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Forcing large allocations here can creates serious fragmentation in
some cases, and since the large allocations are only a small
optimisation we should allow the nursery to hoover up small blocks
before allocating large chunks.
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lnat was originally "long unsigned int" but we were using it when we
wanted a 64-bit type on a 64-bit machine. This broke on Windows x64,
where long == int == 32 bits. Using types of unspecified size is bad,
but what we really wanted was a type with N bits on an N-bit machine.
StgWord is exactly that.
lnat was mentioned in some APIs that clients might be using
(e.g. StackOverflowHook()), so we leave it defined but with a comment
to say that it's deprecated.
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This patch makes two changes to the way stacks are managed:
1. The stack is now stored in a separate object from the TSO.
This means that it is easier to replace the stack object for a thread
when the stack overflows or underflows; we don't have to leave behind
the old TSO as an indirection any more. Consequently, we can remove
ThreadRelocated and deRefTSO(), which were a pain.
This is obviously the right thing, but the last time I tried to do it
it made performance worse. This time I seem to have cracked it.
2. Stacks are now represented as a chain of chunks, rather than
a single monolithic object.
The big advantage here is that individual chunks are marked clean or
dirty according to whether they contain pointers to the young
generation, and the GC can avoid traversing clean stack chunks during
a young-generation collection. This means that programs with deep
stacks will see a big saving in GC overhead when using the default GC
settings.
A secondary advantage is that there is much less copying involved as
the stack grows. Programs that quickly grow a deep stack will see big
improvements.
In some ways the implementation is simpler, as nothing special needs
to be done to reclaim stack as the stack shrinks (the GC just recovers
the dead stack chunks). On the other hand, we have to manage stack
underflow between chunks, so there's a new stack frame
(UNDERFLOW_FRAME), and we now have separate TSO and STACK objects.
The total amount of code is probably about the same as before.
There are new RTS flags:
-ki<size> Sets the initial thread stack size (default 1k) Egs: -ki4k -ki2m
-kc<size> Sets the stack chunk size (default 32k)
-kb<size> Sets the stack chunk buffer size (default 1k)
-ki was previously called just -k, and the old name is still accepted
for backwards compatibility. These new options are documented.
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as well as decommiting it.
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And add a comment about the dangers of int overflow
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The GC had a two-level structure, G generations each of T steps.
Steps are for aging within a generation, mostly to avoid premature
promotion.
Measurements show that more than 2 steps is almost never worthwhile,
and 1 step is usually worse than 2. In theory fractional steps are
possible, so the ideal number of steps is somewhere between 1 and 3.
GHC's default has always been 2.
We can implement 2 steps quite straightforwardly by having each block
point to the generation to which objects in that block should be
promoted, so blocks in the nursery point to generation 0, and blocks
in gen 0 point to gen 1, and so on.
This commit removes the explicit step structures, merging generations
with steps, thus simplifying a lot of code. Performance is
unaffected. The tunable number of steps is now gone, although it may
be replaced in the future by a way to tune the aging in generation 0.
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At the moment, this just saves a memory reference in the GC inner loop
(worth a percent or two of GC time). Later, it will hopefully let me
experiment with partial steps, and simplifying the generation/step
infrastructure.
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The first phase of this tidyup is focussed on the header files, and in
particular making sure we are exposinng publicly exactly what we need
to, and no more.
- Rts.h now includes everything that the RTS exposes publicly,
rather than a random subset of it.
- Most of the public header files have moved into subdirectories, and
many of them have been renamed. But clients should not need to
include any of the other headers directly, just #include the main
public headers: Rts.h, HsFFI.h, RtsAPI.h.
- All the headers needed for via-C compilation have moved into the
stg subdirectory, which is self-contained. Most of the headers for
the rest of the RTS APIs have moved into the rts subdirectory.
- I left MachDeps.h where it is, because it is so widely used in
Haskell code.
- I left a deprecated stub for RtsFlags.h in place. The flag
structures are now exposed by Rts.h.
- Various internal APIs are no longer exposed by public header files.
- Various bits of dead code and declarations have been removed
- More gcc warnings are turned on, and the RTS code is more
warning-clean.
- More source files #include "PosixSource.h", and hence only use
standard POSIX (1003.1c-1995) interfaces.
There is a lot more tidying up still to do, this is just the first
pass. I also intend to standardise the names for external RTS APIs
(e.g use the rts_ prefix consistently), and declare the internal APIs
as hidden for shared libraries.
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When a stack is occupying less than 1/4 of the memory it owns, and is
larger than a megablock, we release half of it. Shrinking is O(1), it
doesn't need to copy the stack.
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eg. use +RTS -g2 -RTS for 2 threads. Only major GCs are parallelised,
minor GCs are still sequential. Don't use more threads than you
have CPUs.
It works most of the time, although you won't see much speedup yet.
Tuning and more work on stability still required.
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The main goal here is to reduce fragmentation, which turns out to be
the case of #743. While I was here I found some opportunities to
improve performance too. The code is rather more complex, but it also
contains a long comment describing the strategy, so please take a look
at that for the details.
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In the free list, we don't strictly speaking need to have every block
in a coalesced group point to the head block, although this is an
invariant for non-free blocks. Dropping this invariant for the free
list means that coalesce() is O(1) rather than O(N), and freeGroup()
is therefore O(N) not O(N^2).
The bad case probably didn't happen most of the time, indeed it has
never shown up in a profile that I've seen. I had a report from a
while back that this was a problem with really large heaps, though.
Fortunately the fix is easy.
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In preparation for parallel GC, split up the monolithic GC.c file into
smaller parts. Also in this patch (and difficult to separate,
unfortunatley):
- Don't include Stable.h in Rts.h, instead just include it where
necessary.
- consistently use STATIC_INLINE in source files, and INLINE_HEADER
in header files. STATIC_INLINE is now turned off when DEBUG is on,
to make debugging easier.
- The GC no longer takes the get_roots function as an argument.
We weren't making use of this generalisation.
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