing on top (path
+splitting, partial inlining, partial escape analysis, etc).
+
+For PyPy with its meta-JIT (and the fact that we don't have particularly much
+funding nor people) I still think tracing was/is a relatively pragmatic choice.
+When I talked with [Sam Tobin-Hochstadt](https://samth.github.io/) about this
+topic recently he characterized it like this: "tracing is a labor-saving device
+for compiler authors".
+
+Performance-wise PyPy is still quite hard to beat in the cases where it works
+well (i.e. pure Python code that doesn't use too many C modules, which are
+[supported but slow in
+PyPy](https://pypy.org/posts/2018/09/inside-cpyext-why-emulating-cpython-c-8083064623681286567.html)).
+In general, there are very few JITs for Python (particularly with the
+constraint of not being "allowed" to change the language), the most competitive
+other ones are [GraalPy](https://www.graalvm.org/python/), also based on a
+meta-JIT approach. Instagram is running on
+[Cinder](https://github.com/facebookincubator/cinder/) and also CPython has
+[grown a JIT
+recently](https://tonybaloney.github.io/posts/python-gets-a-jit.html) which
+was part of the recent [3.13 release, but only as an off-by-default build
+option](https://docs.python.org/3.13/whatsnew/3.13.html#an-experimental-just-in-time-jit-compiler),
+so I'm very excited about how Python's performance will develop in the next
+years!
+