Re: [RFC]Proposal for better garbage collection
On 02/22/2012 08:40 PM, H. S. Teoh wrote:
This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
Those are the same thing. '{ }' is not what introduces a scope.
Re: [RFC]Proposal for better garbage collection
Le 23/02/2012 20:58, H. S. Teoh a écrit : On Thu, Feb 23, 2012 at 01:51:31PM +0200, Manu wrote: [...] I wonder if there are alternative ways to detect a foreign stack. And I'm not sure why it even matters, you can't depend on the extern ABI, how do you unwind the stack reliably in the first place? [...] This is a bit off-topic, but what happens in the current implementation if you pass a D callback to a C function, and then throw an exception from the callback? Does it work? Or does it do something really nasty? T This will work, but has serious drawbacks. First of all, you are not sure the C function will release all resources (free, fclose, etc . . .). So you cannot be sure of the state of your program. This is something that you don't want to do.
Re: [RFC]Proposal for better garbage collection
On Thu, Feb 23, 2012 at 09:18:22PM +0100, Artur Skawina wrote: > On 02/23/12 20:58, H. S. Teoh wrote: [...] > > This is a bit off-topic, but what happens in the current > > implementation if you pass a D callback to a C function, and then > > throw an exception from the callback? Does it work? Or does it do > > something really nasty? > > No, unless you consider a segfault to be really nasty. :) Well, segfaults are nasty, but there are nastier things. :) > Actually, it mostly works - i just tried it in a gtk app, and it works > as long as you catch the exception and only look at the error msg. If > you don't catch it (or try writeln(e) etc), then the result is > something like: > > action.Action!(int).Action.registerNS.MissingActionEx@action.d(54): Action > "GUI" missing symbol 'int AppWindowClosed()' > > ./gtkapp() [0x8054366] > /usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x153052) [0xf7375052] > /usr/lib/i686/sse2/libgobject-2.0.so.0(g_closure_invoke+0x19b) [0xf71e25fd] > /usr/lib/i686/sse2/libgobject-2.0.so.0(+0x1ddc8) [0xf71f2dc8] > /usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit_valist+0x59a) > [0xf71fa37f] > /usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit+0x34) [0xf71fa61f] > /usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x2a17f3) [0xf74c37f3] > /usr/lib/i686/sse2/libgtk-x11-2.0.so.0(gtk_main_do_event+0x8e6) [0xf7373856] > Segmentation fault > > So something appears to get confused while walking the stack; another > thing to investigate later, i guess... [...] Looks like it got confused at the cross-language boundary. T -- Prosperity breeds contempt, and poverty breeds consent. -- Suck.com
Re: [RFC]Proposal for better garbage collection
On 02/23/12 20:58, H. S. Teoh wrote: > On Thu, Feb 23, 2012 at 01:51:31PM +0200, Manu wrote: > [...] >> I wonder if there are alternative ways to detect a foreign stack. And >> I'm not sure why it even matters, you can't depend on the extern ABI, >> how do you unwind the stack reliably in the first place? > [...] > > This is a bit off-topic, but what happens in the current implementation > if you pass a D callback to a C function, and then throw an exception > from the callback? Does it work? Or does it do something really nasty? No, unless you consider a segfault to be really nasty. :) Actually, it mostly works - i just tried it in a gtk app, and it works as long as you catch the exception and only look at the error msg. If you don't catch it (or try writeln(e) etc), then the result is something like: action.Action!(int).Action.registerNS.MissingActionEx@action.d(54): Action "GUI" missing symbol 'int AppWindowClosed()' ./gtkapp() [0x8054366] /usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x153052) [0xf7375052] /usr/lib/i686/sse2/libgobject-2.0.so.0(g_closure_invoke+0x19b) [0xf71e25fd] /usr/lib/i686/sse2/libgobject-2.0.so.0(+0x1ddc8) [0xf71f2dc8] /usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit_valist+0x59a) [0xf71fa37f] /usr/lib/i686/sse2/libgobject-2.0.so.0(g_signal_emit+0x34) [0xf71fa61f] /usr/lib/i686/sse2/libgtk-x11-2.0.so.0(+0x2a17f3) [0xf74c37f3] /usr/lib/i686/sse2/libgtk-x11-2.0.so.0(gtk_main_do_event+0x8e6) [0xf7373856] Segmentation fault So something appears to get confused while walking the stack; another thing to investigate later, i guess... artur
Re: [RFC]Proposal for better garbage collection
On Thu, Feb 23, 2012 at 01:51:31PM +0200, Manu wrote: [...] > I wonder if there are alternative ways to detect a foreign stack. And > I'm not sure why it even matters, you can't depend on the extern ABI, > how do you unwind the stack reliably in the first place? [...] This is a bit off-topic, but what happens in the current implementation if you pass a D callback to a C function, and then throw an exception from the callback? Does it work? Or does it do something really nasty? T -- Elegant or ugly code as well as fine or rude sentences have something in common: they don't depend on the language. -- Luca De Vitis
Re: [RFC]Proposal for better garbage collection
On Thu, Feb 23, 2012 at 06:01:48PM +0100, deadalnix wrote: [...] > Additionnaly, the stack is made like a linked list. Each function > calling another one register the return address. With this > information, we can have data about what is on the stack except for > the very last function called with no runtime overhead. This is > another alternative. Yep. In one of my replies I considered the possibility of storing a function ID on the stack, but that may not be necessary if the GC has access to compile-time static info about each function, so just by seeing the return address it knows which function it is, and can figure out where the pointers are. (Of course there are other issues that need to be addressed... but we can't decide on that without actual data.) > But you have to consider that, even with a mask, you are not sure > that what is marked as a pointer is a pointer. A memory location can > represent different thing during a function execution. So thoses > values can only be considered as probable pointers, or we disable > some compiler optimizations. As we cannot be sure, the point 2/ stay > valid. I believe his proposal was for the function to manually update these bits as it runs. It does introduce a lot of overhead. And like you said, without actual hard data to show whether or not this overhead is justified (offset by improved GC performance), how do we know that we should do this at all? How do we know we aren't making it worse? > Granted the overhead of the operation, it ay not worth it. To know > that, we need actual data on how much data is the stack is actually > pointer, and how much false positive we get. As the future is 64bits, > I'm not sure it is interesting for us. Actually, I believe David Simcha *is* considering the possibility of precise scanning. But the proof is in the actual benchmarks. We don't know if it will help or not unless we have real data to back it up. T -- Real Programmers use "cat > a.out".
Re: [RFC]Proposal for better garbage collection
You didn't mention what is the most important IMO. In D, most data are thread local. Shared data are either shared or immutable. Both thread local data and immutable data lead to very interesting GC optimisations. This is where we need language support.
Re: [RFC]Proposal for better garbage collection
Le 22/02/2012 20:53, Benjamin Thaut a écrit : If you have a better idea for percise stack scanning I'm open for suggestions. This is the problem with your proposal. It doesn't consider pro and cons and actual data. It doesn't consider the alternatives. You go straight to « How can we do that ? » without condidering « should we do that ? » What would be the impact of being precise on the heap but not on the stack ? 1/ It would add some false positives. The future being 64bits, False positive will be way less present than on 32bits machines. I did searched for numbers on that, but couldn't found them. Considering this is only on the stack, this may be neglectible (or not, but it definitively require data). 2/ Data pointed by the stack are not movable.Again, what is the impact of that. How much data could be promoted from young generation to old one (considering we have young and old gen). How much data couldn't be compacted ? What would the overhead on allocators ? This definitively lack the required data and/or analysis of pro and cons. Additionnaly, the stack is made like a linked list. Each function calling another one register the return address. With this information, we can have data about what is on the stack except for the very last function called with no runtime overhead. This is another alternative. But you have to consider that, even with a mask, you are not sure that what is marked as a pointer is a pointer. A memory location can represent different thing during a function execution. So thoses values can only be considered as probable pointers, or we disable some compiler optimizations. As we cannot be sure, the point 2/ stay valid. Granted the overhead of the operation, it ay not worth it. To know that, we need actual data on how much data is the stack is actually pointer, and how much false positive we get. As the future is 64bits, I'm not sure it is interesting for us.
Re: [RFC]Proposal for better garbage collection
On Wed, 22 Feb 2012 15:32:37 -0600, H. S. Teoh wrote: On Wed, Feb 22, 2012 at 07:56:15PM +0100, Benjamin Thaut wrote: [...] 2) Tracking references on the stack: The D compiler always needs to emit a full stack frame so that the GC can walk up the stack at any time in the program. The stack frame of every function generated by the D compiler starts which a bitfield (usually the size of a machine register) where each bit indicates that these bytes are a pointer / reference. The bitfield needs to be large enough to cover the whole stack frame of the function. [...] I was thinking about this a bit more, and I had an idea: why bother with storing bitfields on the stack? Any function's local pointer variables are known at compile-time. So store a function ID (probably a pointer) that maps to some static storage where this information is stored. Then we always only need 1 word of extra storage on the stack frame, and the GC can follow the pointer to get the info it needs. A recursively called function won't incur the cost of duplicated copies of bitfields, its ID points to same place. You can even have two different functions share the same ID if they have pointer variables in exactly the same places. The static storage can then be an array of relative stack offsets to the function's pointer variables, so the GC can easily use this info to find roots. No need to complicate the GC with manipulating bitfields, it's just an int[]. If you want to get fancy, have the compiler reorder local variables so that pointers are clustered together in blocks, then in the static storage you can just encode pointer blocks by offset+length. (Although this may not help much with (pointer,length) pairs on the stack, like slices.) Or the compiler can reorder variables to maximize ID merges. The same thing can be done for scopes, since their local variables are also all known at compile-time. T The break even point between bit-fields and pointers is 512 bytes. Although, if one is thinking about on stack storage this probably doesn't matter since for alignment purposes you'll always end up using at least 1 word if not 2. However, a lot of functions use less then 512 (or 1024) bytes of of stack space. I'd think it would be much more space efficient to have a separate bitfield for the stack. Cache efficiency should be about the same as a on stack representation, and scanning would, in theory, be quicker. IIRC, a separate bit-field was the approach used by at least one precise C GC.
Re: [RFC]Proposal for better garbage collection
On 22 February 2012 20:56, Benjamin Thaut wrote: > 2) Tracking references on the stack: > > The D compiler always needs to emit a full stack frame so that the GC can > walk up the stack at any time in the program. You say "every function needs a stack frame". Can you comment on this with respect to leaf functions? No leaf function should ever generate a stack frame, infact, typical leaf functions will never touch memory at all. This is VERY IMPORTANT for critical loops. I have never worked on a project where I did not depend on the performance of leaf functions to do the hard work in the most critical parts of my application. Obviously such functions would not be making allocations, and shouldn't be interacting with the GC any way, so why is having a stack frame important? > The stack frame of every function generated by the D compiler starts which > a bitfield (*usually the size of a machine register*)... > Oh really? And how do we define that type? ;) (*cough* reference to size_t/ptrdiff_t thread) > For example on x86: 11001... > 1 = bytes 0 to 4 are a pointer > 1 = bytes 4 to 8 are a pointer > 00 = bytes 8 to 16 are not a pointer > 1 = bytes 16 to 20 are a pointer > > The last bit indicates whether the bitfield is continued in the following > bytes or not. 1 means continued 0 means finished. > Every scope generated by the D compiler would need additional code at the > start and end of the scope. When the scope is entered the bitfield would be > patched to represent the new variables inside the scope and when the scope > is left the bitfield is patched again to remove the changes that were made > on entering the scope. > Every time a function gets called that did not get generated by the D > compiler ( C / C++ etc functions) the compiler generates a call into the > runtime and passes the current stack pointer and stack base pointer to it. > > void _d_externalCallStart(void* stackptr, void* baseptr); > > Every time such a function returns the compiler generates a call into > the the runtime too. > > void _d_externalCallEnd(void* stackptr, void* baseptr); > Can you comment on what those functions will actually do? It definitely sounds very worrying to me to be turning every function call into THREE calls. Calling into extern code is certainly not rare... almost everything of any use is an extern C lib. What about interaction with the OS? Trivial libs like zlib? etc... I wonder if there are alternative ways to detect a foreign stack. And I'm not sure why it even matters, you can't depend on the extern ABI, how do you unwind the stack reliably in the first place? > Every time a functions that can get called from other languages (extern(C) > etc) are executed the end callback is inserted at the start of the > functions and the start callback is inserted at the end of the function. > Using these callbacks the GC can mark certain parts of the stack as > "non-D" and ignore them when scanning for bit fields and > references/pointers. It can just skip parts of the stack that are "non-D" > and therefore does not need a full stack frame within these "non-D" > sections. > All these features are required so that the GC can precisely scan > pointers/references on the stack and change them as necessary. > Remaining issues: The D compiler can freely move around value types on the > stack. With such move operations it would be necessary to fix up all the > bit fields. I needs to be investigated if this is doable. > > 3) Tracking references on the heap > > For every class / struct a mixin template which is defined inside the > runtime gets instantiated. This template can then use introspection to > generate the necessary information to allow the GC to scan the pointers > within that struct / class precisely. > > 4) Thread local / global memory > > A callback into the runtime needs to happen in the following cases: > - a __gshared variable is assigned > - a reference / pointer is casted to immutable > - a reference / pointer is casted to shared > > void _d_castToGlobalMem(void* ptr); > > This can be used by the GC to keep thread local pools of memory and move a > memory block to a global memory pool as soon as it is needed there. > > 5) pointer / reference changed callback > > Every time a pointer / reference is changed the D compiler emits a call > into the runtime and passes the new value of the reference / pointer with > it. > > void _d_pointerChanged(void *ptr); > > This can be used when writing a generational GC to have separate pools for > young and old generations. Every time the young generation needs to be > collected it can be avoided to scan the old generations pool because it is > sufficient to only check the pointers that have changed since the last time > the young generation was collected. With the above mentioned callback it is > easily possible to track these references. > > Remaining issues: > -If the GC interrupts a thread right before any of the above mentioned > callbacks happen it will
Re: [RFC]Proposal for better garbage collection
On Feb 22, 2012, at 8:07 PM, "Martin Nowak" wrote: >> 4) Thread local / global memory >> >> A callback into the runtime needs to happen in the following cases: >> - a __gshared variable is assigned >> - a reference / pointer is casted to immutable >> - a reference / pointer is casted to shared >> >> void _d_castToGlobalMem(void* ptr); >> >> This can be used by the GC to keep thread local pools of memory and move a >> memory block to a global memory pool as soon as it is needed there. >> > If we'd want per-thread mark/sweeping then shared memory must never own > unshared memory. I think this could be done using a separate allocator for > shared/immutable data. For casts this would require a transitive move of the > data or it'd need to be prohibited. Casting to/from shared needs a bit more logic anyway, so the proper thread finalizes unshared objects. Casting away shared clears the block's owner and vice versa. Nice perk to this is casting away shared could then detect a sharing violation if a block already has another owner.
Re: [RFC]Proposal for better garbage collection
On 2012-02-22 20:40, H. S. Teoh wrote:
On Wed, Feb 22, 2012 at 07:56:15PM +0100, Benjamin Thaut wrote:
As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the
possible issues with garbage collection I could think of and
possible solutions for them. This document is just a draft and only
a proposal, critics and comments are welcome.
Have you seen this?
http://www.llucax.com.ar/proj/dgc/index.html
[...]
2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.
This adds a lot of overhead to the runtime stack, esp. if you have deep
recursion. It's also not necessarily faster, since the GC now has to
parse a bitfield (a variable-length encoded bitfield, no less), instead
of just scanning words directly, which can be optimized by CPU-specific
microcode depending on the target platform.
[...]
Every scope generated by the D compiler would need additional code
at the start and end of the scope. When the scope is entered the
bitfield would be patched to represent the new variables inside the
scope and when the scope is left the bitfield is patched again to
remove the changes that were made on entering the scope.
This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
which will encourage people to omit {} after if, which makes code more
fragile and hard to read.
Doesn't the "faster" example introduces an implicit scope?
--
/Jacob Carlborg
Re: [RFC]Proposal for better garbage collection
Am 22.02.2012 22:32, schrieb H. S. Teoh:
On Wed, Feb 22, 2012 at 07:56:15PM +0100, Benjamin Thaut wrote:
[...]
2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.
[...]
I was thinking about this a bit more, and I had an idea: why bother with
storing bitfields on the stack? Any function's local pointer variables
are known at compile-time. So store a function ID (probably a pointer)
that maps to some static storage where this information is stored. Then
we always only need 1 word of extra storage on the stack frame, and the
GC can follow the pointer to get the info it needs. A recursively called
function won't incur the cost of duplicated copies of bitfields, its ID
points to same place. You can even have two different functions share
the same ID if they have pointer variables in exactly the same places.
The static storage can then be an array of relative stack offsets to the
function's pointer variables, so the GC can easily use this info to find
roots. No need to complicate the GC with manipulating bitfields, it's
just an int[].
If you want to get fancy, have the compiler reorder local variables so
that pointers are clustered together in blocks, then in the static
storage you can just encode pointer blocks by offset+length. (Although
this may not help much with (pointer,length) pairs on the stack, like
slices.) Or the compiler can reorder variables to maximize ID merges.
The same thing can be done for scopes, since their local variables are
also all known at compile-time.
T
But where would you know from which scope variables are still (or
already) valid and which are not?
void func()
{
void* ptr = gc.alloc(...);
//Ptr2, Ptr3 not valid yet
void* ptr2 = gc.alloc(...);
//ptr3 not valid yet
{
void* ptr3 = ptr1;
}
//ptr 3 not valid anymore
}
Also as the bitfiel is stored on the stack it will most likely ba
already in the cache. Whereas with your approach scanning 1 stackframe
would very likely also cause 1 cache miss because of the additional
indirection. So if you are scanning 30 stack frames it will cause 30
cache misses.
--
Kind Regards
Benjamin Thaut
Re: [RFC]Proposal for better garbage collection
4) Thread local / global memory A callback into the runtime needs to happen in the following cases: - a __gshared variable is assigned - a reference / pointer is casted to immutable - a reference / pointer is casted to shared void _d_castToGlobalMem(void* ptr); This can be used by the GC to keep thread local pools of memory and move a memory block to a global memory pool as soon as it is needed there. If we'd want per-thread mark/sweeping then shared memory must never own unshared memory. I think this could be done using a separate allocator for shared/immutable data. For casts this would require a transitive move of the data or it'd need to be prohibited.
Re: [RFC]Proposal for better garbage collection
I don't know, but after reading this: http://www.cs.purdue.edu/homes/grr/snapshot-gc.ps I think there might be a possibility of (almost) free GC. COWing whole pages is about the last thing you need with a high mutation rate.
Re: [RFC]Proposal for better garbage collection
On Wed, Feb 22, 2012 at 07:48:38PM -0500, Jonathan M Davis wrote: > On Wednesday, February 22, 2012 16:45:29 H. S. Teoh wrote: > > But still, some benchmarks do appear to be showing signs of a large > > performance hit on the GC when there happens to be many integers that > > look like valid pointers. > > As I understand it, this is mitigated considerably on 64-bit platforms > due to the large pointer size. [...] True. You'd have to deliberately want to break the GC in order for coincidental integer values to cause a significant problem. But this problem could be a major issue if D was to become a significant player on handhelds, which, if I understand correctly, are still largely running 32-bit CPUs. It's also a major problem on 16-bit platforms, but the only use of those that I can conceive of are toy applications so it's probably not worth the consideration. :) T -- That's not a bug; that's a feature!
Re: [RFC]Proposal for better garbage collection
On Wednesday, February 22, 2012 16:45:29 H. S. Teoh wrote: > But still, some benchmarks do appear to be showing signs of a large > performance hit on the GC when there happens to be many integers that > look like valid pointers. As I understand it, this is mitigated considerably on 64-bit platforms due to the large pointer size. - Jonathan M Davis
Re: [RFC]Proposal for better garbage collection
On Wed, Feb 22, 2012 at 03:05:38PM -0800, Sean Kelly wrote: > On Feb 22, 2012, at 10:56 AM, Benjamin Thaut wrote: > > > > 5) pointer / reference changed callback > > > > Every time a pointer / reference is changed the D compiler emits a > > call into the runtime and passes the new value of the reference / > > pointer with it. > > > > void _d_pointerChanged(void *ptr); > > D can call assembler, C routines like memset(), plain old opaque C > library code, etc. What should the D compiler do in light of all the > sources of memory changes that it can't monitor? [...] Yeah, the GC should be capable of dealing with non-@safe code. Otherwise it would just be too limited to be used for large non-trivial D projects. But still, some benchmarks do appear to be showing signs of a large performance hit on the GC when there happens to be many integers that look like valid pointers. This may be beyond the programmer's control, since it could be the OS that gives the GC an address segment that just happens to span integer values very commonly used throughout the app. T -- What doesn't kill me makes me stranger.
Re: [RFC]Proposal for better garbage collection
On Feb 22, 2012, at 10:56 AM, Benjamin Thaut wrote: > > 5) pointer / reference changed callback > > Every time a pointer / reference is changed the D compiler emits a call into > the runtime and passes the new value of the reference / pointer with it. > > void _d_pointerChanged(void *ptr); D can call assembler, C routines like memset(), plain old opaque C library code, etc. What should the D compiler do in light of all the sources of memory changes that it can't monitor? > 6) Different interface for the GC > > The current interface to the GC would have to change because the "this block > of memory might contain a pointer" approach wouldn't work anymore. For > example a block of memory and a delegate which iterates over all pointers > within the memory block could be used for user allocated memory blocks. There > should be a separate allocator function provided by the GC that allocates > memory that does not get moved around so it can be used to pass it to non > garbage collected code. I posted a suggested new GC interface do the runtime mailing list 6 or so months ago. In short, I do think the current interface is lacking. Also, CDGC does support precise scanning and runs with Druntime. The big problem there is that CDGC is based on the Tango GC (where Druntime's GC originated) and someone needs to review all the GC changes since the Druntime project was created to see what may need to be merged into CDGC. I started on this once, but it turned out to be more work than I had time for.
Re: [RFC]Proposal for better garbage collection
On Thu, Feb 23, 2012 at 12:03:59AM +0400, Dmitry Olshansky wrote: [...] > >7) Compiler Options > > > >Each of the above mentioned groups of features should be exposed as > >compiler options so that you can turn them on/off depending on which > >type of GC you use. Default on/off states for these features are set > >within a config file depending on which type of GC currently ships > >per default with druntime. > > Combinatorial explosion of sets of options that doesn't necessary > allow a particular GC? Yeah, this is not a good way to go. Better would be for each GC come with a GC description file, that describes what hooks/info it needs from the compiler. Then the compiler can read this description file (passed as a *single* compile flag) and do the right thing for that particular GC. It can even automatically link in that particular GC without needing you to specify anything further. The GC description file can contain info like: - Where/when to insert calls to GC functions (e.g., start collect cycle, start mark cycle) - Which function to use for allocating memory - Any additional info required: - e.g., map of each function's pointer local variables so that the GC knows where the roots are; - Whether or not hooks are needed for function entry/exit, and which GC function to map them to; - Any additional GC-specific info to insert into functions / structs / etc.. - Whether or not pointer reads/writes need to include GC-specific code (the description can include code to insert, if needed). - Path to GC source(s) to be compiled into the program. T -- If Java had true garbage collection, most programs would delete themselves upon execution. -- Robert Sewell
Re: [RFC]Proposal for better garbage collection
On Wed, Feb 22, 2012 at 07:56:15PM +0100, Benjamin Thaut wrote: [...] > 2) Tracking references on the stack: > > The D compiler always needs to emit a full stack frame so that the > GC can walk up the stack at any time in the program. The stack frame > of every function generated by the D compiler starts which a > bitfield (usually the size of a machine register) where each bit > indicates that these bytes are a pointer / reference. The bitfield > needs to be large enough to cover the whole stack frame of the > function. [...] I was thinking about this a bit more, and I had an idea: why bother with storing bitfields on the stack? Any function's local pointer variables are known at compile-time. So store a function ID (probably a pointer) that maps to some static storage where this information is stored. Then we always only need 1 word of extra storage on the stack frame, and the GC can follow the pointer to get the info it needs. A recursively called function won't incur the cost of duplicated copies of bitfields, its ID points to same place. You can even have two different functions share the same ID if they have pointer variables in exactly the same places. The static storage can then be an array of relative stack offsets to the function's pointer variables, so the GC can easily use this info to find roots. No need to complicate the GC with manipulating bitfields, it's just an int[]. If you want to get fancy, have the compiler reorder local variables so that pointers are clustered together in blocks, then in the static storage you can just encode pointer blocks by offset+length. (Although this may not help much with (pointer,length) pairs on the stack, like slices.) Or the compiler can reorder variables to maximize ID merges. The same thing can be done for scopes, since their local variables are also all known at compile-time. T -- Spaghetti code may be tangly, but lasagna code is just cheesy.
Re: [RFC]Proposal for better garbage collection
On Wed, Feb 22, 2012 at 08:53:45PM +0100, Benjamin Thaut wrote:
> Am 22.02.2012 20:40, schrieb H. S. Teoh:
> >On Wed, Feb 22, 2012 at 07:56:15PM +0100, Benjamin Thaut wrote:
> >>As I'm not satisfied with the current GC D has and don't see the
> >>situation improving in the future without significant changes to the
> >>compiler I wrote the following document that points out all the
> >>possible issues with garbage collection I could think of and
> >>possible solutions for them. This document is just a draft and only
> >>a proposal, critics and comments are welcome.
> >
> >Have you seen this?
> >
> > http://www.llucax.com.ar/proj/dgc/index.html
>
> Yes I know about dgc it is better but still not on par with for
> example the GC that is shipped with the .NET 4.0
> All I'm saying is that without propper support from the compiler we
> are not going to get GCs as good as in other modern languages.
I agree. Better compiler support would definitely be beneficial.
[...]
> >>Every scope generated by the D compiler would need additional code
> >>at the start and end of the scope. When the scope is entered the
> >>bitfield would be patched to represent the new variables inside the
> >>scope and when the scope is left the bitfield is patched again to
> >>remove the changes that were made on entering the scope.
> >
> >This would introduce quite a lot of overhead per scope. It will also
> >lead to strange things like:
> >
> > if (x) y(); // faster
> > if (x) { y(); } // slower
> >
> >which will encourage people to omit {} after if, which makes code more
> >fragile and hard to read.
> >
>
> Scopeds that don't have variables declared inside them don't need
> the bitfield patching. so that argument is completely pointless.
> Scopes that contain varaibles that are not pointers or refrences
> also don't need the patching.
That wasn't clear from your description. It makes more sense now.
[...]
> >>5) pointer / reference changed callback
> >>
> >>Every time a pointer / reference is changed the D compiler emits a
> >>call into the runtime and passes the new value of the reference /
> >>pointer with it.
> >
> >This introduces a LOT of overhead, especially in a language like D which
> >manipulates a lot of pointers quite often (esp. if you use slices a
> >lot).
> >
>
> I did not make this up, I know a smalltalk implementation that
> actually does this and is pretty efficient.
OK.
[...]
> >>-If the GC interrupts a thread right before any of the above
> >>mentioned callbacks happen it will cause a invalid state for the GC
> >>and the GC might access invalid pointers. It has to be investigated
> >>if this leads to invalid behavior. It can be fixed by not
> >>interrupting a thread but pause it the next time it calls any of
> >>callbacks, or other functions that can be interrupted by the GC.
> >
> >This adds a lot of intermittent pauses in program execution.
>
> Why should there be pauses, there is just a additional check in
> every callback to the gc there already is. When the gc wants to
> collect he sets the pause flag to true and waits until all required
> threads paused themselfs.
Whereas with a scheme like dgc there is no need for threads to pause at
all.
> >The link I posted at the top has a GC implementation that doesn't
> >introduce *any* of this overhead (the GC runs concurrently with the
> >program), with no pause during a collection cycle (garbage is
> >incrementally collected when allocating new memory).
> >
>
> Any non percise scanning algorithms will not be able to deal with
> memory fragmentation
There are ways to deal with this. Though, granted, they're imperfect.
> and will also have uneccessary overhead for scanning regions of memory
> that don't contain any pointers.
True. But if the scanning is running in its own thread anyway, and no
other thread needs to wait for it, then this doesn't really matter, does
it?
> Also they can leak memory because some int value has the same value as
> a pointer and therefore the gc does not free that block of memory.
Yes, this is definitely a problem. It's not easy to fix this in a
language like D, though, without adding some overhead.
[...]
> >>8) Conclusion
> >>
> >>Garbage Collection brings a lot of advantages for the programmer
> >>using the language but is not free and shouldn't be treated as free.
> >
> >I don't know, but after reading this:
> >
> > http://www.cs.purdue.edu/homes/grr/snapshot-gc.ps
> >
> >I think there might be a possibility of (almost) free GC.
> >
>
> There is no free GC. The only question is which trade offs you want
> to make. Modern implementations like the .NET 4.0 garbage collector
> show that all the things mentioned here are possible and are faster
> then primitve implementations.
True. But then again, D's GC is only a simple implementation. Just
because an advanced implementation of a GC beats D's GC doesn't
necessarily mean that that particular implementation's GC model is the
best.
But I'm not trying
Re: [RFC]Proposal for better garbage collection
On 23.02.2012 0:03, Dmitry Olshansky wrote: On 22.02.2012 22:56, Benjamin Thaut wrote: As I'm not satisfied with the current GC D has and don't see the situation improving in the future without significant changes to the compiler I wrote the following document that points out all the possible issues with garbage collection I could think of and possible solutions for them. This document is just a draft and only a proposal, critics and comments are welcome. Kind Regards Benjamin Thaut BTW check out ideas on GC http://prowiki.org/wiki4d/wiki.cgi?GSOC_2012_Ideas
Re: [RFC]Proposal for better garbage collection
On 22.02.2012 22:56, Benjamin Thaut wrote: As I'm not satisfied with the current GC D has and don't see the situation improving in the future without significant changes to the compiler I wrote the following document that points out all the possible issues with garbage collection I could think of and possible solutions for them. This document is just a draft and only a proposal, critics and comments are welcome. Kind Regards Benjamin Thaut 1) Preface All modern Languages with fast and efficient GCs have build in support for garbage collection within the compiler / virtual machine. Currently the D language does have as much GC support as C++ has but fully relies on the GC with a lot of language features and the standard library. As a result the GC is slow, non parallel and always needs to stop all threads before collection. This document suggests to build in better support for garbage collection into the language so that creating a fast and efficient GC becomes possible. A list of features that would be required is described in this document. 2) Tracking references on the stack: The D compiler always needs to emit a full stack frame so that the GC can walk up the stack at any time in the program. I think walking up the stack to collect this info again and again (the stack has a lot of "heavy frames" on the bottom, right?) sounds like a tremendously slow way of getting necessary memory ranges. I'm no expert though. The stack frame of every function generated by the D compiler starts which a bitfield (usually the size of a machine register) where each bit indicates that these bytes are a pointer / reference. The bitfield needs to be large enough to cover the whole stack frame of the function. For example on x86: 11001... 1 = bytes 0 to 4 are a pointer 1 = bytes 4 to 8 are a pointer 00 = bytes 8 to 16 are not a pointer 1 = bytes 16 to 20 are a pointer The last bit indicates whether the bitfield is continued in the following bytes or not. 1 means continued 0 means finished. Every scope generated by the D compiler would need additional code at the start and end of the scope. When the scope is entered the bitfield would be patched to represent the new variables inside the scope and when the scope is left the bitfield is patched again to remove the changes that were made on entering the scope. Again I'm no expert, but what happens when GC starts collecting a thread stack amid this patching operation? Every time a function gets called that did not get generated by the D compiler ( C / C++ etc functions) the compiler generates a call into the runtime and passes the current stack pointer and stack base pointer to it. void _d_externalCallStart(void* stackptr, void* baseptr); Every time such a function returns the compiler generates a call into the the runtime too. void _d_externalCallEnd(void* stackptr, void* baseptr); Why would you need these? And if you start calling callback on every operation, you may just as well pass direct ranges of memory to GC without stack walk. + you can call D function from C one that in turn calls D one, think extern(C) and callbacks. Every time a functions that can get called from other languages (extern(C) etc) are executed the end callback is inserted at the start of the functions and the start callback is inserted at the end of the function. Using these callbacks the GC can mark certain parts of the stack as "non-D" and ignore them when scanning for bit fields and references/pointers. It can just skip parts of the stack that are "non-D" and therefore does not need a full stack frame within these "non-D" sections. All these features are required so that the GC can precisely scan pointers/references on the stack and change them as necessary. Remaining issues: The D compiler can freely move around value types on the stack. With such move operations it would be necessary to fix up all the bit fields. I needs to be investigated if this is doable. 3) Tracking references on the heap For every class / struct a mixin template which is defined inside the runtime gets instantiated. This template can then use introspection to generate the necessary information to allow the GC to scan the pointers within that struct / class precisely. 4) Thread local / global memory A callback into the runtime needs to happen in the following cases: - a __gshared variable is assigned - a reference / pointer is casted to immutable - a reference / pointer is casted to shared void _d_castToGlobalMem(void* ptr); This can be used by the GC to keep thread local pools of memory and move a memory block to a global memory pool as soon as it is needed there. 5) pointer / reference changed callback Every time a pointer / reference is changed the D compiler emits a call into the runtime and passes the new value of the reference / pointer with it. Bye-bye any speed of p++ ? I mean I'm horrified, and I bet I'm not alone. void _d_pointerChanged(void *ptr); This can be used when
Re: [RFC]Proposal for better garbage collection
Am 22.02.2012 20:40, schrieb H. S. Teoh:
On Wed, Feb 22, 2012 at 07:56:15PM +0100, Benjamin Thaut wrote:
As I'm not satisfied with the current GC D has and don't see the
situation improving in the future without significant changes to the
compiler I wrote the following document that points out all the
possible issues with garbage collection I could think of and
possible solutions for them. This document is just a draft and only
a proposal, critics and comments are welcome.
Have you seen this?
http://www.llucax.com.ar/proj/dgc/index.html
Yes I know about dgc it is better but still not on par with for example
the GC that is shipped with the .NET 4.0
All I'm saying is that without propper support from the compiler we are
not going to get GCs as good as in other modern languages.
[...]
2) Tracking references on the stack:
The D compiler always needs to emit a full stack frame so that the
GC can walk up the stack at any time in the program. The stack frame
of every function generated by the D compiler starts which a
bitfield (usually the size of a machine register) where each bit
indicates that these bytes are a pointer / reference. The bitfield
needs to be large enough to cover the whole stack frame of the
function.
This adds a lot of overhead to the runtime stack, esp. if you have deep
recursion. It's also not necessarily faster, since the GC now has to
parse a bitfield (a variable-length encoded bitfield, no less), instead
of just scanning words directly, which can be optimized by CPU-specific
microcode depending on the target platform.
If you have a better idea for percise stack scanning I'm open for
suggestions.
[...]
Every scope generated by the D compiler would need additional code
at the start and end of the scope. When the scope is entered the
bitfield would be patched to represent the new variables inside the
scope and when the scope is left the bitfield is patched again to
remove the changes that were made on entering the scope.
This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
which will encourage people to omit {} after if, which makes code more
fragile and hard to read.
Scopeds that don't have variables declared inside them don't need the
bitfield patching. so that argument is completely pointless. Scopes that
contain varaibles that are not pointers or refrences also don't need the
patching.
Every time a function gets called that did not get generated by the
D compiler ( C / C++ etc functions) the compiler generates a call
into the runtime and passes the current stack pointer and stack base
pointer to it.
void _d_externalCallStart(void* stackptr, void* baseptr);
Every time such a function returns the compiler generates a call
into the the runtime too.
void _d_externalCallEnd(void* stackptr, void* baseptr);
Every time a functions that can get called from other languages
(extern(C) etc) are executed the end callback is inserted at the
start of the functions and the start callback is inserted at the end
of the function.
Using these callbacks the GC can mark certain parts of the stack as
"non-D" and ignore them when scanning for bit fields and
references/pointers. It can just skip parts of the stack that are
"non-D" and therefore does not need a full stack frame within these
"non-D" sections.
This may not be a bad idea, though it does introduce some overhead when
you cross language boundaries.
All these features are required so that the GC can precisely scan
pointers/references on the stack and change them as necessary.
Remaining issues: The D compiler can freely move around value types
on the stack. With such move operations it would be necessary to fix
up all the bit fields. I needs to be investigated if this is doable.
This can only make the GC slower, especially if it needs to update
variable-length encoded bitfields. Of course, you may be able to offset
this by making it possible to do real-time GC, (reduced throughput but
less waiting time for collection cycles) but that's a very complex
problem.
3) Tracking references on the heap
For every class / struct a mixin template which is defined inside
the runtime gets instantiated. This template can then use
introspection to generate the necessary information to allow the GC
to scan the pointers within that struct / class precisely.
So basically you're proposing a compacting precise-scanning GC instead
of the current conservative GC. There are pros and cons in either
approach; it'd be nice if you could compare them.
I'm proposing a compacting percise-scanning generantional gc that has
thread local pools and can scan these thread local pools without
stopping the other threads. Also it will be able to collect young
generations without the need to scan the old generations.
[...]
5) pointer / reference changed callback
Every time a pointer / reference is change
Re: [RFC]Proposal for better garbage collection
On Wed, Feb 22, 2012 at 07:56:15PM +0100, Benjamin Thaut wrote:
> As I'm not satisfied with the current GC D has and don't see the
> situation improving in the future without significant changes to the
> compiler I wrote the following document that points out all the
> possible issues with garbage collection I could think of and
> possible solutions for them. This document is just a draft and only
> a proposal, critics and comments are welcome.
Have you seen this?
http://www.llucax.com.ar/proj/dgc/index.html
[...]
> 2) Tracking references on the stack:
>
> The D compiler always needs to emit a full stack frame so that the
> GC can walk up the stack at any time in the program. The stack frame
> of every function generated by the D compiler starts which a
> bitfield (usually the size of a machine register) where each bit
> indicates that these bytes are a pointer / reference. The bitfield
> needs to be large enough to cover the whole stack frame of the
> function.
This adds a lot of overhead to the runtime stack, esp. if you have deep
recursion. It's also not necessarily faster, since the GC now has to
parse a bitfield (a variable-length encoded bitfield, no less), instead
of just scanning words directly, which can be optimized by CPU-specific
microcode depending on the target platform.
[...]
> Every scope generated by the D compiler would need additional code
> at the start and end of the scope. When the scope is entered the
> bitfield would be patched to represent the new variables inside the
> scope and when the scope is left the bitfield is patched again to
> remove the changes that were made on entering the scope.
This would introduce quite a lot of overhead per scope. It will also
lead to strange things like:
if (x) y(); // faster
if (x) { y(); } // slower
which will encourage people to omit {} after if, which makes code more
fragile and hard to read.
> Every time a function gets called that did not get generated by the
> D compiler ( C / C++ etc functions) the compiler generates a call
> into the runtime and passes the current stack pointer and stack base
> pointer to it.
>
> void _d_externalCallStart(void* stackptr, void* baseptr);
>
> Every time such a function returns the compiler generates a call
> into the the runtime too.
>
> void _d_externalCallEnd(void* stackptr, void* baseptr);
>
> Every time a functions that can get called from other languages
> (extern(C) etc) are executed the end callback is inserted at the
> start of the functions and the start callback is inserted at the end
> of the function.
> Using these callbacks the GC can mark certain parts of the stack as
> "non-D" and ignore them when scanning for bit fields and
> references/pointers. It can just skip parts of the stack that are
> "non-D" and therefore does not need a full stack frame within these
> "non-D" sections.
This may not be a bad idea, though it does introduce some overhead when
you cross language boundaries.
> All these features are required so that the GC can precisely scan
> pointers/references on the stack and change them as necessary.
> Remaining issues: The D compiler can freely move around value types
> on the stack. With such move operations it would be necessary to fix
> up all the bit fields. I needs to be investigated if this is doable.
This can only make the GC slower, especially if it needs to update
variable-length encoded bitfields. Of course, you may be able to offset
this by making it possible to do real-time GC, (reduced throughput but
less waiting time for collection cycles) but that's a very complex
problem.
> 3) Tracking references on the heap
>
> For every class / struct a mixin template which is defined inside
> the runtime gets instantiated. This template can then use
> introspection to generate the necessary information to allow the GC
> to scan the pointers within that struct / class precisely.
So basically you're proposing a compacting precise-scanning GC instead
of the current conservative GC. There are pros and cons in either
approach; it'd be nice if you could compare them.
[...]
> 5) pointer / reference changed callback
>
> Every time a pointer / reference is changed the D compiler emits a
> call into the runtime and passes the new value of the reference /
> pointer with it.
This introduces a LOT of overhead, especially in a language like D which
manipulates a lot of pointers quite often (esp. if you use slices a
lot).
[...]
> -If the GC interrupts a thread right before any of the above mentioned
> callbacks happen it will cause a invalid state for the GC and the GC
> might access invalid pointers. It has to be investigated if this leads
> to invalid behavior. It can be fixed by not interrupting a thread but
> pause it the next time it calls any of callbacks, or other functions
> that can be interrupted by the GC.
This adds a lot of intermittent pauses in program execution.
The link I posted at the top has a GC implementation th
