mirror of git://gcc.gnu.org/git/gcc.git
375 lines
17 KiB
Plaintext
375 lines
17 KiB
Plaintext
BITMAPPED ALLOCATOR
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===================
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2004-03-11 Dhruv Matani <dhruvbird@HotPOP.com>
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---------------------------------------------------------------------
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As this name suggests, this allocator uses a bit-map to keep track of
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the used and unused memory locations for it's book-keeping purposes.
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This allocator will make use of 1 single bit to keep track of whether
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it has been allocated or not. A bit 1 indicates free, while 0
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indicates allocated. This has been done so that you can easily check a
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collection of bits for a free block. This kind of Bitmapped strategy
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works best for single object allocations, and with the STL type
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parameterized allocators, we do not need to choose any size for the
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block which will be represented by a single bit. This will be the size
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of the parameter around which the allocator has been
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parameterized. Thus, close to optimal performance will result. Hence,
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this should be used for node based containers which call the allocate
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function with an argument of 1.
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The bitmapped allocator's internal pool is exponentially
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growing. Meaning that internally, the blocks acquired from the Free
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List Store will double every time the bitmapped allocator runs out of
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memory.
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--------------------------------------------------------------------
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The macro __GTHREADS decides whether to use Mutex Protection around
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every allocation/deallocation. The state of the macro is picked up
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automatically from the gthr abstration layer.
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----------------------------------------------------------------------
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What is the Free List Store?
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----------------------------
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The Free List Store (referred to as FLS for the remaining part of this
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document) is the Global memory pool that is shared by all instances of
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the bitmapped allocator instantiated for any type. This maintains a
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sorted order of all free memory blocks given back to it by the
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bitmapped allocator, and is also responsible for giving memory to the
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bitmapped allocator when it asks for more.
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Internally, there is a Free List threshold which indicates the Maximum
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number of free lists that the FLS can hold internally
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(cache). Currently, this value is set at 64. So, if there are more
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than 64 free lists coming in, then some of them will be given back to
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the OS using operator delete so that at any given time the Free List's
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size does not exceed 64 entries. This is done because a Binary Search
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is used to locate an entry in a free list when a request for memory
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comes along. Thus, the run-time complexity of the search would go up
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given an increasing size, for 64 entries however, lg(64) == 6
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comparisons are enough to locate the correct free list if it exists.
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Suppose the free list size has reached it's threshold, then the
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largest block from among those in the list and the new block will be
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selected and given back to the OS. This is done because it reduces
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external fragmentation, and allows the OS to use the larger blocks
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later in an orderly fashion, possibly merging them later. Also, on
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some systems, large blocks are obtained via calls to mmap, so giving
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them back to free system resources becomes most important.
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The function _S_should_i_give decides the policy that determines
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whether the current block of memory should be given to the allocator
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for the request that it has made. That's because we may not always
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have exact fits for the memory size that the allocator requests. We do
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this mainly to prevent external fragmentation at the cost of a little
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internal fragmentation. Now, the value of this internal fragmentation
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has to be decided by this function. I can see 3 possibilities right
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now. Please add more as and when you find better strategies.
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1. Equal size check. Return true only when the 2 blocks are of equal
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size.
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2. Difference Threshold: Return true only when the _block_size is
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greater than or equal to the _required_size, and if the _BS is >
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_RS by a difference of less than some THRESHOLD value, then return
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true, else return false.
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3. Percentage Threshold. Return true only when the _block_size is
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greater than or equal to the _required_size, and if the _BS is >
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_RS by a percentage of less than some THRESHOLD value, then return
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true, else return false.
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Currently, (3) is being used with a value of 36% Maximum wastage per
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Super Block.
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--------------------------------------------------------------------
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1) What is a super block? Why is it needed?
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A super block is the block of memory acquired from the FLS from
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which the bitmap allocator carves out memory for single objects and
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satisfies the user's requests. These super blocks come in sizes that
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are powers of 2 and multiples of 32 (_Bits_Per_Block). Yes both at
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the same time! That's because the next super block acquired will be
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2 times the previous one, and also all super blocks have to be
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multiples of the _Bits_Per_Block value.
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2) How does it interact with the free list store?
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The super block is contained in the FLS, and the FLS is responsible
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for getting / returning Super Bocks to and from the OS using
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operator new as defined by the C++ standard.
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---------------------------------------------------------------------
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How does the allocate function Work?
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------------------------------------
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The allocate function is specialized for single object allocation
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ONLY. Thus, ONLY if n == 1, will the bitmap_allocator's specialized
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algorithm be used. Otherwise, the request is satisfied directly by
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calling operator new.
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Suppose n == 1, then the allocator does the following:
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1. Checks to see whether the a free block exists somewhere in a region
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of memory close to the last satisfied request. If so, then that
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block is marked as allocated in the bit map and given to the
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user. If not, then (2) is executed.
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2. Is there a free block anywhere after the current block right upto
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the end of the memory that we have? If so, that block is found, and
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the same procedure is applied as above, and returned to the
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user. If not, then (3) is executed.
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3. Is there any block in whatever region of memory that we own free?
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This is done by checking (a) The use count for each super block,
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and if that fails then (b) The individual bit-maps for each super
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block. Note: Here we are never touching any of the memory that the
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user will be given, and we are confining all memory accesses to a
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small region of memory! This helps reduce cache misses. If this
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succeeds then we apply the same procedure on that bit-map as (1),
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and return that block of memory to the user. However, if this
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process fails, then we resort to (4).
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4. This process involves Refilling the internal exponentially growing
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memory pool. The said effect is achieved by calling _S_refill_pool
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which does the following:
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(a). Gets more memory from the Global Free List of the
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Required size.
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(b). Adjusts the size for the next call to itself.
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(c). Writes the appropriate headers in the bit-maps.
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(d). Sets the use count for that super-block just allocated
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to 0 (zero).
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(e). All of the above accounts to maintaining the basic
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invariant for the allocator. If the invariant is
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maintained, we are sure that all is well.
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Now, the same process is applied on the newly acquired free blocks,
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which are dispatched accordingly.
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Thus, you can clearly see that the allocate function is nothing but a
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combination of the next-fit and first-fit algorithm optimized ONLY for
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single object allocations.
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-------------------------------------------------------------------------
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How does the deallocate function work?
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--------------------------------------
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The deallocate function again is specialized for single objects ONLY.
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For all n belonging to > 1, the operator delete is called without
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further ado, and the deallocate function returns.
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However for n == 1, a series of steps are performed:
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1. We first need to locate that super-block which holds the memory
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location given to us by the user. For that purpose, we maintain a
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static variable _S_last_dealloc_index, which holds the index into
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the vector of block pairs which indicates the index of the last
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super-block from which memory was freed. We use this strategy in
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the hope that the user will deallocate memory in a region close to
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what he/she deallocated the last time around. If the check for
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belongs_to succeeds, then we determine the bit-map for the given
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pointer, and locate the index into that bit-map, and mark that bit
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as free by setting it.
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2. If the _S_last_dealloc_index does not point to the memory block
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that we're looking for, then we do a linear search on the block
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stored in the vector of Block Pairs. This vector in code is called
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_S_mem_blocks. When the corresponding super-block is found, we
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apply the same procedure as we did for (1) to mark the block as
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free in the bit-map.
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Now, whenever a block is freed, the use count of that particular super
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block goes down by 1. When this use count hits 0, we remove that super
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block from the list of all valid super blocks stored in the
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vector. While doing this, we also make sure that the basic invariant
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is maintained by making sure that _S_last_request and
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_S_last_dealloc_index point to valid locations within the vector.
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--------------------------------------------------------------------
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Data Layout for a Super Block:
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==============================
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Each Super Block will be of some size that is a multiple of the number
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of Bits Per Block. Typically, this value is chosen as Bits_Per_Byte X
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sizeof(unsigned int). On an X86 system, this gives the figure
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8 X 4 = 32. Thus, each Super Block will be of size 32 X Some_Value.
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This Some_Value is sizeof(value_type). For now, let it be called 'K'.
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Thus, finally, Super Block size is 32 X K bytes.
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This value of 32 has been chosen because each unsigned int has 32-bits
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and Maximum use of these can be made with such a figure.
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Consider a block of size 32 ints.
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In memory, it would look like this:
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---------------------------------------------------------------------
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| 136 | 0 | 4294967295 | Data-> Space for 32-ints |
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---------------------------------------------------------------------
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The first Columns represents the size of the Block in bytes as seen by
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the Bitmap Allocator. Internally, a global free list is used to keep
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track of the free blocks used and given back by the bitmap
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allocator. It is this Free List Store that is responsible for writing
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and managing this information. Actually the number of bytes allocated
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in this case would be: 4 + 4 + 4 + 32*4 = 140 bytes, but the first 4
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bytes are an addition by the Free List Store, so the Bitmap Allocator
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sees only 136 bytes. These first 4 bytes about which the bitmapped
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allocator is not aware hold the value 136.
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What do the remaining values represent?
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---------------------------------------
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The 2nd 4 in the expression is the sizeof(unsigned int) because the
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Bitmapped Allocator maintains a used count for each Super Block, which
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is initially set to 0 (as indicated in the diagram). This is
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incremented every time a block is removed from this super block
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(allocated), and decremented whenever it is given back. So, when the
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used count falls to 0, the whole super block will be given back to the
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Free List Store.
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The value 4294967295 represents the integer corresponding to the
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bit representation of all bits set: 11111111111111111111111111111111.
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The 3rd 4 is size of the bitmap itself, which is the size of 32-bits,
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which is 4-bytes, or 1 X sizeof(unsigned int).
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--------------------------------------------------------------------
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Another issue would be whether to keep the all bitmaps in a separate
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area in memory, or to keep them near the actual blocks that will be
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given out or allocated for the client. After some testing, I've
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decided to keep these bitmaps close to the actual blocks. this will
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help in 2 ways.
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1. Constant time access for the bitmap themselves, since no kind of
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look up will be needed to find the correct bitmap list or it's
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equivalent.
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2. And also this would preserve the cache as far as possible.
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So in effect, this kind of an allocator might prove beneficial from a
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purely cache point of view. But this allocator has been made to try
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and roll out the defects of the node_allocator, wherein the nodes get
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skewed about in memory, if they are not returned in the exact reverse
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order or in the same order in which they were allocated. Also, the
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new_allocator's book keeping overhead is too much for small objects
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and single object allocations, though it preserves the locality of
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blocks very well when they are returned back to the allocator.
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-------------------------------------------------------------------
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Expected overhead per block would be 1 bit in memory. Also, once
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the address of the free list has been found, the cost for
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allocation/deallocation would be negligible, and is supposed to be
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constant time. For these very reasons, it is very important to
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minimize the linear time costs, which include finding a free list
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with a free block while allocating, and finding the corresponding
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free list for a block while deallocating. Therefore, I have decided
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that the growth of the internal pool for this allocator will be
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exponential as compared to linear for node_allocator. There, linear
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time works well, because we are mainly concerned with speed of
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allocation/deallocation and memory consumption, whereas here, the
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allocation/deallocation part does have some linear/logarithmic
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complexity components in it. Thus, to try and minimize them would
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be a good thing to do at the cost of a little bit of memory.
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Another thing to be noted is the the pool size will double every time
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the internal pool gets exhausted, and all the free blocks have been
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given away. The initial size of the pool would be sizeof(unsigned
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int)*8 which is the number of bits in an integer, which can fit
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exactly in a CPU register. Hence, the term given is exponential growth
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of the internal pool.
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---------------------------------------------------------------------
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After reading all this, you may still have a few questions about the
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internal working of this allocator, like my friend had!
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Well here are the exact questions that he posed:
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1) The "Data Layout" section is cryptic. I have no idea of what you
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are trying to say. Layout of what? The free-list? Each bitmap? The
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Super Block?
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The layout of a Super Block of a given size. In the example, a super
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block of size 32 X 1 is taken. The general formula for calculating
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the size of a super block is 32*sizeof(value_type)*2^n, where n
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ranges from 0 to 32 for 32-bit systems.
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2) And since I just mentioned the term `each bitmap', what in the
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world is meant by it? What does each bitmap manage? How does it
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relate to the super block? Is the Super Block a bitmap as well?
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Good question! Each bitmap is part of a Super Block which is made up
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of 3 parts as I have mentioned earlier. Re-iterating, 1. The use
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count, 2. The bit-map for that Super Block. 3. The actual memory
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that will be eventually given to the user. Each bitmap is a multiple
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of 32 in size. If there are 32*(2^3) blocks of single objects to be
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given, there will be '32*(2^3)' bits present. Each 32 bits managing
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the allocated / free status for 32 blocks. Since each unsigned int
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contains 32-bits, one unsigned int can manage upto 32 blocks'
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status. Each bit-map is made up of a number of unsigned ints, whose
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exact number for a super-block of a given size I have just
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mentioned.
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3) How do the allocate and deallocate functions work in regard to
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bitmaps?
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The allocate and deallocate functions manipulate the bitmaps and have
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nothing to do with the memory that is given to the user. As I have
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earlier mentioned, a 1 in the bitmap's bit field indicates free,
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while a 0 indicates allocated. This lets us check 32 bits at a time
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to check whether there is at lease one free block in those 32 blocks
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by testing for equality with (0). Now, the allocate function will
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given a memory block find the corresponding bit in the bitmap, and
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will reset it (ie. make it re-set (0)). And when the deallocate
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function is called, it will again set that bit after locating it to
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indicate that that particular block corresponding to this bit in the
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bit-map is not being used by anyone, and may be used to satisfy
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future requests.
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----------------------------------------------------------------------
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(Tech-Stuff, Please stay out if you are not interested in the
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selection of certain constants. This has nothing to do with the
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algorithm per-se, only with some vales that must be chosen correctly
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to ensure that the allocator performs well in a real word scenario,
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and maintains a good balance between the memory consumption and the
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allocation/deallocation speed).
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The formula for calculating the maximum wastage as a percentage:
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(32 X k + 1) / (2 X (32 X k + 1 + 32 X c)) X 100.
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Where,
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k => The constant overhead per node. eg. for list, it is 8
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bytes, and for map it is 12 bytes.
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c => The size of the base type on which the map/list is
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instantiated. Thus, suppose the the type1 is int and type2 is
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double, they are related by the relation sizeof(double) ==
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2*sizeof(int). Thus, all types must have this double size
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relation for this formula to work properly.
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Plugging-in: For List: k = 8 and c = 4 (int and double), we get:
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33.376%
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For map/multimap: k = 12, and c = 4 (int and double), we get:
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37.524%
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Thus, knowing these values, and based on the sizeof(value_type), we
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may create a function that returns the Max_Wastage_Percentage for us
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to use.
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