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annotate packages/services/memalloc/common/current/doc/notes.txt @ 115:6ed91473a1cd ecos-sw-2000-08-21

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Merge from eCos master repository on 2000-08-21-22:40:54-BST
author jlarmour
date Fri, 25 Aug 2000 17:32:38 +0000
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115
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1 Memory allocation package - Implementation Notes
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2 ------------------------------------------------
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3
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4
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5
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6 Made with loving care by Jonathan Larmour (jlarmour@redhat.com)
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7 Initial version: 2000-07-03
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8 Last updated: 2000-07-03
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9
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10
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11
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12 Meta
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13 ----
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14
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15 This document describes some interesting bits and pieces about the memory
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16 allocation package - CYGPKG_MEMALLOC. It is intended as a guide to
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17 developers, not users. This isn't (yet) in formal documentation format,
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18 and probably should be.
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19
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20
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21 Philosophy
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22 ----------
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23
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24 The object of this package is to provide everything required for dynamic
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25 memory allocation, some sample implementations, the ability to plug in
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26 more implementations, and a standard malloc() style interface to those
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27 allocators.
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28
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29 The classic Unix-style view of a heap is using brk()/sbrk() to extend the
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30 data segment of the application. However this is inappropriate for an
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31 embedded system because:
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32
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33 - you may not have an MMU, which means memory may be disjoint, thus breaking
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34 this paradigm
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35
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36 - in a single process system there is no need to play tricks since there
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37 is only the one address space and therefore heap area to use.
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38
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39 Therefore instead, we base the heap on the idea of fixed size memory pools.
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40 The size of each pool is known in advance.
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41
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42
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43 Overview
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44 --------
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45
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46 Most of the infrastructure this package provides is geared towards
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47 supporting the ISO standard malloc() family of functions. A "standard"
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48 eCos allocator should be able to plug in to this infrastructure and
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49 transparently work. The interface is based on simple use of C++ - nothing
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50 too advanced.
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51
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52 The allocator to use is dictated by the
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53 CYGBLD_MEMALLOC_MALLOC_IMPLEMENTATION_HEADER option. Choosing the
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54 allocator can be done by ensuring the CDL for the new allocator
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55 has a "requires" that sets the location of the header to use when that
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56 allocator is enabled. New allocators should default to disabled, so they
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57 don't have to worry about which one is the default, thus causing CDL
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58 conflicts. When enabled the new allocator should also claim to implement
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59 CYGINT_MEMALLOC_MALLOC_ALLOCATORS.
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60
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61 The implementation header file that is set must have a special property
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62 though - it may be included with __MALLOC_IMPL_WANTED defined. If this
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63 is the case, then this means the infrastructure wants to find out the
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64 name of the class that is implemented in this header file. This is done
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65 by setting CYGCLS_MEMALLOC_MALLOC_IMPL. If __MALLOC_IMPL_WANTED is defined
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66 then no non-preprocessor output should be generated, as this will be included
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67 in a TCL script in due course. An existing example from this package would
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68 be:
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69
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70 #define CYGCLS_MEMALLOC_MALLOC_IMPL Cyg_Mempool_dlmalloc
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71
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72 // if the implementation is all that's required, don't output anything else
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73 #ifndef __MALLOC_IMPL_WANTED
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74
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75 class Cyg_Mempool_dlmalloc
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76 {
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77 [etc.]
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78
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79 To meet the expectations of malloc, the class should have the following
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80 public interfaces (for details it is best to look at some of the
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81 examples in this package):
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82
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83 - a constructor taking arguments of the form:
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84
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85 ALLOCATORNAME( cyg_uint8 *base, cyg_int32 size );
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86
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87 If you want to be able to support other arguments for when accessing
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88 the allocator directly you can add them, but give them default values,
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89 or use overloading
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90
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91 - a destructor
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92
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93 - a try_alloc() function that returns new memory, or NULL on failure:
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94
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95 cyg_uint8 *
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96 try_alloc( cyg_int32 size );
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97
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98 - a free() function taking one pointer argument that returns a boolean
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99 for success or failure:
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100
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101 cyg_bool
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102 free( cyg_uint8 *ptr );
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103
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104 Again, extra arguments can be added, as long as they are defaulted.
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105
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106
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107 - resize_alloc() which is designed purely to support realloc(). It
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108 has the prototype:
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109 cyg_uint8 *
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110 resize_alloc( cyg_uint8 *alloc_ptr, cyg_int32 newsize,
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111 cyg_int32 *oldsize );
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112
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113 The idea is that if alloc_ptr can be adjusted to newsize, then it will
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114 be. If oldsize is non-NULL the old size (possibly rounded) is placed
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115 there. However what this *doesn't* do (unlike the real realloc()) is
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116 fall back to doing a new malloc(). All it does is try to do tricks
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117 inside the allocator. It's up to higher layers to call malloc().
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118
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119 - get_status() allows the retrieval of info from the allocator. The idea
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120 is to pass in the bitmask OR of the flags defined in common.hxx, which
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121 selects what information is requested. If the request is supported by
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122 the allocator, the approriate structure fields are filled in; otherwise
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123 unsupported fields will be left with the value -1. (The constructor for
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124 Cyg_Mempool_Status initializes them to -1). If you want to reinitialize
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125 the structure and deliberately lose the data in a Cyg_Mempool_Status
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126 object, you need to invoke the init() method of the status object to
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127 reinitialize it.
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128
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129 void
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130 get_status( cyg_mempool_status_flag_t flags, Cyg_Mempool_Status &status );
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131
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132 A subset of the available stats are exported via mallinfo()
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133
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134
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135 Cyg_Mempolt2 template
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136 ---------------------
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137
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138 If using the eCos kernel with multiple threads accessing the allocators,
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139 then obviously you need to be sure that the allocator is accessed in a
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140 thread-safe way. The malloc() wrappers do not make any assumptions
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141 about this. One helpful approach currently used by all the allocators
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142 in this package is to (optionally) use a template (Cyg_Mempolt2) that
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143 provides extra functions like a blocking alloc() that waits for memory
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144 to be freed before returning, and a timed variant. Other calls are
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145 generally passed straight through, but with the kernel scheduler locked
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146 to prevent pre-emption.
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147
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148 You don't have to use this facility to fit into the infrastructure though,
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149 and thread safety is not a prerequisite for the rest of the infrastructure.
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150 And indeed certain allocators will be able to do scheduling at a finer
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151 granularity than just locking the scheduler every time.
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152
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153 The odd name is because of an original desire to keep 8.3 filenames, which
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154 was reflected in the class name to make it correspond to the filename.
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155 There used to be an alternative Cyg_Mempoolt template, but that has fallen
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156 into disuse and is no longer supported.
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157
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158
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159 Automatic heap sizing
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160 ---------------------
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161
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162 This package contains infrastructure to allow the automatic definition
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163 of memory pools that occupy all available memory. In order to do this
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164 you must use the eCos Memory Layout Tool to define a user-defined section.
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165 These sections *must* have the prefix "heap", for example "heap1", "heap2",
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166 "heapdram" etc. otherwise they will be ignored.
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167
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168 The user-defined section may be of fixed size, or of unknown size. If it
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169 has unknown size then its size is dictated by either the location of
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170 the next following section with an absolute address, or if there are
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171 no following sections, the end of the memory region. The latter should
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172 be the norm.
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173
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174 If no user-defined sections starting with "heap" are found, a fallback
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175 static array (i.e. allocated in the BSS) will be used, whose size can
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176 be set in the configuration.
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177
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178 It is also possible to define multiple heap sections. This is
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179 necessary when you have multiple disjoint memory regions, and no MMU
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180 to join it up into one contiguous memory space. In which case
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181 a special wrapper allocator object is automatically used. This object
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182 is an instantiation of the Cyg_Mempool_Joined template class,
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183 defined in memjoin.hxx. It is instantiated with a list of every heap
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184 section, which it then records. It's sole purpose is to act as a go
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185 between to the underlying implementation, and does the right thing by
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186 using pointer addresses to determine which memory pool the pointer
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187 allocator, and therefore which memory pool instantiation to use.
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188
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189 Obviously using the Cyg_Mempool_Joined class adds overhead, but if this
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190 is a problem, then in that case you shouldn't define multiple disjoint
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191 heaps!
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192
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193
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194 Run-time heap sizing
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195 --------------------
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196
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197 As a special case, some platforms support the addition of memory in the
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198 field, in which case it is desirable to automatically make this
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199 available to malloc. The mechanism for this is to define a macro in
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200 the HAL, specifically, defined in hal_intr.h:
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201
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202 HAL_MEM_REAL_REGION_TOP( cyg_uint8 *regionend )
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203
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204 This macro takes the address of the "normal" end of the region. This
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205 corresponds with the size of the memory region in the MLT, and would
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206 be end of the "unexpanded" region. This makes sense because the memory
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207 region must be determined by the "worst case" of what memory will be
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208 installed.
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209
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210 This macro then returns a pointer which is the *real* region end,
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211 as determined by the HAL at run-time.
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212
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213 By having the macro in this form, it is therefore flexible enough to
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214 work with multiple memory regions.
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215
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216 There is an example in the ARM HAL - specifically the EBSA285.
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217
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218
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219 How it works
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220 ------------
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221
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222 The MLT outputs macros providing information about user-defined sections
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223 into a header file, available via system.h with the CYGHWR_MEMORY_LAYOUT_H
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224 define. When the user-defined section has no known size, it determines
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225 the size correctly relative to the end of the region, and sets the SIZE
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226 macro accordingly.
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227
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228 A custom build rule preprocesses src/heapgen.cpp to generate heapgeninc.tcl
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229 This contains TCL "set"s to allow access to the values of various
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230 bits of configuration data. heapgen.cpp also includes the malloc
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231 implementation header (as defined by
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232 CYGBLD_MEMALLOC_MALLOC_IMPLEMENTATION_HEADER) with __MALLOC_IMPL_WANTED
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233 defined. This tells the header that it should define the macro
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234 CYGCLS_MEMALLOC_MALLOC_IMPL to be the name of the actual class. This
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235 is then also exported with a TCL "set".
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236
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237 src/heapgen.tcl then includes heapgeninc.tcl which gives it access to
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238 the configuration values. heapgen.tcl then searches the LDI file for
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239 any sections beginning with "heap" (with possibly leading underscores).
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240 It records each one it finds and then generates a file heaps.cxx in the
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241 build tree to instantiate a memory pool object of the required class for
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242 each heap. It also generates a list containing the addresses of each
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243 pool that was instantiated. A header file heaps.hxx is then generated
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244 that exports the number of pools, a reference to this list array and
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245 includes the implementation header.
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246
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247 Custom build rules then copy the heaps.hxx into the include/pkgconf
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248 subdir of the install tree, and compile the heaps.cxx.
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249
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250 To access the generated information, you must #include <pkgconf/heaps.hxx>
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251 The number of heaps is given by the CYGMEM_HEAP_COUNT macro. The type of
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252 the pools is given by CYGCLS_MEMALLOC_MALLOC_IMPL, and the array of
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253 instantiated pools is available with cygmem_memalloc_heaps. For example,
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254 here is a sample heaps.hxx:
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255
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256 #ifndef CYGONCE_PKGCONF_HEAPS_HXX
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257 #define CYGONCE_PKGCONF_HEAPS_HXX
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258 /* <pkgconf/heaps.hxx> */
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259
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260 /* This is a generated file - do not edit! */
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261
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262 #define CYGMEM_HEAP_COUNT 1
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263 #include <cyg/memalloc/dlmalloc.hxx>
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264
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265 extern Cyg_Mempool_dlmalloc *cygmem_memalloc_heaps[ 2 ];
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266
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267 #endif
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268 /* EOF <pkgconf/heaps.hxx> */
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269
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270 The array has size 2 because it consists of one pool, plus a terminating
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271 NULL.
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272
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273 In future the addition of cdl_get() available from TCL scripts contained
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274 within the CDL scripts will remove the need for a lot of this magic.
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275
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276
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277 dlmalloc
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278 --------
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279
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280 A port of dlmalloc is included. Far too many changes were required to make
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281 it fit within the scheme above, so therefore there was no point
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282 trying to preserve the layout to make it easier to merge in new versions.
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283 However dlmalloc rarely changes any more - it is very stable.
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284
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285 The version of dlmalloc used was a mixture of 2.6.6 and the dlmalloc from
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286 newlib (based on 2.6.4). In the event, most of the patches merged were
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287 of no consequence to the final version.
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288
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289 For reference, the various versions examined are included in the
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290 doc/dlmalloc subdirectory: dlmalloc-2.6.4.c, dlmalloc-2.6.6.c,
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291 dlmalloc-newlib.c and dlmalloc-merged.c (which is the result of merging
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292 the changes between 2.6.4 and the newlib version into 2.6.6). Note it
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293 was not tested at that point.
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294
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295
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296 Remaining issues
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297 ----------------
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298
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299 You should be allowed to have different allocators for different memory
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300 regions. The biggest hurdle here is host tools support to express this.
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301
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302 Currently the "joined" allocator wrapper simply treats each memory pool
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303 as an equal. It doesn't understand that some memory pools may be faster
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304 than others, and cannot make decisions about which pools (and therefore
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305 regions and therefore possibly speeds of memory) to use on the basis
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306 of allocation size. This should be (configurably) possible.
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307
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308
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309 History
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310 -------
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311
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312
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313 A long, long time ago, in a galaxy far far away.... the situation used to
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314 be that the kernel package contained the fixed block and simple variable
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315 block memory allocators, and those were the only memory allocator
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316 implementations. This was all a bit incongruous as it meant that any code
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317 wanting dynamic memory allocation had to include the whole kernel, even
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318 though the dependencies could be encapsulated. This was particularly silly
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319 because the implementation of malloc() (etc.) in the C library didn't use
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320 any of the features that *did* depend on the kernel, such as timed waits
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321 while allocating memory, etc.
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322
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323 The C library malloc was pretty naff then too. It used a static buffer
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324 as the basis of the memory pool, with a hard-coded size, set in the
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325 configuration. You couldn't make it fit into all of memory.
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326
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327 Jifl
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328 2000-07-03
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329
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330 //####COPYRIGHTBEGIN####
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331 //
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332 // -------------------------------------------
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333 // The contents of this file are subject to the Red Hat eCos Public License
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334 // Version 1.1 (the "License"); you may not use this file except in
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335 // compliance with the License. You may obtain a copy of the License at
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336 // http://www.redhat.com/
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337 //
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338 // Software distributed under the License is distributed on an "AS IS"
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339 // basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the
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340 // License for the specific language governing rights and limitations under
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341 // the License.
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342 //
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343 // The Original Code is eCos - Embedded Configurable Operating System,
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344 // released September 30, 1998.
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345 //
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346 // The Initial Developer of the Original Code is Red Hat.
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347 // Portions created by Red Hat are
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348 // Copyright (C) 1998, 1999, 2000 Red Hat, Inc.
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349 // All Rights Reserved.
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350 // -------------------------------------------
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351 //
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352 //####COPYRIGHTEND####