malloc内存分配字节对齐问题
2016-01-04 22:58
405 查看
最近看了一些开源的C/C++库,其中都对于内存分配这块做出了自己的一些优化和说明,也涉及到了一些内存分配字节对齐以及内存分页的问题。
对于内存分配的字节对齐问题,一直都是只知其事,不知其解,平时也很少关注这一块会带来的性能问题。但是要是放在一个高并发,快速以及资源最大化利用的系统里面,这一块往往是需要注意的,所以也就趁着这次机会,大概的了解一下。
我们先来看一下glibc里面malloc.c的定义
实际上,对齐参数(MALLOC_ALIGNMENT)大小的设定需要满足以下两点:
1. 必须是2的幂
2. 必须是void *的整数倍
所以从request2size可知,在64位系统,如果申请内存为1~24字节,系统内存消耗32字节,当申请25字节的内存时,系统内存消耗48字节。而对于32位系统,申请内存为1~12字节时,系统内存消耗为16字节,当申请内存为13字节时,系统内存消耗为24字节。
这里分享一个别人写的怎么实现一个简单的malloc函数:http://blog.codinglabs.org/articles/a-malloc-tutorial.html
对于内存分配的字节对齐问题,一直都是只知其事,不知其解,平时也很少关注这一块会带来的性能问题。但是要是放在一个高并发,快速以及资源最大化利用的系统里面,这一块往往是需要注意的,所以也就趁着这次机会,大概的了解一下。
我们先来看一下glibc里面malloc.c的定义
1100 /* 1101 ----------------------- Chunk representations ----------------------- 1102 */ 1103 1104 1105 /* 1106 This struct declaration is misleading (but accurate and necessary). 1107 It declares a "view" into memory allowing access to necessary 1108 fields at known offsets from a given base. See explanation below. 1109 */ 1110 1111 struct malloc_chunk { 1112 1113 INTERNAL_SIZE_T prev_size; /* Size of previous chunk (if free). */ 1114 INTERNAL_SIZE_T size; /* Size in bytes, including overhead. */ 1115 1116 struct malloc_chunk* fd; /* double links -- used only if free. */ 1117 struct malloc_chunk* bk; 1118 1119 /* Only used for large blocks: pointer to next larger size. */ 1120 struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */ 1121 struct malloc_chunk* bk_nextsize; 1122 }; 1123 1124 1125 /* 1126 malloc_chunk details: 1127 1128 (The following includes lightly edited explanations by Colin Plumb.) 1129 1130 Chunks of memory are maintained using a `boundary tag' method as 1131 described in e.g., Knuth or Standish. (See the paper by Paul 1132 Wilson ftp://ftp.cs.utexas.edu/pub/garbage/allocsrv.ps for a 1133 survey of such techniques.) Sizes of free chunks are stored both 1134 in the front of each chunk and at the end. This makes 1135 consolidating fragmented chunks into bigger chunks very fast. The 1136 size fields also hold bits representing whether chunks are free or 1137 in use. 1138 1139 An allocated chunk looks like this: 1140 1141 1142 chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1143 | Size of previous chunk, if allocated | | 1144 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1145 | Size of chunk, in bytes |M|P| 1146 mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1147 | User data starts here... . 1148 . . 1149 . (malloc_usable_size() bytes) . 1150 . | 1151 nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1152 | Size of chunk | 1153 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1154 1155 1156 Where "chunk" is the front of the chunk for the purpose of most of 1157 the malloc code, but "mem" is the pointer that is returned to the 1158 user. "Nextchunk" is the beginning of the next contiguous chunk. 1159 1160 Chunks always begin on even word boundaries, so the mem portion 1161 (which is returned to the user) is also on an even word boundary, and 1162 thus at least double-word aligned. 1163 1164 Free chunks are stored in circular doubly-linked lists, and look like this: 1165 1166 chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1167 | Size of previous chunk | 1168 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1169 `head:' | Size of chunk, in bytes |P| 1170 mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1171 | Forward pointer to next chunk in list | 1172 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1173 | Back pointer to previous chunk in list | 1174 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1175 | Unused space (may be 0 bytes long) . 1176 . . 1177 . | 1178 nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1179 `foot:' | Size of chunk, in bytes | 1180 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1181 1182 The P (PREV_INUSE) bit, stored in the unused low-order bit of the 1183 chunk size (which is always a multiple of two words), is an in-use 1184 bit for the *previous* chunk. If that bit is *clear*, then the 1185 word before the current chunk size contains the previous chunk 1186 size, and can be used to find the front of the previous chunk. 1187 The very first chunk allocated always has this bit set, 1188 preventing access to non-existent (or non-owned) memory. If 1189 prev_inuse is set for any given chunk, then you CANNOT determine 1190 the size of the previous chunk, and might even get a memory 1191 addressing fault when trying to do so. 1192 1193 Note that the `foot' of the current chunk is actually represented 1194 as the prev_size of the NEXT chunk. This makes it easier to 1195 deal with alignments etc but can be very confusing when trying 1196 to extend or adapt this code. 1197 1198 The two exceptions to all this are 1199 1200 1. The special chunk `top' doesn't bother using the 1201 trailing size field since there is no next contiguous chunk 1202 that would have to index off it. After initialization, `top' 1203 is forced to always exist. If it would become less than 1204 MINSIZE bytes long, it is replenished. 1205 1206 2. Chunks allocated via mmap, which have the second-lowest-order 1207 bit M (IS_MMAPPED) set in their size fields. Because they are 1208 allocated one-by-one, each must contain its own trailing size field. 1209 1210 */ 1211 1212 /* 1213 ---------- Size and alignment checks and conversions ---------- 1214 */ 1215 1216 /* conversion from malloc headers to user pointers, and back */ 1217 1218 #define chunk2mem(p) ((void*)((char*)(p) + 2*SIZE_SZ)) 1219 #define mem2chunk(mem) ((mchunkptr)((char*)(mem) - 2*SIZE_SZ)) 1220 1221 /* The smallest possible chunk */ 1222 #define MIN_CHUNK_SIZE (offsetof(struct malloc_chunk, fd_nextsize)) 1223 1224 /* The smallest size we can malloc is an aligned minimal chunk */ 1225 1226 #define MINSIZE \ 1227 (unsigned long)(((MIN_CHUNK_SIZE+MALLOC_ALIGN_MASK) & ~MALLOC_ALIGN_MASK)) 1228 1229 /* Check if m has acceptable alignment */ 1230 1231 #define aligned_OK(m) (((unsigned long)(m) & MALLOC_ALIGN_MASK) == 0) 1232 1233 #define misaligned_chunk(p) \ 1234 ((uintptr_t)(MALLOC_ALIGNMENT == 2 * SIZE_SZ ? (p) : chunk2mem (p)) \ 1235 & MALLOC_ALIGN_MASK) 1236 1237 1238 /* 1239 Check if a request is so large that it would wrap around zero when 1240 padded and aligned. To simplify some other code, the bound is made 1241 low enough so that adding MINSIZE will also not wrap around zero. 1242 */ 1243 1244 #define REQUEST_OUT_OF_RANGE(req) \ 1245 ((unsigned long) (req) >= \ 1246 (unsigned long) (INTERNAL_SIZE_T) (-2 * MINSIZE)) 1247 1248 /* pad request bytes into a usable size -- internal version */ 1249 1250 #define request2size(req) \ 1251 (((req) + SIZE_SZ + MALLOC_ALIGN_MASK < MINSIZE) ? \ 1252 MINSIZE : \ 1253 ((req) + SIZE_SZ + MALLOC_ALIGN_MASK) & ~MALLOC_ALIGN_MASK) 1254 1255 /* Same, except also perform argument check */ 1256 1257 #define checked_request2size(req, sz) \ 1258 if (REQUEST_OUT_OF_RANGE (req)) { \ 1259 __set_errno (ENOMEM); \ 1260 return 0; \ 1261 } \ 1262 (sz) = request2size (req); 1263其中,有很多的宏定义,我们只看最主要的几个。request2size负责内存对齐操作,MINSIZE是malloc时内存占用的最小内存单元,32位系统为16字节,64位系统为32字节,MALLOC_ALIGNMENT为内存对齐字节数,由于在32和64位系统中,size_t为4字节和8字节,所以MALLOC_ALIGNMENT在32位和64位系统中,分别为8和16.
实际上,对齐参数(MALLOC_ALIGNMENT)大小的设定需要满足以下两点:
1. 必须是2的幂
2. 必须是void *的整数倍
所以从request2size可知,在64位系统,如果申请内存为1~24字节,系统内存消耗32字节,当申请25字节的内存时,系统内存消耗48字节。而对于32位系统,申请内存为1~12字节时,系统内存消耗为16字节,当申请内存为13字节时,系统内存消耗为24字节。
这里分享一个别人写的怎么实现一个简单的malloc函数:http://blog.codinglabs.org/articles/a-malloc-tutorial.html
相关文章推荐
- 解析C语言中位字段内存分配的问题
- C语言编程中分配内存空间的相关函数
- c语言中malloc、realloc与calloc 的区别以及联系
- C++中new与delete、malloc与free应用分析
- C#字符串内存分配与驻留池学习分享
- C语言中的malloc使用详解
- 基于C++内存分配、函数调用与返回值的深入分析
- PHP变量内存分配问题记录整理
- 理解Javascript_01_理解内存分配原理分析
- new和malloc的区别深入解析
- C/C++语言中结构体的内存分配小例子
- C/C++动态分配与释放内存的区别详细解析
- JavaScript对内存分配及管理机制详细解析
- Java中内存分配的几种方法
- 关于C语言程序的内存分配的入门知识学习
- new、delelte和malloc、free的关系
- 关于操作系统各种数据类型的内存分配的一点探讨
- 初识英特尔线程构建模块之内存管理tbb_malloc
- VMware内存分配初探
- java内存管理