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Reworked allocation APIs, STW machinery, and replaced semispace with Immix #3616

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merged 131 commits into from Mar 25, 2016
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@brixen brixen commented Feb 5, 2016

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brixen added 30 commits Aug 31, 2015
This was a well-intentioned idea but not practical or useful.

The idea was to have the compiler help check where in call paths a
garbage-collection cycle could run. Unfortnately, adding this in as an
after-thought resulted in all the places where GCTokens are created from thin
air deep in some call path. It didn't change the fact that GC could happen
pretty much anywhere.

In a managed runtime, either GC can happen everywhere or it should only happen
at a very small number of extremely well-defined points.  The middle ground of
"it can happen at all these places" is an invitation for a low budget horror
movie, dismembered objects strewn throughout the code.

Along with the rework of the stop-the-world mechanism, the removal of GCToken
and restricting the invocation of a GC cycle to a single well-defined method
call in a few well-defined locations, and finally, making all allocation paths
GC-safe (ie GC will NOT run when allocating an object), Rubinius will have much
better defined GC behavior.

The GC safe allocation path is important for cases like the string_dup
instruction, where a young GC cycle could run when allocating the dup and the
original String (eg a literal String in a method) is in the young generation
and moved. Since the original String is on the C stack and not in a GC root
object, the dup fails when copying the contents of the original String. It's
better to make allocation GC-safe than to accept the performance cost of the GC
root in these sorts of cases. Also, that case is only one well-defined instance
of the issue. There are more complicated ones.
These changes introduce a couple things:

1. All allocation paths are GC-safe.

What that means is that when requesting a new object be created, the request
will be fulfilled *unless* the system (or process limits prevent it) *without*
GC running. In other words, there are two possible results of allocating an
object: 1) a new object, or 2) an exception because no more memory is
available to the process.

In either case, from the point the object is requested until that request
returns (or the return is bypassed by the exception unwind), the GC will not
run.

There is a trade-off here between running the GC at the instant that some
threshold is breached (eg the eden space is exhausted) and loosening some
requirements that must be maintained for a generational, moving garbage
collector (ie every object reference must be known to the GC at the time the
GC runs). Since we run GC on method entry and loop back branches, there is no
reasonable scenario in which deferring GC until allocation has completed will
result in unwanted object graph thresholds being breached pathologically (eg
an execution path where allocation can grow unbounded).

2. All objects are allocated from the various heaps *uninitialized* and a
protocol is established to call an initialization routine on the objects.

The initialization routine is `T::initialize(State* state, T* obj)`, where T is
the type of object being allocated. The method is a static method of the class
of the object. This breaks with the protocol that Ruby uses where `new` is a
module method and `initialize` is an instance method. The primary reason for
choosing a static (ie C++ class) is to avoid an instance method operating on
an incompletely initialized object.

One purpose of this initialization protocol is to eliminate or reduce the
double initialization that we were doing (ie setting all fields to nil and
then initializing them to other default values). The main initialization
method shown above may be an empty body, in which case the compiler will elide
it anyway and there's no overhead to the protocol. In that case, another
initialization method should be called on the newly created object. Since the
allocation method is templated and if the initialization method is visible (ie
in the header file), the compiler should be able to elide remaining double
initialization in most contexts.
In the case of `Thread.new`, the OS thread will never run because a
ThreadError exception is raised when no block is passed. If we track the VM
object that would ultimately contain the reference to the OS thread, we either
need a way to remove the VM object when eg `Thread.new` raises an exception or
we will leak these objects. Instead of tracking and then untracking the VM
object, we create the object untracked and track it if the OS thread starts
executing.
brixen added 28 commits Mar 14, 2016
Since a SpinLock is a simple integer on which CAS operations are performed,
there is no way to go afoul of 'ownership' during fork(). This appears to
solve a spordic issue where the child was not able to reset the fork_exec_lock_
inherited from the parent process.
Before returning from Thread.new (or any similar methods), we ensure that
all the state for tracking Thread instances and any related state are
completely initialized. This prevents a case where the process calls fork()
immediately after creating a Thread and the Thread's state is only partially
initialized before the fork() call completes.
Basically, almost everything that is in util/ should not be. These major
components need to be well-integrated with the rest of Rubinius. As in the
previous case of Immix and the present case of the logger here, that means
things like taking STATE, VM, etc as parameters and interfacing with things
like process locks and process phases around fork/exec, etc.
@brixen brixen merged commit 636abb1 into master Mar 25, 2016
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@brixen brixen deleted the stw branch Mar 26, 2016
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2 participants