Hi Stef, I have skimmed the proposal in the included class comment. It is beyond my knowledge really to comment specifically. Looks great! :-)
I picked up on the reference to 64k classes limit. Whilst at the moment I can't imagine a Pharo system needing that many classes, the VW system I am working on already has ~64k classes. I just thought it an interesting data point. I don't know what happens in the scheme below if the limit is a hard one or not. For my stats I did "roots of the system, gather all classes, go" style loop and skimmed the result for sanity. This includes metaclasses. So I don't know if the number is really half or not. Do metaclasses count towards this total in this scheme? Also our number is quite high because for tool/browser support we also have proxies for all GemStone classes. Anyway I don't mention this to adjust any technical decisions but it was more just a real (if 50%) data point. Cheers, Mike On 30 May 2012, at 07:48, Stéphane Ducasse <[email protected]> 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 >> >
