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diff --git a/gcc-4.8.1/libstdc++-v3/doc/html/manual/bitmap_allocator_impl.html b/gcc-4.8.1/libstdc++-v3/doc/html/manual/bitmap_allocator_impl.html deleted file mode 100644 index 2f16969ab..000000000 --- a/gcc-4.8.1/libstdc++-v3/doc/html/manual/bitmap_allocator_impl.html +++ /dev/null @@ -1,312 +0,0 @@ -<?xml version="1.0" encoding="UTF-8" standalone="no"?> -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Implementation</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.77.1" /><meta name="keywords" content="ISO C++, allocator" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="bitmap_allocator.html" title="Chapter 21. The bitmap_allocator" /><link rel="prev" href="bitmap_allocator.html" title="Chapter 21. The bitmap_allocator" /><link rel="next" href="policy_data_structures.html" title="Chapter 22. Policy-Based Data Structures" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Implementation</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bitmap_allocator.html">Prev</a> </td><th width="60%" align="center">Chapter 21. The bitmap_allocator</th><td width="20%" align="right"> <a accesskey="n" href="policy_data_structures.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="allocator.bitmap.impl"></a>Implementation</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.free_list_store"></a>Free List Store</h3></div></div></div><p> - 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. - </p><p> - 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. - </p><p> - Suppose the free list size has reached its 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. - </p><p> - 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. - </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>Equal size check. Return true only when the 2 blocks are of equal -size.</p></li><li class="listitem"><p>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. </p></li><li class="listitem"><p>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.</p></li></ol></div><p> - Currently, (3) is being used with a value of 36% Maximum wastage per - Super Block. - </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.super_block"></a>Super Block</h3></div></div></div><p> - 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. - </p><p> - How does it interact with the free list store? - </p><p> - 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. - </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.super_block_data"></a>Super Block Data Layout</h3></div></div></div><p> - 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. - </p><p> - 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. - </p><p> - 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). - </p><div class="table"><a id="idp17500848"></a><p class="title"><strong>Table 21.1. Bitmap Allocator Memory Map</strong></p><div class="table-contents"><table summary="Bitmap Allocator Memory Map" border="1"><colgroup><col align="left" class="c1" /><col align="left" class="c2" /><col align="left" class="c3" /><col align="left" class="c4" /><col align="left" class="c5" /></colgroup><tbody><tr><td align="left">268</td><td align="left">0</td><td align="left">4294967295</td><td align="left">4294967295</td><td align="left">Data -> Space for 64 ints</td></tr></tbody></table></div></div><br class="table-break" /><p> - 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. - </p><p> - What do the remaining values represent?</p><p> - 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. - </p><p> - The value 4294967295 represents the integer corresponding to the bit - representation of all bits set: 11111111111111111111111111111111. - </p><p> - 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). - </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.max_wasted"></a>Maximum Wasted Percentage</h3></div></div></div><p> - 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. - </p><p> - The formula for calculating the maximum wastage as a percentage: - </p><p> -(32 x k + 1) / (2 x (32 x k + 1 + 32 x c)) x 100. - </p><p> - 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. - </p><p> - Plugging-in: For List: k = 8 and c = 4 (int and double), we get: - 33.376% - </p><p> -For map/multimap: k = 12, and c = 4 (int and double), we get: 37.524% - </p><p> - 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. - </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.allocate"></a><code class="function">allocate</code></h3></div></div></div><p> - 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. - </p><p> - Suppose n == 1, then the allocator does the following: - </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p> - 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. - </p></li><li class="listitem"><p> - 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. - </p></li><li class="listitem"><p> - Is there any block in whatever region of memory that we own - free? This is done by checking - </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> - The use count for each super block, and if that fails then - </p></li><li class="listitem"><p> - The individual bit-maps for each super block. - </p></li></ul></div><p> - 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). - </p></li><li class="listitem"><p> - This process involves Refilling the internal exponentially - growing memory pool. The said effect is achieved by calling - _S_refill_pool which does the following: - </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> - Gets more memory from the Global Free List of the Required - size. - </p></li><li class="listitem"><p> - Adjusts the size for the next call to itself. - </p></li><li class="listitem"><p> - Writes the appropriate headers in the bit-maps. - </p></li><li class="listitem"><p> - Sets the use count for that super-block just allocated to 0 - (zero). - </p></li><li class="listitem"><p> - 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. - </p></li></ul></div></li></ol></div><p> -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. -</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.deallocate"></a><code class="function">deallocate</code></h3></div></div></div><p> - 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. - </p><p> - However for n == 1, a series of steps are performed: - </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p> - 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. - </p></li><li class="listitem"><p> - 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. - </p></li></ol></div><p> - 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. - </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.questions"></a>Questions</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="bitmap.impl.question.1"></a>1</h4></div></div></div><p> -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? - </p><p> - 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. - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="bitmap.impl.question.2"></a>2</h4></div></div></div><p> - 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? - </p><p> - 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. - </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="bitmap.impl.question.3"></a>3</h4></div></div></div><p> - How do the allocate and deallocate functions work in regard to - bitmaps? - </p><p> - 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. - </p><p> - e.g.: Consider a bit-map of 64-bits as represented below: - 1111111111111111111111111111111111111111111111111111111111111111 - </p><p> - 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. - </p><p> - The bit-map now looks like this: - 1111111111111111111111111111111111111111111111111111111111111110 - </p></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.locality"></a>Locality</h3></div></div></div><p> - 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. - </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>Constant time access for the bitmap themselves, since no kind of -look up will be needed to find the correct bitmap list or its -equivalent.</p></li><li class="listitem"><p>And also this would preserve the cache as far as possible.</p></li></ol></div><p> - 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. - </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="bitmap.impl.grow_policy"></a>Overhead and Grow Policy</h3></div></div></div><p> - 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. - </p><p> - 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. - </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bitmap_allocator.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bitmap_allocator.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="policy_data_structures.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 21. The bitmap_allocator </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 22. Policy-Based Data Structures</td></tr></table></div></body></html>
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