Commit 83c008fa authored by reid's avatar reid
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[project @ 1998-10-27 23:35:33 by reid]

Added CAF text to rts document
parent bff3893a
......@@ -2715,6 +2715,270 @@ are evacuated and @NULL@-values are chained together to form a new free list.
There's no need to link the stable pointer table onto the mutable
list because we always treat it as a root.
\Subsection{Garbage Collecting CAFs}{CAF}
% begin{direct quote from current paper}
A CAF (constant applicative form) is a top-level expression with no
arguments. The expression may need a large, even unbounded, amount of
storage when it is fully evaluated.
CAFs are represented by closures in static memory that are updated
with indirections to objects in the heap space once the expression is
evaluated. Previous version of GHC maintained a list of all evaluated
CAFs and traversed them during GC, the result being that the storage
allocated by a CAF would reside in the heap until the program ended.
% end{direct quote from current paper}
% begin{elaboration on why CAFs are very very bad}
Treating CAFs this way has two problems:
It can cause a very large space leak. For example, this program
should run in constant space but, instead, will run out of memory.
> main :: IO ()
> main = print nats
> nats :: [Int]
> nats = [0..maxInt]
Expressions with no arguments have very different space behaviour
depending on whether or not they occur at the top level. For example,
if we make \verb+nats+ a local definition, the space leak goes away
and the resulting program runs in constant space, as expected.
> main :: IO ()
> main = print nats
> where
> nats :: [Int]
> nats = [0..maxInt]
This huge change in the operational behaviour of the program
is a problem for optimising compilers and for programmers.
For example, GHC will normally flatten a set of let bindings using
this transformation:
let x1 = let x2 = e2 in e1 ==> let x2 = e2 in let x1 = e1
but it does not do so if this would raise \verb+x2+ to the top level
since that may create a CAF. Many Haskell programmers avoid creating
large CAFs by adding a dummy argument to a CAF or by moving a CAF away
from the top level.
% end{elaboration on why CAFs are very very bad}
Solving the CAF problem requires different treatment in interactive
systems such as Hugs than in batch-mode systems such as GHC
In a batch-mode the program the runtime system is terminated
after every execution of the runtime system. In such systems,
the garbage collector can completely ``destroy'' a CAF when it
is no longer live --- in much the same way as it ``destroys''
normal closures when they are no longer live.
In an interactive system, many expressions are evaluated without
restarting the runtime system between each evaluation. In such
systems, the garbage collector cannot completely ``destroy'' a CAF
when it is no longer live because, whilst it might not be required in
the evaluation of the current expression, it might be required in the
next evaluation.
There are two possible behaviours we migth want:
When a CAF is no longer required for the current evaluation, the CAF
should be reverted to its original form. This behaviour ensures that
the operational behaviour of the interactive system is a reasonable
predictor of the operational behaviour of the batch-mode system. This
allows us to use Hugs for performance debugging (in particular, trying
to understand and reduce the heap usage of a program) --- an area of
increasing importance as Haskell is used more and more to solve ``real
problems'' in ``real problem domains''.
Even if a CAF is no longer required for the current evaluation, we might
choose to hang onto it by collecting it in the normal way. This keeps
the space leak but might be useful in a teaching environment when
trying to teach the difference between call by name evaluation (which
doesn't share work) and lazy evaluation (which does share work).
It turns out that it is easy to support both styles of use, so the
runtime system provides a switch which lets us turn this on and off
during execution. \ToDo{What is this switch called?} It would also
be easy to provide a function \verb+RevertCAF+ to let the interpreter
revert any CAF it wanted between (but not during) executions, if we so
desired. Running \verb+RevertCAF+ during execution would lose some sharing
but is otherwise harmless.
% % begin{even more pointless observation?}
% The simplest fix would be to remove the special treatment of
% top level variables. This works but is very inefficient.
% ToDo: say why.
% (Note: delete this paragraph from final version.)
% % end{even more pointless observation?}
% begin{pointless observation?}
An easy but inefficient fix to the CAF problem would be to make a
complete copy of the heap before every evaluation and discard the copy
after evaluation. This works but is inefficient.
% end{pointless observation?}
An efficient way to achieve a similar effect is to revert all
updatable thunks to their original form as they become unnecessary for
the current evaluation. To do this, we modify the compiler to ensure
that the only updatable thunks generated by the compiler are CAFs and
we modify the garbage collector to revert entered CAFs to unentered
CAFs as their value becomes unnecessary.
\subsubsection{New Heap Objects}
We add three new kinds of heap object: unentered CAF closures, entered
CAF objects and CAF blackholes. We first describe how they are
evaluated and then how they are garbage collected.
Unentered CAF closures contain a pointer to closure representing the
body of the CAF. The ``body closure'' is not updatable.
Unentered CAF closures contain two unused fields to make them the same
size as entered CAF closures --- which allows us to perform an inplace
update. \ToDo{Do we have to add another kind of inplace update operation
to the storage manager interface or do we consider this to be internal
to the SM?}
\verb+CAF_unentered+ & \emph{body closure} & \emph{unused} & \emph{unused} \\ \hline
When an unentered CAF is entered, we do the following:
allocate a CAF black hole;
push an update frame (to update the CAF black hole) onto the stack;
overwrite the CAF with an entered CAF object (see below) with the same
body and whose value field points to the black hole;
add the CAF to a list of all entered CAFs (called ``the CAF list'');
the closure representing the value of the CAF is entered.
When evaluation of the CAF body returns a value, the update frame
causes the CAF black hole to be updated with the value in the normal
\ToDo{Add a picture}
Entered CAF closures contain two pointers: a pointer to the CAF body
(the same as for unentered CAF closures); a pointer to the CAF value
(this is initialised with a CAF blackhole, as previously described);
and a link to the next CAF in the CAF list
\ToDo{How is the end of the list marked? Null pointer or sentinel value?}.
\verb+CAF_entered+ & \emph{body closure} & \emph{value} & \emph{link} \\ \hline
When an entered CAF is entered, it enters its value closure.
CAF blackholes are identical to normal blackholes except that they
have a different infotable. The only reason for having CAF blackholes
is to allow an optimisation of lazy blackholing where we stop scanning
the stack when we see the first {\em normal blackhole} but not
when we see a {\em CAF blackhole.}
\ToDo{The optimisation we want to allow should be described elsewhere
so that all we have to do here is describe the difference.}
Instead of allocating a blackhole to update with the value of the CAF,
it might seem simpler to update the CAF directly. This would require
a new kind of update frame which would update the value field of the
CAF with a pointer to the value and wouldn't catch blackholes caused
by CAFs that depend on themselves so we chose not to do so.
\subsubsection{Garbage Collection}
To avoid the space leak, each run of the garbage collector must revert
the entered CAFs which are not required to complete the current
evaluation (that is all the closures reachable from the set of
runnable threads and the stable pointer table).
It does this by performing garbage collection in three phases:
During the first phase, we ``mark'' all closures reachable from the
scheduler state.
How we ``mark'' closures depends on the garbage collector. For
example, in a 2-space collector, closures are ``marked'' by copying
them into ``to-space'', overwriting them with a forwarding node and
``marking'' all the closures reachable from the copy. The only
requirements are that we can test whether a closure is marked and if a
closure is marked then so are all closures reachable from it.
\ToDo{At present we say that the scheduler state includes any state
that Hugs may have. This is not true anymore.}
Performing this phase first provides us with a cheap test for
execution closures: at this stage in execution, the execution closures
are precisely the marked closures.
During the second phase, we revert all unmarked CAFs on the CAF list
and remove them from the CAF list.
Since the CAF list is exactly the set of all entered CAFs, this reverts
all entered CAFs which are not execution closures.
During the third phase, we mark all top level objects (including CAFs)
by calling \verb+MarkHugsRoots+ which will call \verb+MarkRoot+ for
each top level object known to Hugs.
To implement the second style of interactive behaviour (where we
deliberately keep the CAF-related space leak), we simply omit the
second phase. Omitting the second phase causes the third phase to
mark any unmarked CAF value closures.
So far, we have been describing a pure Hugs system which contains no
machine generated code. The main difference in a hybrid system is
that GHC-generated code is statically allocated in memory instead of
being dynamically allocated on the heap. We split both
\verb+CAF_unentered+ and \verb+CAF_entered+ into two versions: a
static and a dynamic version. The static and dynamic versions of each
CAF differ only in whether they are moved during garbage collection.
When reverting CAFs, we revert dynamic entered CAFs to dynamic
unentered CAFs and static entered CAFs to static unentered CAFs.
\Section{The Bytecode Evaluator}{bytecode-evaluator}
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