1 Memory allocation package - Implementation Notes
2 ------------------------------------------------
6 Made with loving care by Jonathan Larmour (jlarmour@redhat.com)
7 Initial version: 2000-07-03
8 Last updated: 2000-07-03
15 This document describes some interesting bits and pieces about the memory
16 allocation package - CYGPKG_MEMALLOC. It is intended as a guide to
17 developers, not users. This isn't (yet) in formal documentation format,
18 and probably should be.
24 The object of this package is to provide everything required for dynamic
25 memory allocation, some sample implementations, the ability to plug in
26 more implementations, and a standard malloc() style interface to those
29 The classic Unix-style view of a heap is using brk()/sbrk() to extend the
30 data segment of the application. However this is inappropriate for an
31 embedded system because:
33 - you may not have an MMU, which means memory may be disjoint, thus breaking
36 - in a single process system there is no need to play tricks since there
37 is only the one address space and therefore heap area to use.
39 Therefore instead, we base the heap on the idea of fixed size memory pools.
40 The size of each pool is known in advance.
46 Most of the infrastructure this package provides is geared towards
47 supporting the ISO standard malloc() family of functions. A "standard"
48 eCos allocator should be able to plug in to this infrastructure and
49 transparently work. The interface is based on simple use of C++ - nothing
52 The allocator to use is dictated by the
53 CYGBLD_MEMALLOC_MALLOC_IMPLEMENTATION_HEADER option. Choosing the
54 allocator can be done by ensuring the CDL for the new allocator
55 has a "requires" that sets the location of the header to use when that
56 allocator is enabled. New allocators should default to disabled, so they
57 don't have to worry about which one is the default, thus causing CDL
58 conflicts. When enabled the new allocator should also claim to implement
59 CYGINT_MEMALLOC_MALLOC_ALLOCATORS.
61 The implementation header file that is set must have a special property
62 though - it may be included with __MALLOC_IMPL_WANTED defined. If this
63 is the case, then this means the infrastructure wants to find out the
64 name of the class that is implemented in this header file. This is done
65 by setting CYGCLS_MEMALLOC_MALLOC_IMPL. If __MALLOC_IMPL_WANTED is defined
66 then no non-preprocessor output should be generated, as this will be included
67 in a TCL script in due course. An existing example from this package would
70 #define CYGCLS_MEMALLOC_MALLOC_IMPL Cyg_Mempool_dlmalloc
72 // if the implementation is all that's required, don't output anything else
73 #ifndef __MALLOC_IMPL_WANTED
75 class Cyg_Mempool_dlmalloc
79 To meet the expectations of malloc, the class should have the following
80 public interfaces (for details it is best to look at some of the
81 examples in this package):
83 - a constructor taking arguments of the form:
85 ALLOCATORNAME( cyg_uint8 *base, cyg_int32 size );
87 If you want to be able to support other arguments for when accessing
88 the allocator directly you can add them, but give them default values,
93 - a try_alloc() function that returns new memory, or NULL on failure:
96 try_alloc( cyg_int32 size );
98 - a free() function taking one pointer argument that returns a boolean
99 for success or failure:
102 free( cyg_uint8 *ptr );
104 Again, extra arguments can be added, as long as they are defaulted.
107 - resize_alloc() which is designed purely to support realloc(). It
110 resize_alloc( cyg_uint8 *alloc_ptr, cyg_int32 newsize,
111 cyg_int32 *oldsize );
113 The idea is that if alloc_ptr can be adjusted to newsize, then it will
114 be. If oldsize is non-NULL the old size (possibly rounded) is placed
115 there. However what this *doesn't* do (unlike the real realloc()) is
116 fall back to doing a new malloc(). All it does is try to do tricks
117 inside the allocator. It's up to higher layers to call malloc().
119 - get_status() allows the retrieval of info from the allocator. The idea
120 is to pass in the bitmask OR of the flags defined in common.hxx, which
121 selects what information is requested. If the request is supported by
122 the allocator, the approriate structure fields are filled in; otherwise
123 unsupported fields will be left with the value -1. (The constructor for
124 Cyg_Mempool_Status initializes them to -1). If you want to reinitialize
125 the structure and deliberately lose the data in a Cyg_Mempool_Status
126 object, you need to invoke the init() method of the status object to
130 get_status( cyg_mempool_status_flag_t flags, Cyg_Mempool_Status &status );
132 A subset of the available stats are exported via mallinfo()
135 Cyg_Mempolt2 template
136 ---------------------
138 If using the eCos kernel with multiple threads accessing the allocators,
139 then obviously you need to be sure that the allocator is accessed in a
140 thread-safe way. The malloc() wrappers do not make any assumptions
141 about this. One helpful approach currently used by all the allocators
142 in this package is to (optionally) use a template (Cyg_Mempolt2) that
143 provides extra functions like a blocking alloc() that waits for memory
144 to be freed before returning, and a timed variant. Other calls are
145 generally passed straight through, but with the kernel scheduler locked
146 to prevent pre-emption.
148 You don't have to use this facility to fit into the infrastructure though,
149 and thread safety is not a prerequisite for the rest of the infrastructure.
150 And indeed certain allocators will be able to do scheduling at a finer
151 granularity than just locking the scheduler every time.
153 The odd name is because of an original desire to keep 8.3 filenames, which
154 was reflected in the class name to make it correspond to the filename.
155 There used to be an alternative Cyg_Mempoolt template, but that has fallen
156 into disuse and is no longer supported.
159 Automatic heap sizing
160 ---------------------
162 This package contains infrastructure to allow the automatic definition
163 of memory pools that occupy all available memory. In order to do this
164 you must use the eCos Memory Layout Tool to define a user-defined section.
165 These sections *must* have the prefix "heap", for example "heap1", "heap2",
166 "heapdram" etc. otherwise they will be ignored.
168 The user-defined section may be of fixed size, or of unknown size. If it
169 has unknown size then its size is dictated by either the location of
170 the next following section with an absolute address, or if there are
171 no following sections, the end of the memory region. The latter should
174 If no user-defined sections starting with "heap" are found, a fallback
175 static array (i.e. allocated in the BSS) will be used, whose size can
176 be set in the configuration.
178 It is also possible to define multiple heap sections. This is
179 necessary when you have multiple disjoint memory regions, and no MMU
180 to join it up into one contiguous memory space. In which case
181 a special wrapper allocator object is automatically used. This object
182 is an instantiation of the Cyg_Mempool_Joined template class,
183 defined in memjoin.hxx. It is instantiated with a list of every heap
184 section, which it then records. It's sole purpose is to act as a go
185 between to the underlying implementation, and does the right thing by
186 using pointer addresses to determine which memory pool the pointer
187 allocator, and therefore which memory pool instantiation to use.
189 Obviously using the Cyg_Mempool_Joined class adds overhead, but if this
190 is a problem, then in that case you shouldn't define multiple disjoint
197 As a special case, some platforms support the addition of memory in the
198 field, in which case it is desirable to automatically make this
199 available to malloc. The mechanism for this is to define a macro in
200 the HAL, specifically, defined in hal_intr.h:
202 HAL_MEM_REAL_REGION_TOP( cyg_uint8 *regionend )
204 This macro takes the address of the "normal" end of the region. This
205 corresponds with the size of the memory region in the MLT, and would
206 be end of the "unexpanded" region. This makes sense because the memory
207 region must be determined by the "worst case" of what memory will be
210 This macro then returns a pointer which is the *real* region end,
211 as determined by the HAL at run-time.
213 By having the macro in this form, it is therefore flexible enough to
214 work with multiple memory regions.
216 There is an example in the ARM HAL - specifically the EBSA285.
222 The MLT outputs macros providing information about user-defined sections
223 into a header file, available via system.h with the CYGHWR_MEMORY_LAYOUT_H
224 define. When the user-defined section has no known size, it determines
225 the size correctly relative to the end of the region, and sets the SIZE
228 A custom build rule preprocesses src/heapgen.cpp to generate heapgeninc.tcl
229 This contains TCL "set"s to allow access to the values of various
230 bits of configuration data. heapgen.cpp also includes the malloc
231 implementation header (as defined by
232 CYGBLD_MEMALLOC_MALLOC_IMPLEMENTATION_HEADER) with __MALLOC_IMPL_WANTED
233 defined. This tells the header that it should define the macro
234 CYGCLS_MEMALLOC_MALLOC_IMPL to be the name of the actual class. This
235 is then also exported with a TCL "set".
237 src/heapgen.tcl then includes heapgeninc.tcl which gives it access to
238 the configuration values. heapgen.tcl then searches the LDI file for
239 any sections beginning with "heap" (with possibly leading underscores).
240 It records each one it finds and then generates a file heaps.cxx in the
241 build tree to instantiate a memory pool object of the required class for
242 each heap. It also generates a list containing the addresses of each
243 pool that was instantiated. A header file heaps.hxx is then generated
244 that exports the number of pools, a reference to this list array and
245 includes the implementation header.
247 Custom build rules then copy the heaps.hxx into the include/pkgconf
248 subdir of the install tree, and compile the heaps.cxx.
250 To access the generated information, you must #include <pkgconf/heaps.hxx>
251 The number of heaps is given by the CYGMEM_HEAP_COUNT macro. The type of
252 the pools is given by CYGCLS_MEMALLOC_MALLOC_IMPL, and the array of
253 instantiated pools is available with cygmem_memalloc_heaps. For example,
254 here is a sample heaps.hxx:
256 #ifndef CYGONCE_PKGCONF_HEAPS_HXX
257 #define CYGONCE_PKGCONF_HEAPS_HXX
258 /* <pkgconf/heaps.hxx> */
260 /* This is a generated file - do not edit! */
262 #define CYGMEM_HEAP_COUNT 1
263 #include <cyg/memalloc/dlmalloc.hxx>
265 extern Cyg_Mempool_dlmalloc *cygmem_memalloc_heaps[ 2 ];
268 /* EOF <pkgconf/heaps.hxx> */
270 The array has size 2 because it consists of one pool, plus a terminating
273 In future the addition of cdl_get() available from TCL scripts contained
274 within the CDL scripts will remove the need for a lot of this magic.
280 A port of dlmalloc is included. Far too many changes were required to make
281 it fit within the scheme above, so therefore there was no point
282 trying to preserve the layout to make it easier to merge in new versions.
283 However dlmalloc rarely changes any more - it is very stable.
285 The version of dlmalloc used was a mixture of 2.6.6 and the dlmalloc from
286 newlib (based on 2.6.4). In the event, most of the patches merged were
287 of no consequence to the final version.
289 For reference, the various versions examined are included in the
290 doc/dlmalloc subdirectory: dlmalloc-2.6.4.c, dlmalloc-2.6.6.c,
291 dlmalloc-newlib.c and dlmalloc-merged.c (which is the result of merging
292 the changes between 2.6.4 and the newlib version into 2.6.6). Note it
293 was not tested at that point.
299 You should be allowed to have different allocators for different memory
300 regions. The biggest hurdle here is host tools support to express this.
302 Currently the "joined" allocator wrapper simply treats each memory pool
303 as an equal. It doesn't understand that some memory pools may be faster
304 than others, and cannot make decisions about which pools (and therefore
305 regions and therefore possibly speeds of memory) to use on the basis
306 of allocation size. This should be (configurably) possible.
313 A long, long time ago, in a galaxy far far away.... the situation used to
314 be that the kernel package contained the fixed block and simple variable
315 block memory allocators, and those were the only memory allocator
316 implementations. This was all a bit incongruous as it meant that any code
317 wanting dynamic memory allocation had to include the whole kernel, even
318 though the dependencies could be encapsulated. This was particularly silly
319 because the implementation of malloc() (etc.) in the C library didn't use
320 any of the features that *did* depend on the kernel, such as timed waits
321 while allocating memory, etc.
323 The C library malloc was pretty naff then too. It used a static buffer
324 as the basis of the memory pool, with a hard-coded size, set in the
325 configuration. You couldn't make it fit into all of memory.
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