On Wed, May 30, 2012 at 10:22 PM, Eliot Miranda <[email protected]>wrote:
> > > > On Wed, May 30, 2012 at 12:59 PM, Stéphane Ducasse < > [email protected]> wrote: > >> I would like to be sure that we can have >> - bit for immutable objects >> - bits for experimenting. >> > > There will be quite a few. And one will be able to steal bits from the > class field if one needs fewer classes. I'm not absolutely sure of the > layout yet. But for example > > 8: slot size (255 => extra header word with large size) > 3: odd bytes/fixed fields (odd bytes for non-pointer, fixed fields for > pointer, 7 => # fixed fields is in the class) > 4 bits: format (pointers, indexable, bytes/shorts/longs/doubles indexable, > compiled method, ephemerons, weak, etc) > 1: immutability > 3: GC 2 mark bits. 1 forwarded bit > 20: identity hash > and we can make it lazy, that is, we compute it not at instantiation time but rather the first time the primitive is call. > 20: class index > This would probably work for a while. I think that it would be good to let an "open door" so that in the future we can just add one more word for a class pointer. > > still leaves 5 bits unused. And stealing 4 bits each from class index > still leaves 64k classes. So this format is simple and provides lots of > unused bits. The format field is a great idea as it combines a number of > orthogonal properties in very few bits. I don't want to include odd bytes > in format because I think a separate field that holds odd bytes and fixed > fields is better use of space. But we can gather statistics before > deciding. > > >> Stef >> >> On May 30, 2012, at 8:48 AM, Stéphane Ducasse wrote: >> >> > Hi guys >> > >> > Here is an important topic I would like to see discussed so that we see >> how we can improve and join forces. >> > May a mail discussion then a wiki for the summary would be good. >> > >> > >> > stef >> > >> > >> > >> > Begin forwarded message: >> > >> >> From: Eliot Miranda <[email protected]> >> >> Subject: Re: Plan/discussion/communication around new object format >> >> Date: May 27, 2012 10:49:54 PM GMT+02:00 >> >> To: Stéphane Ducasse <[email protected]> >> >> >> >> >> >> >> >> On Sat, May 26, 2012 at 1:46 AM, Stéphane Ducasse < >> [email protected]> wrote: >> >> Hi eliot >> >> >> >> do you have a description of the new object format you want to >> introduce? >> >> >> >> The design is in the class comment of CogMemoryManager in the Cog >> VMMaker packages. >> >> >> >> Then what is your schedule? >> >> >> >> This is difficult. I have made a small start and should be able to >> spend time on it starting soon. I want to have it finished by early next >> year. But it depends on work schedules etc. >> >> >> >> >> >> I would like to see if we can allocate igor/esteban time before we run >> out of money >> >> to help on that important topic. >> >> Now the solution is unclear and I did not see any document where we >> can evaluate >> >> and plan how we can help. So do you want help on that topic? Then how >> can people >> >> contribute if any? >> >> >> >> The first thing to do is to read the design document, to see if the >> Pharo community thinks it is the right direction, and to review it, spot >> deficiencies etc. So please get those interested to read the class comment >> of CogMemoryManager in the latest VMMaker.oscog. >> >> >> >> Here's the current version of it: >> >> >> >> CogMemoryManager is currently a place-holder for the design of the new >> Cog VM's object representation and garbage collector. The goals for the GC >> are >> >> >> >> - efficient object representation a la Eliot Miranda's VisualWorks >> 64-bit object representation that uses a 64-bit header, eliminating direct >> class references so that all objects refer to their classes indirectly. >> Instead the header contains a constant class index, in a field smaller >> than a full pointer, These class indices are used in inline and first-level >> method caches, hence they do not have to be updated on GC (although they do >> have to be traced to be able to GC classes). Classes are held in a sparse >> weak table. The class table needs only to be indexed by an instance's >> class index in class hierarchy search, in the class primitive, and in >> tracing live objects in the heap. The additional header space is allocated >> to a much expanded identity hash field, reducing hash efficiency problems >> in identity collections due to the extremely small (11 bit) hash field in >> the old Squeak GC. The identity hash field is also a key element of the >> class index scheme. A class's identity hash is its index into the class >> table, so to create an instance of a class one merely copies its identity >> hash into the class index field of the new instance. This implies that >> when classes gain their identity hash they are entered into the class table >> and their identity hash is that of a previously unused index in the table. >> It also implies that there is a maximum number of classes in the table. >> At least for a few years 64k classes should be enough. A class is entered >> into the class table in the following operations: >> >> behaviorHash >> >> adoptInstance >> >> instantiate >> >> become (i.e. if an old class becomes a new class) >> >> if target class field's = to original's id hash >> >> and replacement's id hash is zero >> >> enter replacement in class table >> >> behaviorHash is a special version of identityHash that must be >> implemented in the image by any object that can function as a class (i.e. >> Behavior). >> >> >> >> - more immediate classes. An immediate Character class would speed up >> String accessing, especially for WideString, since no instatiation needs to >> be done on at:put: and no dereference need be done on at:. In a 32-bit >> system tag checking is complex since it is thought important to retain >> 31-bit SmallIntegers. Hence, as in current Squeak, the least significant >> bit set implies a SmallInteger, but Characters would likely have a tag >> pattern of xxx10. Hence masking with 11 results in two values for >> SmallInteger, xxx01 and xxx11. 30-bit characters are more than adequate >> for Unicode. In a 64-bit system we can use the full three bits and >> usefully implement an immediate Float. As in VisualWorks a functional >> representation takes three bits away from the exponent. Rotating to put >> the sign bit in the least significant non-tag bit makes expanding and >> contracting the 8-bit exponent to the 11-bit IEEE double exponent easy ad >> makes comparing negative and positive zero easier (an immediate Float is >> zero if its unsigned 64-bits are < 16). So the representation looks like >> >> | 8 bit exponent | 52 bit mantissa | sign bit | 3 tag bits | >> >> For details see "60-bit immediate Floats" below. >> >> >> >> >> >> - efficient scavenging. The current Squeak GC uses a slow >> pointer-reversal collector that writes every field in live objects three >> times in each collection, twice in the pointer-reversing heap traversal to >> mark live objects and once to update the pointer to its new location. A >> scavenger writes every field of live data twice in each collection, once as >> it does a block copy of the object when copying to to space, once as it >> traverses the live pointers in the to space objects. Of course the block >> copy is a relatively cheap write. >> >> >> >> - lazy become. The JIT's use of inline cacheing provides a cheap way >> of avoiding scanning the heap as part of a become (which is the simple >> approach to implementing become in a system with direct pointers). A >> becomeForward: on a (set of) non-zero-sized object(s) turns the object into >> a "corpse" or "forwarding object" whose first (non-header) word/slot is >> replaced by a pointer to the target of the becomeForward:. The corpse's >> class index is set to one that identifies corpses and, because it is a >> hidden class index, will always fail an inline cache test. The inline >> cache failure code is then responsible for following the forwarding pointer >> chain (these are Iliffe vectors :) ) and resolving to the actual target. >> We have yet to determine exactly how this is done (e.g. change the >> receiver register and/or stack contents and retry the send, perhaps >> scanning the current activation). See below on how we deal with becomes on >> objects with named inst vars. Note that we probably don't have to worry >> about zero-sized objects. These are unlikely to be passed through the FFI >> (there is nothing to pass :) ) and so will rarely be becommed. If they do, >> they can become slowly. Alternatively we can insist that objects are at >> least 16 bytes in size (see a8-byte alignment below) so that there will >> always be space for a forwarding pointer. Since none of the immediate >> classes can have non-immediate instances and since we allocate the >> immediate classes indices corresponding to their tag pattern (SmallInteger >> = 1, Character = 3, SmallFloat = 4?) we can use all the class indices from >> 0 to 7 for special uses, 0 = forward, and e.g. 1 = header-sized filler. >> >> >> >> - pinning. To support a robust and easy-to-use FFI the memory manager >> must support temporary pinning where individual objects can be prevented >> from being moved by the GC for as long as required, either by being one of >> an in-progress FFI call's arguments, or by having pinning asserted by a >> primitive (allowing objects to be passed to external code that retains a >> reference to the object after returning). Pinning probably implies a >> per-object "is-pinned" bit in the object header. Pinning will be done via >> lazy become; i..e an object in new space will be becommed into a pinned >> object in old space. We will only support pinning in old space. >> >> >> >> - efficient old space collection. An incremental collector (a la >> Dijkstra's three colour algorithm) collects old space, e.g. via an amount >> of tracing being hung off scavenges and/or old space allocations at an >> adaptive rate that keeps full garbage collections to a minimum. (see free >> space/free list below) >> >> >> >> - 8-byte alignment. It is advantageous for the FFI, for >> floating-point access, for object movement and for 32/64-bit >> compatibility to keep object sizes in units of 8 bytes. For the FFI, >> 8-byte alignment means passing objects to code that expects that >> requirement (such as modern x86 numeric processing instructions). This >> implies that >> >> - the starts of all spaces are aligned on 8-byte boundaries >> >> - object allocation rounds up the requested size to a multiple of >> 8 bytes >> >> - the overflow size field is also 8 bytes >> >> We shall probably keep the minimum object size at 16 bytes so that >> there is always room for a forwarding pointer. But this implies that we >> will need to implement an 8-byte filler to fill holes between objects > 16 >> bytes whose length mod 16 bytes is 8 bytes and following pinned objects. >> We can do this using a special class index, e.g. 1, so that the method >> that answers the size of an object looks like, e.g. >> >> chunkSizeOf: oop >> >> <var: #oop type: #'object *'> >> >> ^object classIndex = 1 >> >> ifTrue: [BaseHeaderSize] >> >> ifFalse: [BaseHeaderSize >> >> + (object slotSize = OverflowSlotSize >> >> ifTrue: >> [OverflowSizeBytes] >> >> ifFalse: [0]) >> >> + (object slotSize * BytesPerSlot)] >> >> >> >> chunkStartOf: oop >> >> <var: #oop type: #'object *'> >> >> ^(self cCoerceSimple: oop to: #'char *') >> >> - ((object classIndex = 1 >> >> or: [object slotSize ~= OverflowSlotSize]) >> >> ifTrue: [0] >> >> ifFalse: [OverflowSizeBytes]) >> >> >> >> For the moment we do not tackle the issue of heap growth and shrinkage >> with the ability to allocate and deallocate heap segments via >> memory-mapping. This technique allows space to be released back to the OS >> by unmapping empty segments. We may revisit this but it is not a key >> requirement for the first implementation. >> >> >> >> The basic approach is to use a fixed size new space and a growable old >> space. The new space is a classic three-space nursery a la Ungar's >> Generation Scavenging, a large eden for new objects and two smaller >> survivor spaces that exchange roles on each collection, one being the to >> space to which surviving objects are copied, the other being the from space >> of the survivors of the previous collection, i.e. the previous to space. >> >> >> >> To provide apparent pinning in new space we rely on lazy become. >> Since most pinned objects will be byte data and these do not require stack >> zone activation scanning, the overhead is simply an old space allocation >> and corpsing. >> >> >> >> To provide pinning in old space, large objects are implicitly pinned >> (because it is expensive to move large objects and, because they are both >> large and relatively rare, they contribute little to overall fragmentation >> - as in aggregates, small objects can be used to fill-in the spaces between >> karge objects). Hence, objects above a particular size are automatically >> allocated in old space, rather than new space. Small objects are pinned as >> per objects in new space, by asserting the pin bit, which will be set >> automaticaly when allocating a large object. As a last resort, or by >> programmer control (the fullGC primitive) old space is collected via >> mark-sweep (mark-compact) and so the mark phase must build the list of >> pinned objects around which the sweep/compact phase must carefully step. >> >> >> >> Free space in old space is organized by a free list/free tree as in >> Eliot's VisualWorks 5i old space allocator. There are 64 free lists, >> indices 1 through 63 holding blocks of space of that size, index 0 holding >> a semi-balanced ordered tree of free blocks, each node being the head of >> the list of free blocks of that size. At the start of the mark phase the >> free list is thrown away and the sweep phase coallesces free space and >> steps over pinned objects as it proceeds. We can reuse the forwarding >> pointer compaction scheme used in the old collector. Incremental >> collections merely move unmarked objects to the free lists (as well as >> nilling weak pointers in weak arrays and scheduling them for finalization). >> The occupancy of the free lists is represented by a bitmap in a 64-bit >> integer so that an allocation of size 63 or less can know whether there >> exists a free chunk of that size, but more importantly can know whether a >> free chunk larger than it exists in the fixed size free lists without >> having to search all larger free list heads. >> >> >> >> The incremental collector (a la Dijkstra's three colour algorithm) >> collects old space via an amount of tracing being hung off scavenges and/or >> old space allocations at an adaptive rate that keeps full garbage >> collections to a minimum. [N.B. Not sure how to do this yet. The >> incremental collector needs to complete a pass often enough to reclaim >> objects, but infrequent enough not to waste time. So some form of feedback >> should work. In VisualWorks tracing is broken into quanta or work where >> image-level code determines the size of a quantum based on how fast the >> machine is, and how big the heap is. This code could easily live in the >> VM, controllable through vmParameterAt:put:. An alternative would be to >> use the heartbeat to bound quanta by time. But in any case some amount of >> incremental collection would be done on old space allocation and >> scavenging, the ammount being chosen to keep pause times acceptably short, >> and at a rate to reclaim old space before a full GC is required, i.e. at a >> rate proportional to the growth in old space]. The incemental collector is >> a state machine, being either marking, nilling weak pointers, or freeing. >> If nilling weak pointers is not done atomically then there must be a read >> barrier in weak array at: so that reading from an old space weak array that >> is holding stale un-nilled references to unmarked objects. Tricks such as >> including the weak bit in bounds calculations can make this cheap for >> non-weak arrays. Alternatively nilling weak pointers can be made an atomic >> part of incremental collection, which can be made cheaper by maintaining >> the set of weak arrays (e.g. on a list). >> >> >> >> The incremental collector implies a more complex write barrier. >> Objects are of three colours, black, having been scanned, grey, being >> scanned, and white, unreached. A mark stack holds the grey objects. If >> the incremental collector is marking and an unmarked white object is stored >> into a black object then the stored object must become grey, being added to >> the mark stack. So the wrte barrier is essentially >> >> target isYoung ifFalse: >> >> [newValue isYoung >> >> ifTrue: [target isInRememberedSet ifFalse: >> >> [target addToRememberedSet]] >> "target now refers to a young object; it is a root for scavenges" >> >> ifFalse: >> >> [(target isBlack >> >> and: [igc marking >> >> and: [newValue isWhite]]) ifTrue: >> >> [newValue beGrey]]] "add newValue >> to IGC's markStack for subsequent scanning" >> >> >> >> The incremental collector does not detect already marked objects all >> of whose references have been overwritten by other stores (e.g. in the >> above if newValue overwrites the sole remaining reference to a marked >> object). So the incremental collector only guarantees to collect all >> garbage created in cycle N at the end of cycle N + 1. The cost is hence >> slightly worse memory density but the benefit, provided the IGC works hard >> enough, is the elimination of long pauses due to full garbage collections, >> which become actions of last resort or programmer desire. >> >> >> >> Lazy become. >> >> >> >> As described earlier the basic idea behind lazy become is to use >> corpses (forwarding objects) that are followed lazily during GC and inline >> cache miss. However, a lazy scheme cannot be used on objects with named >> inst vars without adding checking to all inst var accesses, which we judge >> too expensive. Instead, when becomming objects with named inst vars, we >> scan all activations in the stack zone, eagerly becomming these references, >> and we check for corpses when faulting in a context into the stack zone. >> Essentially, the invariant is that there are no references to corpses from >> the receiver slots of stack activations. A detail is whether we allow or >> forbid pinning of closure indirection vectors, or scan the entire stack of >> each activation. Using a special class index pun for indirection vectors >> is a cheap way of preventing their becomming/pinning etc. Although "don't >> do that" (don't attempt to pin/become indirection vectors) is also an >> acceptable response. >> >> >> >> 60-bit immediate Floats >> >> Representation for immediate doubles, only used in the 64-bit >> implementation. Immediate doubles have the same 52 bit mantissa as IEEE >> double-precision floating-point, but only have 8 bits of exponent. So >> they occupy just less than the middle 1/8th of the double range. They >> overlap the normal single-precision floats which also have 8 bit exponents, >> but exclude the single-precision denormals (exponent-127) and the >> single-precsion NaNs (exponent +127). +/- zero is just a pair of values >> with both exponent and mantissa 0. >> >> So the non-zero immediate doubles range from >> >> +/- 0x3800,0000,0000,0001 / 5.8774717541114d-39 >> >> to +/- 0x47ff,ffff,ffff,ffff / 6.8056473384188d+38 >> >> The encoded tagged form has the sign bit moved to the least >> significant bit, which allows for faster encode/decode because offsetting >> the exponent can't overflow into the sign bit and because testing for +/- 0 >> is an unsigned compare for <= 0xf: >> >> msb >> lsb >> >> [8 exponent subset bits][52 mantissa bits ][1 sign bit][3 tag bits] >> >> So assuming the tag is 5, the tagged non-zero bit patterns are >> >> 0x0000,0000,0000,001[d/5] >> >> to 0xffff,ffff,ffff,fff[d/5] >> >> and +/- 0d is 0x0000,0000,0000,000[d/5] >> >> Encode/decode of non-zero values in machine code looks like: >> >> msb >> lsb >> >> Decode: >> [8expsubset][52mantissa][1s][3tags] >> >> shift away tags: [ 000 >> ][8expsubset][52mantissa][1s] >> >> add exponent offset: [ 11 exponent ][52mantissa][1s] >> >> rot sign: [1s][ 11 exponent >> ][52mantissa] >> >> >> >> Encode: [1s][ 11 exponent >> ][52mantissa] >> >> rot sign: [ 11 exponent >> ][52mantissa][1s] >> >> sub exponent offset: [ 000 ][8expsubset][52 mantissa][1s] >> >> shift: [8expsubset][52 >> mantissa][1s][ 000 ] >> >> or/add tags: [8expsubset][52mantissa][1s][3tags] >> >> but is slower in C because >> >> a) there is no rotate, and >> >> b) raw conversion between double and quadword must (at least in the >> source) move bits through memory ( quadword = *(q64 *)&doubleVariable). >> >> >> >> Issues: >> >> How do we avoid the Size4Bit for 64-bits? The format word encodes the >> number of odd bytes, but currently has only 4 bits and hence only supports >> odd bytes of 0 - 3. We need odd bytes of 0 - 7. But I don't like the >> separate Size4Bit. Best to change the VI code and have a 5 bit format? We >> loose one bit but save two bits (isEphemeron and isWeak (or three, if >> isPointers)) for a net gain of one (or two) >> >> >> >> Further, keep Squeak's format idea or go for separate bits? For >> 64-bits we need a 5 bit format field. This contrasts with isPointers, >> isWeak, isEphemeron, 3 odd size bits (or byte size).. format field is >> quite economical. >> >> >> >> Are class indices in inline caches strong references to classes or >> weak references? >> >> If strong then they must be scanned during GC and the methodZone must >> be flushed on fullGC to reclaim all classes (this looks to be a bug in the >> V3 Cogit). >> >> If weak then when the class table loses references, PICs containing >> freed classes must be freed and then sends to freed PICs or containing >> freed clases must be unlinked. >> >> The second approach is faster; the common case is scanning the class >> table, the uncommon case is freeing classes. The second approach is >> better; in-line caches do not prevent reclamation of classes. >> >> >> >> >> >> Stef >> >> >> >> >> >> >> >> -- >> >> best, >> >> Eliot >> >> >> > >> >> >> > > > -- > best, > Eliot > > > -- Mariano http://marianopeck.wordpress.com
