X-Git-Url: https://oss.titaniummirror.com/gitweb?a=blobdiff_plain;f=libstdc%2B%2B-v3%2Fdoc%2Fxml%2Fmanual%2Fbitmap_allocator.xml;fp=libstdc%2B%2B-v3%2Fdoc%2Fxml%2Fmanual%2Fbitmap_allocator.xml;h=1815a39c4c0257303df1e4b28392c24f261f5b52;hb=6fed43773c9b0ce596dca5686f37ac3fc0fa11c0;hp=0000000000000000000000000000000000000000;hpb=27b11d56b743098deb193d510b337ba22dc52e5c;p=msp430-gcc.git diff --git a/libstdc++-v3/doc/xml/manual/bitmap_allocator.xml b/libstdc++-v3/doc/xml/manual/bitmap_allocator.xml new file mode 100644 index 00000000..1815a39c --- /dev/null +++ b/libstdc++-v3/doc/xml/manual/bitmap_allocator.xml @@ -0,0 +1,559 @@ + + + + + + + ISO C++ + + + allocator + + + + +bitmap_allocator + + + + + +Design + + + As this name suggests, this allocator uses a bit-map to keep track + of the used and unused memory locations for it's book-keeping + purposes. + + + This allocator will make use of 1 single bit to keep track of + whether it has been allocated or not. A bit 1 indicates free, + while 0 indicates allocated. This has been done so that you can + easily check a collection of bits for a free block. This kind of + Bitmapped strategy works best for single object allocations, and + with the STL type parameterized allocators, we do not need to + choose any size for the block which will be represented by a + single bit. This will be the size of the parameter around which + the allocator has been parameterized. Thus, close to optimal + performance will result. Hence, this should be used for node based + containers which call the allocate function with an argument of 1. + + + + The bitmapped allocator's internal pool is exponentially growing. + Meaning that internally, the blocks acquired from the Free List + Store will double every time the bitmapped allocator runs out of + memory. + + + + The macro __GTHREADS decides whether to use + Mutex Protection around every allocation/deallocation. The state + of the macro is picked up automatically from the gthr abstraction + layer. + + + + + +Implementation + + + Free List Store + + + The Free List Store (referred to as FLS for the remaining part of this + document) is the Global memory pool that is shared by all instances of + the bitmapped allocator instantiated for any type. This maintains a + sorted order of all free memory blocks given back to it by the + bitmapped allocator, and is also responsible for giving memory to the + bitmapped allocator when it asks for more. + + + Internally, there is a Free List threshold which indicates the + Maximum number of free lists that the FLS can hold internally + (cache). Currently, this value is set at 64. So, if there are + more than 64 free lists coming in, then some of them will be given + back to the OS using operator delete so that at any given time the + Free List's size does not exceed 64 entries. This is done because + a Binary Search is used to locate an entry in a free list when a + request for memory comes along. Thus, the run-time complexity of + the search would go up given an increasing size, for 64 entries + however, lg(64) == 6 comparisons are enough to locate the correct + free list if it exists. + + + Suppose the free list size has reached it's threshold, then the + largest block from among those in the list and the new block will + be selected and given back to the OS. This is done because it + reduces external fragmentation, and allows the OS to use the + larger blocks later in an orderly fashion, possibly merging them + later. Also, on some systems, large blocks are obtained via calls + to mmap, so giving them back to free system resources becomes most + important. + + + The function _S_should_i_give decides the policy that determines + whether the current block of memory should be given to the + allocator for the request that it has made. That's because we may + not always have exact fits for the memory size that the allocator + requests. We do this mainly to prevent external fragmentation at + the cost of a little internal fragmentation. Now, the value of + this internal fragmentation has to be decided by this function. I + can see 3 possibilities right now. Please add more as and when you + find better strategies. + + + + Equal size check. Return true only when the 2 blocks are of equal +size. + Difference Threshold: Return true only when the _block_size is +greater than or equal to the _required_size, and if the _BS is > _RS +by a difference of less than some THRESHOLD value, then return true, +else return false. + Percentage Threshold. Return true only when the _block_size is +greater than or equal to the _required_size, and if the _BS is > _RS +by a percentage of less than some THRESHOLD value, then return true, +else return false. + + + + Currently, (3) is being used with a value of 36% Maximum wastage per + Super Block. + + + + + Super Block + + + A super block is the block of memory acquired from the FLS from + which the bitmap allocator carves out memory for single objects + and satisfies the user's requests. These super blocks come in + sizes that are powers of 2 and multiples of 32 + (_Bits_Per_Block). Yes both at the same time! That's because the + next super block acquired will be 2 times the previous one, and + also all super blocks have to be multiples of the _Bits_Per_Block + value. + + + How does it interact with the free list store? + + + The super block is contained in the FLS, and the FLS is responsible for + getting / returning Super Bocks to and from the OS using operator new + as defined by the C++ standard. + + + + + Super Block Data Layout + + Each Super Block will be of some size that is a multiple of the + number of Bits Per Block. Typically, this value is chosen as + Bits_Per_Byte x sizeof(size_t). On an x86 system, this gives the + figure 8 x 4 = 32. Thus, each Super Block will be of size 32 + x Some_Value. This Some_Value is sizeof(value_type). For now, let + it be called 'K'. Thus, finally, Super Block size is 32 x K bytes. + + + This value of 32 has been chosen because each size_t has 32-bits + and Maximum use of these can be made with such a figure. + + + Consider a block of size 64 ints. In memory, it would look like this: + (assume a 32-bit system where, size_t is a 32-bit entity). + + + +Bitmap Allocator Memory Map + + + + + + + + + + 268 + 0 + 4294967295 + 4294967295 + Data -> Space for 64 ints + + + +
+ + + The first Column(268) represents the size of the Block in bytes as + seen by the Bitmap Allocator. Internally, a global free list is + used to keep track of the free blocks used and given back by the + bitmap allocator. It is this Free List Store that is responsible + for writing and managing this information. Actually the number of + bytes allocated in this case would be: 4 + 4 + (4x2) + (64x4) = + 272 bytes, but the first 4 bytes are an addition by the Free List + Store, so the Bitmap Allocator sees only 268 bytes. These first 4 + bytes about which the bitmapped allocator is not aware hold the + value 268. + + + + What do the remaining values represent? + + The 2nd 4 in the expression is the sizeof(size_t) because the + Bitmapped Allocator maintains a used count for each Super Block, + which is initially set to 0 (as indicated in the diagram). This is + incremented every time a block is removed from this super block + (allocated), and decremented whenever it is given back. So, when + the used count falls to 0, the whole super block will be given + back to the Free List Store. + + + The value 4294967295 represents the integer corresponding to the bit + representation of all bits set: 11111111111111111111111111111111. + + + The 3rd 4x2 is size of the bitmap itself, which is the size of 32-bits + x 2, + which is 8-bytes, or 2 x sizeof(size_t). + + + + + Maximum Wasted Percentage + + + This has nothing to do with the algorithm per-se, + only with some vales that must be chosen correctly to ensure that the + allocator performs well in a real word scenario, and maintains a good + balance between the memory consumption and the allocation/deallocation + speed. + + + The formula for calculating the maximum wastage as a percentage: + + + +(32 x k + 1) / (2 x (32 x k + 1 + 32 x c)) x 100. + + + + where k is the constant overhead per node (e.g., for list, it is + 8 bytes, and for map it is 12 bytes) and c is the size of the + base type on which the map/list is instantiated. Thus, suppose the + type1 is int and type2 is double, they are related by the relation + sizeof(double) == 2*sizeof(int). Thus, all types must have this + double size relation for this formula to work properly. + + + Plugging-in: For List: k = 8 and c = 4 (int and double), we get: + 33.376% + + + +For map/multimap: k = 12, and c = 4 (int and double), we get: 37.524% + + + Thus, knowing these values, and based on the sizeof(value_type), we may + create a function that returns the Max_Wastage_Percentage for us to use. + + + + + + <function>allocate</function> + + + The allocate function is specialized for single object allocation + ONLY. Thus, ONLY if n == 1, will the bitmap_allocator's + specialized algorithm be used. Otherwise, the request is satisfied + directly by calling operator new. + + + Suppose n == 1, then the allocator does the following: + + + + + Checks to see whether a free block exists somewhere in a region + of memory close to the last satisfied request. If so, then that + block is marked as allocated in the bit map and given to the + user. If not, then (2) is executed. + + + + + Is there a free block anywhere after the current block right + up to the end of the memory that we have? If so, that block is + found, and the same procedure is applied as above, and + returned to the user. If not, then (3) is executed. + + + + + Is there any block in whatever region of memory that we own + free? This is done by checking + + + + + The use count for each super block, and if that fails then + + + + + The individual bit-maps for each super block. + + + + + + Note: Here we are never touching any of the memory that the + user will be given, and we are confining all memory accesses + to a small region of memory! This helps reduce cache + misses. If this succeeds then we apply the same procedure on + that bit-map as (1), and return that block of memory to the + user. However, if this process fails, then we resort to (4). + + + + + This process involves Refilling the internal exponentially + growing memory pool. The said effect is achieved by calling + _S_refill_pool which does the following: + + + + + Gets more memory from the Global Free List of the Required + size. + + + + + Adjusts the size for the next call to itself. + + + + + Writes the appropriate headers in the bit-maps. + + + + + Sets the use count for that super-block just allocated to 0 + (zero). + + + + + All of the above accounts to maintaining the basic invariant + for the allocator. If the invariant is maintained, we are + sure that all is well. Now, the same process is applied on + the newly acquired free blocks, which are dispatched + accordingly. + + + + + + + +Thus, you can clearly see that the allocate function is nothing but a +combination of the next-fit and first-fit algorithm optimized ONLY for +single object allocations. + + + + + + <function>deallocate</function> + + The deallocate function again is specialized for single objects ONLY. + For all n belonging to > 1, the operator delete is called without + further ado, and the deallocate function returns. + + + However for n == 1, a series of steps are performed: + + + + + We first need to locate that super-block which holds the memory + location given to us by the user. For that purpose, we maintain + a static variable _S_last_dealloc_index, which holds the index + into the vector of block pairs which indicates the index of the + last super-block from which memory was freed. We use this + strategy in the hope that the user will deallocate memory in a + region close to what he/she deallocated the last time around. If + the check for belongs_to succeeds, then we determine the bit-map + for the given pointer, and locate the index into that bit-map, + and mark that bit as free by setting it. + + + If the _S_last_dealloc_index does not point to the memory block + that we're looking for, then we do a linear search on the block + stored in the vector of Block Pairs. This vector in code is + called _S_mem_blocks. When the corresponding super-block is + found, we apply the same procedure as we did for (1) to mark the + block as free in the bit-map. + + + + + Now, whenever a block is freed, the use count of that particular + super block goes down by 1. When this use count hits 0, we remove + that super block from the list of all valid super blocks stored in + the vector. While doing this, we also make sure that the basic + invariant is maintained by making sure that _S_last_request and + _S_last_dealloc_index point to valid locations within the vector. + + + + + Questions + + + 1 + +Q1) The "Data Layout" section is +cryptic. I have no idea of what you are trying to say. Layout of what? +The free-list? Each bitmap? The Super Block? + + + The layout of a Super Block of a given +size. In the example, a super block of size 32 x 1 is taken. The +general formula for calculating the size of a super block is +32 x sizeof(value_type) x 2^n, where n ranges from 0 to 32 for 32-bit +systems. + + + + + 2 + + And since I just mentioned the +term `each bitmap', what in the world is meant by it? What does each +bitmap manage? How does it relate to the super block? Is the Super +Block a bitmap as well? + + + Each bitmap is part of a Super Block which is made up of 3 parts + as I have mentioned earlier. Re-iterating, 1. The use count, + 2. The bit-map for that Super Block. 3. The actual memory that + will be eventually given to the user. Each bitmap is a multiple + of 32 in size. If there are 32 x (2^3) blocks of single objects + to be given, there will be '32 x (2^3)' bits present. Each 32 + bits managing the allocated / free status for 32 blocks. Since + each size_t contains 32-bits, one size_t can manage up to 32 + blocks' status. Each bit-map is made up of a number of size_t, + whose exact number for a super-block of a given size I have just + mentioned. + + + + + 3 + + How do the allocate and deallocate functions work in regard to + bitmaps? + + + The allocate and deallocate functions manipulate the bitmaps and + have nothing to do with the memory that is given to the user. As + I have earlier mentioned, a 1 in the bitmap's bit field + indicates free, while a 0 indicates allocated. This lets us + check 32 bits at a time to check whether there is at lease one + free block in those 32 blocks by testing for equality with + (0). Now, the allocate function will given a memory block find + the corresponding bit in the bitmap, and will reset it (i.e., + make it re-set (0)). And when the deallocate function is called, + it will again set that bit after locating it to indicate that + that particular block corresponding to this bit in the bit-map + is not being used by anyone, and may be used to satisfy future + requests. + + + e.g.: Consider a bit-map of 64-bits as represented below: + 1111111111111111111111111111111111111111111111111111111111111111 + + + + Now, when the first request for allocation of a single object + comes along, the first block in address order is returned. And + since the bit-maps in the reverse order to that of the address + order, the last bit (LSB if the bit-map is considered as a + binary word of 64-bits) is re-set to 0. + + + + The bit-map now looks like this: + 1111111111111111111111111111111111111111111111111111111111111110 + + + + + + Locality + + Another issue would be whether to keep the all bitmaps in a + separate area in memory, or to keep them near the actual blocks + that will be given out or allocated for the client. After some + testing, I've decided to keep these bitmaps close to the actual + blocks. This will help in 2 ways. + + + + Constant time access for the bitmap themselves, since no kind of +look up will be needed to find the correct bitmap list or it's +equivalent. + And also this would preserve the cache as far as possible. + + + + So in effect, this kind of an allocator might prove beneficial from a + purely cache point of view. But this allocator has been made to try and + roll out the defects of the node_allocator, wherein the nodes get + skewed about in memory, if they are not returned in the exact reverse + order or in the same order in which they were allocated. Also, the + new_allocator's book keeping overhead is too much for small objects and + single object allocations, though it preserves the locality of blocks + very well when they are returned back to the allocator. + + + + + Overhead and Grow Policy + + Expected overhead per block would be 1 bit in memory. Also, once + the address of the free list has been found, the cost for + allocation/deallocation would be negligible, and is supposed to be + constant time. For these very reasons, it is very important to + minimize the linear time costs, which include finding a free list + with a free block while allocating, and finding the corresponding + free list for a block while deallocating. Therefore, I have + decided that the growth of the internal pool for this allocator + will be exponential as compared to linear for + node_allocator. There, linear time works well, because we are + mainly concerned with speed of allocation/deallocation and memory + consumption, whereas here, the allocation/deallocation part does + have some linear/logarithmic complexity components in it. Thus, to + try and minimize them would be a good thing to do at the cost of a + little bit of memory. + + + + Another thing to be noted is the pool size will double every time + the internal pool gets exhausted, and all the free blocks have + been given away. The initial size of the pool would be + sizeof(size_t) x 8 which is the number of bits in an integer, + which can fit exactly in a CPU register. Hence, the term given is + exponential growth of the internal pool. + + + + + +