asterius/docs/reports.md

Status reports

This page maintains a list of weekly status reports for the project.

2019-03-04

Covers last week.

Completed work:

• Thorough refactorings in the linker to improve performance & modularity.
  • The linker used to produce small code faithfully, but the speed was slow. Moreover, the code was quite messy, with multiple pieces of whole-program traversal code scattered across a single module.
  • In order to deliver TH support, we need the linker to be:
    • Fast, loading archives & objects and performing all necessary rewriting passes quickly. Most likely the expensive dead code elimination optimization will hurt us in this scenario and we need a switch to disable it.
    • Incremental. Loading some new object code into the linker state must not trigger expensive re-compilation of old code.
  • We tidied up the linker code, moved each rewriting pass to a separate module and fused them into a single pipeline. The fusion guarantees that after dependency analysis, the AST is only traversed once to perform all necessary rewritings and produce executable wasm code.
• Updated ghc and standard libraries to a recent revision (8.7.20190217 -> 8.9.20190301). This includes Moritz Angermann's work to improve iserv.

Planned work for the week:

• Start experimenting with iserv stuff.
• Continue working to improve the linker:
  • The linker code now has some space for adding incremental linking logic. Whether/how it works for TH depends on more knowledge accumulated from iserv experiments.
  • Besides rewriting passes, another major performance bottleneck is dependency analysis, where we start from a global "store" and root symbols to scrap a self-contained module. We'll deal with this one for this week. For TH, we'll explore the possibility of adding a switch for faster linking & larger output code.
  • All rewriting passes for non-debug mode are migrated to the new pipeline, but two additional passes for debug mode are left untouched for now: "memory traps" & "tracing". They are tricky to get right in the new framework, and given debug mode doesn't impact regular users, these two may be left as they are for a bit more time.

2019-02-22

Covers this week.

Completed work:

• Finished preliminary Cabal support.
  • The executable targets are implemented. It's possible to call ahc --make directly or via ahc-cabal new-build to get an "executable". The "executable" can be quickly converted to node/web artifacts by ahc-dist.
  • ahc-cabal is a simple wrapper of cabal. stack is possibly also supported if we provide the same configure flags. Might worth a try in the future.
  • Cabal tests/benchmarks/documentation is not implemented yet.
    • haddock won't work yet.
    • Tests/benchmarks should build fine like normal executables, but Cabal can't run them like vanilla executables yet. The executables can still be "run" with ahc-dist --run.
  • ahc-dist works similarly like the legacy ahc-link tool. They share most command line arguments, except ahc-dist expects --input-exe; it starts from executable files, where ahc-link starts from Haskell sources.

Third-party contributions:

• Thanks to Piotr Majkrzak(@majkrzak) for a PR fixing a --browser problem (#73), and issue #70 for reducing Docker image size, #74 for simplifying export module interface.

Planned work for next week:

• Start working on Template Haskell/GHCi/Plugins (#54). This is the last major planned feature of 2019 Q1.
• Other potential work, in case my main thread become stalled like it always did in the past:
  • Easy improvements in gc, e.g. adding stats.
  • Experiment on creating a more asynchronous runtime. A relevant issue will be added shortly.

2019-02-18

Covers last week.

Completed work, mainly routine maintainence:

• Updated ghc and standard libraries to a recent revision (8.7.20181115 -> 8.7.20190217)
• Updated binaryen and wabt toolchains
• Added the experimental bulk memory opcodes in the binaryen Haskell bindings

Ongoing work:

• Finished implementation of library targets for Cabal.
  • The ahc executable was only meant to be invoked from the boot script. Now it can be run to compile any user input Haskell/Cmm code and produce object files.
  • ahc-ar is implemented to generate .a static libs, and those libs contain symbol indices required by asterius linker. The static libs can later be used as linker inputs.
  • Currently, no patching to Cabal is required.

Planned work for this week:

• Finish implementation of executable targets for Cabal.
  • When compiling the "executable" targets, final linking (including any necessary LTO & rewriting passes) is done. The resulting file can be quickly converted to node/web artifacts by an external tool.
  • The legacy mechanism for storing/retrieving wasm objects will be removed, and we'll only rely on the static libs and input object files for linking.
  • Add unit tests for compiling & running stuff via cabal-install.
• Some improvements in gc, if we manage to finish Cabal stuff.

2019-02-11

Covers last week.

Completed work:

• Fixed known regressions of GC & released the initial version.

Additional known drawbacks of the initial version (see report of previous week):

• There is currently no runtime logs/stats about garbage collection.
• There is currently no tunable parameters; at least we should allow specifying when "real" gc happens and when we just allocate a new nursery instead of traversing the heap (e.g. "real" gc happens when the live semispace size grows beyond a threshold)
• StgTSO/StgStack objects are unnecessarily pinned to simplify scheduler implementation a bit, but they really should be unpinned.

Planned work for this week:

• Start working on Cabal support.
• Some easy improvements in gc, e.g. adding stats/logs, implementing parameters.

2019-02-04

Covers last week.

Ongoing work:

• Finished the preliminary implementation of GC.
  • To increase reliability and catch regressions, after each GC pass, the recycled space is zeroed. If the tospace still contains pointers to recycled space (which is definitely a bug), the program is likely to crash early.
  • This has helped us to identify & fix a few bugs in the GC implementation. Right now there is only one regression left: the todomvc example crashes after initial loading completes. The crash goes away if we don't zero recycled space, but it's not a good idea to just turn that off and pretend there's no bug!

Remaining work for GC:

• Fix the todomvc regression. Given GC is such a critical component in the runtime, it's probably also good timing to integrate some more unit tests from the GHC test suite.
  • This also needs some improvement in our debugging infrastructure: our memory traps (wasm read/write barriers) is currently unaware of recycled/live space, and now we should make it cooperate with the allocator to catch invalid access to recycled space earlier.

Known drawbacks of current GC implementation once it's fully fixed & merged:

• No generational GC yet, so high GC overhead if a large volume of long-lived data is retained through program execution.
• Heap fragmentation is more severe when allocating a lot of small pinned objects.
• Weak# support is expected to split into two different stages and will land after initial GC merge:
  • Support for running "C finalizers" added by the addCFinalizerToWeak# primop. Here, the "C finalizers" are really just JavaScript functions, and the "function pointers" are JavaScript references.
  • Support for running arbitrary IO action as finalizers. This task requires support for Haskell multi-threading, and given multi-threading is not a scheduled goal of 2019 Q1, this will come later.
• Haskell closures exported to JavaScript using makeHaskellCallback* cannot be automatically recycled when they aren't used anymore. This is due to JavaScript's lacking of finalizers; users will need to call freeHaskellCallback* by hand to prevent leaking on the Haskell side.

We'll yield to Cabal support & TH/GHCi/Plugins support after the first version of GC is delivered. There's definitely room for improvement later (e.g. reduce heap fragmentation, different GC algorithms for different workloads, more detailed GC statistics, etc), but those will be managed by separate tickets.

2019-01-28

Covers last week.

Ongoing work:

• More work to improve the sanity checker and get GC near completion:
  • The sanity checker spotted a fatal situation where previously unreachable static closures become reachable again. More experiments in this direction invalidated a previous conjecture that no special treatments for static closure is required as long as they have valid block descriptors and can be moved just like dynamic ones.
  • The solution to the problem above is not hard: when scanning an info table, we also follow the SRT if it's present, and we still need to identify static/dynamic closures and prevent moving static ones. This is implemented in the sanity checker.
  • The sanity checker is no longer backed by explicit recursion. When scanning a long chain of closures, we won't run out of JavaScript stack space.
  • Adjusting the codegen & standard libraries to cope with upcoming GC:
    • The makeHaskellCallback* interfaces now properly allocate a stable pointer for the exported Haskell closure. This is to ensure that they remain valid when later called from JavaScript, even after GC runs.
    • The function closures of foreign export javascript clauses are recognized and become GC roots for similar reasons.
    • Asterius.Types is moved from ghc-prim to base, so the JSVal type can be backed by StablePtr. The GC will use a tag bit to identify regular stable pointers and JavaScript references, and automatically free unused references.
    • Integer is promoted to a standalone datatype, so the GC can scan reachable Integers and free unreachable ones which point to BigInts.

Remaining work for GC:

• Implement evac/scav functionality.
• Implement support for Weak#s based on the constraint that no Haskell execution is required when firing a finalizer.

2019-01-21

Covers last week.

Ongoing work:

• Bugfixes & partially done GC work of later stages (recycling Haskell heap space & JavaScript references):
  • Unified treatment of regular StablePtr#s and JSVal in the runtime. They are identified by a tag bit, and GC will be able to recognize live JSVals when scanning the Haskell heap.
  • Added the SPT as sanity check/garbage collection roots.
  • Fixed a sanity check bug related to AP/PAP heap objects. This could be triggered when checking the SPT after passing a closure built by chained rts_apply calls to rts_evalStableIO.
  • Reimplemented the linker layout code. We now put non-closures & closures to separate regions, and the regions are statically allocated block groups which is handled by the sm uniformly like dynamically allocated groups. This enables us to treat static closures as if they're dynamic ones without any special hack.
  • Simplified the block allocator. We no longer manage block at 4K granularity; now we only manage 1M sized ones. Pros & cons:
    • Much fewer blocks needed to manage, so simpler/faster runtime code.
    • Larger nurseries mean the Haskell mutator code signals HeapOverflow much less frequently. Should reduce amortized GC cost.
    • The main drawback is increated heap fragmentation when it comes to allocating lots of small pinned objects. This is not yet a primary concern, and can be addressed later by a hybrid GC which switches to non-moving mark-sweep algorithm for block groups with pinned objects.
  • Added functionality to free block groups, so they can later be reused without growing the linear memory. Their payloads can be zeroed to eliminate a potential attack surface (or for better reproduction of bugs in case something goes wrong)
  • Moved allocate* to the JavaScript runtime and properly implemented allocatePinned. Previously it was simply an alias of allocate since we didn't move anything around.

Planned work for next week:

• Wrap up all GC work and get a fully functional GC up & running. This was originally planned to finish by end of last week, but fell behind schedule due to the hidden workload described above. Required work:
  • Implement evac/scav functionalities in the runtime.
  • Remove the now obsolete symbol table exporting mechanism, and any closure required to survive all GC scans need to be explicitly present in the SPT upon startup.
  • Remove the terrible hacks of directly coercing between GC pointers of boxed types and regular Addr#s when crossing the FFI boundary. Now we must properly pass StablePtr#s.
  • Breaking refactorings in the current boot libs:
    • The JSVal family of types need to be moved from ghc-prim to base (or a separate package depending on base), since it needs to be a newtype wrapper of StablePtr which is defined in base.
    • The Integer type gets promoted to a standalone datatype. We still use tagging to identify small Integers and BigInts which is managed by SPT just like other JSVals.

2019-01-13

Covers the last week. The week before was new year vacation; happy new year everyone!

Completed & ongoing work:

• Completed the "modularized runtime" refactorings. (#50)
• Drafted three feature roadmaps:
  • Implement proper garbage collection (#52)
  • Implement Cabal support (#53)
  • Implement support for Template Haskell/GHCi/Plugins (#54)
  • The above proposals are scheduled to be completed on 2019 Q1.
• Began working on GC, and finished the first stage: accurate heap objects traversal.
  • Identify different types of data sections in object files (regular bytes/info tables/closures). The info table addresses are emitted into generated JavaScript to allow an accurate info table sanity check.
  • Implemented runtime utils for directly manipulating the linear memory with tagged addresses.
  • Implemented the sanity check which traverses the whole heap and visits every live object. All existing unit tests pass this check.

Planned work for next week:

• Finish the second stage of GC support: evacuate/scavenge.
  • See #52 for details. After this is finished, GC will be operational.
  • Support for handling JSVal and Weak# is scheduled in later stages.

Originally scheduled but lowered priority:

• Improving the Cloudflare worker demo. We're prioritizing more pressing issues like GC over specific use cases right now.

Special thanks to Moritz Angermann (@angerman) for contributing a patch (#55) for fixing ar problem on macOS, and helping to improve macOS & cabal support, discovering a GHC bug related to tables-next-to-code (#16174).

2018-12-28

Covers the last two weeks.

Completed work:

• Significant refactorings in the runtime.
  • Pruned ~500 loc weed code in Asterius.Builtins without breaking tests.
  • Enhanced the scheduler.
    • Previously, when entering a Haskell thread, we evaluated to completion and checked the return code; if something goes wrong, we would just throw an error.
    • Now, the scheduler is capable of handling certain scenarios like heap overflow and resuming Haskell execution.
  • Enhanced the storage manager.
    • Previously, the block allocator always triggered a grow_memory opcode when requesting blocks, making a lot of Array#/ByteArray# related primops rather in-efficient. Also, we allocated a huge heap (defaults to 1G) upon startup and pretended it won't run out.
    • Now, the block allocator grows the linear memory efficiently. And the initial heap is small (4K for both the nursery and the object pool); an overflow condition automatically extends it.
• Implemented the "persistent vault" feature.
  • Every asterius instance has a KV store called a "vault" which can be accessed in both Haskell/JavaScript. It can be used to transfer state across instances, so when an instance throws errors we can't handle, we can restart a new one without losing context.
  • This is a part of the work for Cloudflare Worker showcase.
• Delivered a working TodoMVC example and issued a blog post.
• Other notable bugfixes/improvements:
  • Fixed the dirty_MUT_VAR write barrier for MutVar#s. All non-atomic MutVar#/IORef/STRef operations now work. This is a part of the work for TodoMVC showcase.
  • We implemented UTF8/UTF16-LE/UTF32-LE/Latin-1 encoding/decoding in the runtime. This is a part of the work for text support.
  • The makeHaskellCallback functions are slightly more efficient by avoiding the overhead of allocating StablePtrs.

On-going work not completed yet:

• Modularizing the runtime.
  • Previously, the runtime is a single monolithic JavaScript script which is pasted into the output script. We'd like to split it to modules, and allow users to supply their own module to override the default behavior (evaluating Main.main once).
  • Rationales:
    • For users, it's much more convenient to implement custom logic via a proper module file. Especially in the Cloudflare Worker case, where we need:
      • Fully synchronous initialization
      • Capturing errors/rebooting a new instance
    • It's now possible to write tests for individual pieces of the runtime. This is critical to improve the runtime's reliability.
    • There were some pasted parts in the monolithic runtime; now we can properly reuse code.
    • It's also convenient to inject link-time information into the runtime.
  • We've introduced parcel into our toolchain to implement a "bundling" functionality: at link-time, re-generating a standalone .js file containing all the runtime modules. This is already implemented.
  • We're gradually splitting the monolithic runtime to modules, taking care not to break stuff. Not completed; so far so good.
• Delivering a non-trivial Cloudflare Worker demo.
  • We already have a trivial one working. It's trivial because it only does synchronous request -> response computation; more "real-world" ones will need to invoke asynchronous JavaScript computation (e.g. using Fetch API)
  • Dealing with JavaScript asynchronous computation is not quite tolerable yet; we need to litter the code with makeHaskellCallback*, at least one such call for a JavaScript await.
  • We currently have two potential approaches of improving user experience with async js code:
    • Implement some CPS-based EDSL to automatically deal with callback marshaling.
    • Implement a simple IO manager in the runtime which is capable of suspending Haskell threads when calling async js code and resuming them upon resolving/rejecting.
    • The second one sounds much more decent, but has a high difficulty level. We'll start from the first one.

Rough plans for next week:

• Finish the work on modularizing the runtime, document new behavior of JavaScript generation, then merge to master.
• Deliver a more decent Cloudflare worker demo which calls some async js code.