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This reverts commit d85044f6b201eae0a9e453b89c0433608e0778f0.
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When servicing a stack overflows, only throw an exception to the given
thread if the user explicitly set a max stack size, using +RTS -K.
Otherwise just service it normally and grow the stack.
In case we actually run out of *heap* (stack chuncks are allocated on
the heap), then we need to bail by calling the stackOverflow() hook and
exit immediately.
Authored-by: Ben Gamari <bgamari.foss@gmail.com>
Signed-off-by: Austin Seipp <aseipp@pobox.com>
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We have various problems with reallocating the array of Capabilities,
due to threads in waitForReturnCapability that are already holding a
pointer to a Capability.
Rather than add more locking to make this safer, I decided it would be
easier to ensure that we never move the Capabilities at all. The
capabilities array is now an array of pointers to Capabaility. There
are extra indirections, but it rarely matters - we don't often access
Capabilities via the array, normally we already have a pointer to
one. I ran the parallel benchmarks and didn't see any difference.
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We add the invariant to the MVar blocked threads queue that
threads blocked on an atomic read are always at the front of
the queue. This invariant is easy to maintain, since takers
are only ever added to the end of the queue.
Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
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Previously, stacks were always attributed to CCCS_SYSTEM. Now, we
attribute them to the CCS when the stack was allocated. If a stack
grows, new stack chunks inherit the CCS of the old stack.
Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
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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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If the old stack is only half full, then the next chunk we allocate
will be twice as large, to accommodate large requests for stack. In
that case, make sure that the chunk we allocate is at least as large
as the usual chunk size - there's no point allocating a 2k chunk
(double the default initial chunk size of 1k) if in the normal case we
would allocate a 32k chunk.
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If we overflow the current stack chunk and copy its entire contents
into the next stack chunk, we could end up with two UNDERFLOW_FRAMEs.
We had a special case to catch this in the case when the old stack
chunk was the last one (ending in STOP_FRAME), but it went wrong for
other chunks.
I found this bug while poking around in the core dump attached to
options and running the nofib suite: imaginary/wheel_sieve2 crashed
with +RTS -kc600 -kb300.
I don't know if this is the cause of all the symptoms reported in
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pseudo-register
Needed by #5357
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Terminology cleanup: the type "Ticks" has been renamed "Time", which
is an StgWord64 in units of TIME_RESOLUTION (currently nanoseconds).
The terminology "tick" is now used consistently to mean the interval
between timer signals.
The ticker now always ticks in realtime (actually CLOCK_MONOTONIC if
we have it). Before it used CPU time in the non-threaded RTS and
realtime in the threaded RTS, but I've discovered that the CPU timer
has terrible resolution (at least on Linux) and isn't much use for
profiling. So now we always use realtime. This should also fix
The default tick interval is now 10ms, except when profiling where we
drop it to 1ms. This gives more accurate profiles without affecting
runtime too much (<1%).
Lots of cleanups - the resolution of Time is now in one place
only (Rts.h) rather than having calculations that depend on the
resolution scattered all over the RTS. I hope I found them all.
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Now we keep any partially-full blocks in the gc_thread[] structs after
each GC, rather than moving them to the generation. This should give
us slightly better locality (though I wasn't able to measure any
difference).
Also in this patch: better sanity checking with THREADED.
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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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This is patch that adds support for interruptible FFI calls in the form
of a new foreign import keyword 'interruptible', which can be used
instead of 'safe' or 'unsafe'. Interruptible FFI calls act like safe
FFI calls, except that the worker thread they run on may be interrupted.
Internally, it replaces BlockedOnCCall_NoUnblockEx with
BlockedOnCCall_Interruptible, and changes the behavior of the RTS
to not modify the TSO_ flags on the event of an FFI call from
a thread that was interruptible. It also modifies the bytecode
format for foreign call, adding an extra Word16 to indicate
interruptibility.
The semantics of interruption vary from platform to platform, but the
intent is that any blocking system calls are aborted with an error code.
This is most useful for making function calls to system library
functions that support interrupting. There is no support for pre-Vista
Windows.
There is a partner testsuite patch which adds several tests for this
functionality.
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This was leading to looping and excessive allocation, when the
computation should have just blocked on the black hole.
Reported by Christian Höner zu Siederdissen <choener@tbi.univie.ac.at>
on glasgow-haskell-users.
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The list of threads blocked on an MVar is now represented as a list of
separately allocated objects rather than being linked through the TSOs
themselves. This lets us remove a TSO from the list in O(1) time
rather than O(n) time, by marking the list object. Removing this
linear component fixes some pathalogical performance cases where many
threads were blocked on an MVar and became unreachable simultaneously
(nofib/smp/threads007), or when sending an asynchronous exception to a
TSO in a long list of thread blocked on an MVar.
MVar performance has actually improved by a few percent as a result of
this change, slightly to my surprise.
This is the final cleanup in the sequence, which let me remove the old
way of waking up threads (unblockOne(), MSG_WAKEUP) in favour of the
new way (tryWakeupThread and MSG_TRY_WAKEUP, which is idempotent). It
is now the case that only the Capability that owns a TSO may modify
its state (well, almost), and this simplifies various things. More of
the RTS is based on message-passing between Capabilities now.
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This replaces the global blackhole_queue with a clever scheme that
enables us to queue up blocked threads on the closure that they are
blocked on, while still avoiding atomic instructions in the common
case.
Advantages:
- gets rid of a locked global data structure and some tricky GC code
(replacing it with some per-thread data structures and different
tricky GC code :)
- wakeups are more prompt: parallel/concurrent performance should
benefit. I haven't seen anything dramatic in the parallel
benchmarks so far, but a couple of threading benchmarks do improve
a bit.
- waking up a thread blocked on a blackhole is now O(1) (e.g. if
it is the target of throwTo).
- less sharing and better separation of Capabilities: communication
is done with messages, the data structures are strictly owned by a
Capability and cannot be modified except by sending messages.
- this change will utlimately enable us to do more intelligent
scheduling when threads block on each other. This is what started
off the whole thing, but it isn't done yet (#3838).
I'll be documenting all this on the wiki in due course.
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This replaces some complicated locking schemes with message-passing
in the implementation of throwTo. The benefits are
- previously it was impossible to guarantee that a throwTo from
a thread running on one CPU to a thread running on another CPU
would be noticed, and we had to rely on the GC to pick up these
forgotten exceptions. This no longer happens.
- the locking regime is simpler (though the code is about the same
size)
- threads can be unblocked from a blocked_exceptions queue without
having to traverse the whole queue now. It's a rare case, but
replaces an O(n) operation with an O(1).
- generally we move in the direction of sharing less between
Capabilities (aka HECs), which will become important with other
changes we have planned.
Also in this patch I replaced several STM-specific closure types with
a generic MUT_PRIM closure type, which allowed a lot of code in the GC
and other places to go away, hence the line-count reduction. The
message-passing changes resulted in about a net zero line-count
difference.
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The idea is that this leaves Tasks and OSThread in one-to-one
correspondence. The part of a Task that represents a call into
Haskell from C is split into a separate struct InCall, pointed to by
the Task and the TSO bound to it. A given OSThread/Task thus always
uses the same mutex and condition variable, rather than getting a new
one for each callback. Conceptually it is simpler, although there are
more types and indirections in a few places now.
This improves callback performance by removing some of the locks that
we had to take when making in-calls. Now we also keep the current Task
in a thread-local variable if supported by the OS and gcc (currently
only Linux).
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- Defines a DTrace provider, called 'HaskellEvent', that provides a probe
for every event of the eventlog framework.
- In contrast to the original eventlog, the DTrace probes are available in
all flavours of the runtime system (DTrace probes have virtually no
overhead if not enabled); when -DTRACING is defined both the regular
event log as well as DTrace probes can be used.
- Currently, Mac OS X only. User-space DTrace probes are implemented
differently on Mac OS X than in the original DTrace implementation.
Nevertheless, it shouldn't be too hard to enable these probes on other
platforms, too.
- Documentation is at http://hackage.haskell.org/trac/ghc/wiki/DTrace
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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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This is a batch of refactoring to remove some of the GC's global
state, as we move towards CPU-local GC.
- allocateLocal() now allocates large objects into the local
nursery, rather than taking a global lock and allocating
then in gen 0 step 0.
- allocatePinned() was still allocating from global storage and
taking a lock each time, now it uses local storage.
(mallocForeignPtrBytes should be faster with -threaded).
- We had a gen 0 step 0, distinct from the nurseries, which are
stored in a separate nurseries[] array. This is slightly strange.
I removed the g0s0 global that pointed to gen 0 step 0, and
removed all uses of it. I think now we don't use gen 0 step 0 at
all, except possibly when there is only one generation. Possibly
more tidying up is needed here.
- I removed the global allocate() function, and renamed
allocateLocal() to allocate().
- the alloc_blocks global is gone. MAYBE_GC() and
doYouWantToGC() now check the local nursery only.
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- tracing facilities are now enabled with -DTRACING, and -DDEBUG
additionally enables debug-tracing. -DEVENTLOG has been
removed.
- -debug now implies -eventlog
- events can be printed to stderr instead of being sent to the
binary .eventlog file by adding +RTS -v (which is implied by the
+RTS -Dx options).
- -Dx debug messages can be sent to the binary .eventlog file
by adding +RTS -l. This should help debugging by reducing
the impact of debug tracing on execution time.
- Various debug messages that duplicated the information in events
have been removed.
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There were two bugs, and had it not been for the first one we would
not have noticed the second one, so this is quite fortunate.
The first bug is in stg_unblockAsyncExceptionszh_ret, when we found a
pending exception to raise, but don't end up raising it, there was a
missing adjustment to the stack pointer.
The second bug was that this case was actually happening at all: it
ought to be incredibly rare, because the pending exception thread
would have to be killed between us finding it and attempting to raise
the exception. This made me suspicious. It turned out that there was
a race condition on the tso->flags field; multiple threads were
updating this bitmask field non-atomically (one of the bits is the
dirty-bit for the generational GC). The fix is to move the dirty bit
into its own field of the TSO, making the TSO one word larger (sadly).
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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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This is not a bug fix, it just makes better use of memory
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Generate binary log files from the RTS containing a log of runtime
events with timestamps. The log file can be visualised in various
ways, for investigating runtime behaviour and debugging performance
problems. See for example the forthcoming ThreadScope viewer.
New GHC option:
-eventlog (link-time option) Enables event logging.
+RTS -l (runtime option) Generates <prog>.eventlog with
the binary event information.
This replaces some of the tracing machinery we already had in the RTS:
e.g. +RTS -vg for GC tracing (we should do this using the new event
logging instead).
Event logging has almost no runtime cost when it isn't enabled, though
in the future we might add more fine-grained events and this might
change; hence having a link-time option and compiling a separate
version of the RTS for event logging. There's a small runtime cost
for enabling event-logging, for most programs it shouldn't make much
difference.
(Todo: docs)
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This reduces the latency between a context-switch being triggered and
the thread returning to the scheduler, which in turn should reduce the
cost of the GC barrier when there are many cores.
We still retain the old context_switch flag which is checked at the
end of each block of allocation. The idea is that setting HpLim may
fail if the the target thread is modifying HpLim at the same time; the
context_switch flag is a fallback. It also allows us to "context
switch soon" without forcing an immediate switch, which can be costly.
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Fixes a long-standing bug that could in some cases cause sub-optimal
scheduling behaviour.
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wakeupThreadOnCapbility() is used to signal another capability that
there is a thread waiting to be added to its run queue. It adds the
thread to the (locked) wakeup queue on the remote capability. In
order to do this, it has to modify the TSO's link field, which has a
write barrier. The write barrier might put the TSO on the mutable
list, and the bug was that it was using the mutable list of the
*target* capability, which we do not have exclusive access to. We
should be using the current Capabilty's mutable list in this case.
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Instead of keeping a single list of all threads, keep one per step
and only look at the threads belonging to steps that we are
collecting.
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