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 20: class index 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
