memcache slabs .
来源:互联网 发布:java游戏开发技术 编辑:程序博客网 时间:2024/05/01 00:56
http://blog.csdn.net/benbendy1984/article/details/6216453
memcache 使用slab机制对内存进行分配和管理的。
一 整体结构
把内存分成多个层次,在每个层次中,可以有多个page, 所有层次的page大小基本是一直的(为什么不相同,后面介绍),每个page中又分成相同大小的item,里面有多少个item,取决于page大小和item所在的层次,其中相邻层次直接item大小具有如下关系
size(item: n+1 ) = size( item: n) * grow_factor
其中的grow_factor 是可配置的参数。
在内存分配时,如果需要大小为k的空间,那么就从第n个层次中得到一个item ,其中n需要满足
size(item:n-1) < k <= size(item:n)
那么从n个层次中的这么多个item中取哪个?所有被取光了怎么办?使用完了释放了怎么办? 这些都会在后面进行详细的介绍
二 结构成员
- typedef struct {
- unsigned int size; /* 每个item大小, sizes of items */
- unsigned int perslab; /* 每个page中包含多少个item , how many items per slab */
- void **slots; /* 空闲的item指针, list of item ptrs */
- unsigned int sl_total; /* 以分配空闲的item 个数, size of previous array */
- unsigned int sl_curr; /* 当前空闲的item位置(也就是实际空闲item个数),从后往前的, first free slot */
- void *end_page_ptr; /* 指向最后一个页面中空闲的item开始位置, pointer to next free item at end of page, or 0 */
- unsigned int end_page_free; /* 最后一个页面,item个数, number of items remaining at end of last alloced page */
- unsigned int slabs; /* 实际使用slab(page)个数 how many slabs were allocated for this class */
- void **slab_list; /* 所有page的指针, array of slab pointers */
- unsigned int list_size; /* 已经分配page指针个数,size of prev array */
- unsigned int killing; /* index+1 of dying slab, or zero if none */
- size_t requested; /* 所有被使用了的内存的大小, The number of requested bytes */
- } slabclass_t;
三 函数介绍
3.1 slabs_clsid()
- /*
- * Figures out which slab class (chunk size) is required to store an item of
- * a given size.
- *
- * Given object size, return id to use when allocating/freeing memory for object
- * 0 means error: can't store such a large object
- * 根据大小,返回满足条件的item所在的层次
- */
- unsigned int slabs_clsid(const size_t size) {
- int res = POWER_SMALLEST;
- if (size == 0)
- return 0;
- while (size > slabclass[res].size)
- if (res++ == power_largest) /* won't fit in the biggest slab */
- return 0;
- return res;
- }
3.2 slabs_init
- /**
- * Determines the chunk sizes and initializes the slab class descriptors
- * accordingly.
- * slabs 初始化 1 如果需要预先分配的,那就预先分配内存 2
- * 对每个类别的slab,初始化该slab中每个item大小,item个数 3
- * 调用slabs_preallocate 预先分配slabs(给每个层次创建一个page)
- */
- void slabs_init(const size_t limit, const double factor, const bool prealloc) {
- int i = POWER_SMALLEST - 1;
- unsigned int size = sizeof(item) + settings.chunk_size;
- mem_limit = limit;
- if (prealloc) {
- //预先分配内存,并把开始地址和可使用空间大小记录
- /* Allocate everything in a big chunk with malloc */
- mem_base = malloc(mem_limit);
- if (mem_base != NULL) {
- mem_current = mem_base;
- mem_avail = mem_limit;
- } else {
- fprintf(stderr, "Warning: Failed to allocate requested memory in"
- " one large chunk./nWill allocate in smaller chunks/n");
- }
- }
- memset(slabclass, 0, sizeof(slabclass));
- while (++i < POWER_LARGEST && size <= settings.item_size_max / factor) {
- /* Make sure items are always n-byte aligned */
- //对齐
- if (size % CHUNK_ALIGN_BYTES)
- size += CHUNK_ALIGN_BYTES - (size % CHUNK_ALIGN_BYTES);
- //每个item大小以及item个数
- slabclass[i].size = size;
- slabclass[i].perslab = settings.item_size_max / slabclass[i].size;
- size *= factor;
- if (settings.verbose > 1) {
- fprintf(stderr, "slab class %3d: chunk size %9u perslab %7u/n",
- i, slabclass[i].size, slabclass[i].perslab);
- }
- }
- //最后一个slab
- power_largest = i;
- slabclass[power_largest].size = settings.item_size_max;
- slabclass[power_largest].perslab = 1;
- if (settings.verbose > 1) {
- fprintf(stderr, "slab class %3d: chunk size %9u perslab %7u/n",
- i, slabclass[i].size, slabclass[i].perslab);
- }
- /* for the test suite: faking of how much we've already malloc'd */
- {
- char *t_initial_malloc = getenv("T_MEMD_INITIAL_MALLOC");
- if (t_initial_malloc) {
- mem_malloced = (size_t)atol(t_initial_malloc);
- }
- }
- #ifndef DONT_PREALLOC_SLABS
- {
- char *pre_alloc = getenv("T_MEMD_SLABS_ALLOC");
- if (pre_alloc == NULL || atoi(pre_alloc) != 0) {
- slabs_preallocate(power_largest);
- }
- }
- #endif
- }
3.3 slabs_preallocate
- //对于每个层次的slab,预先分配一个page
- static void slabs_preallocate (const unsigned int maxslabs) {
- int i;
- unsigned int prealloc = 0;
- /* pre-allocate a 1MB slab in every size class so people don't get
- confused by non-intuitive "SERVER_ERROR out of memory"
- messages. this is the most common question on the mailing
- list. if you really don't want this, you can rebuild without
- these three lines. */
- for (i = POWER_SMALLEST; i <= POWER_LARGEST; i++) {
- if (++prealloc > maxslabs)
- return;
- do_slabs_newslab(i);
- }
- }
3.4 grow_slab_list
- /确保page指针空间有空闲空间。如果可使用的空间满了,那么扩大成原来的两倍
- static int grow_slab_list (const unsigned int id) {
- slabclass_t *p = &slabclass[id];
- if (p->slabs == p->list_size) {
- size_t new_size = (p->list_size != 0) ? p->list_size * 2 : 16;
- void *new_list = realloc(p->slab_list, new_size * sizeof(void *));
- if (new_list == 0) return 0;
- p->list_size = new_size;
- p->slab_list = new_list;
- }
- return 1;
- }
3.5 do_slabs_newslab
- //分配一个类别id为id的page
- //1 分配空间 2 最后page赋值,以及该page空闲item个数 3 page数目++
- //,把page指针指向当前page
- static int do_slabs_newslab(const unsigned int id) {
- slabclass_t *p = &slabclass[id];
- int len = p->size * p->perslab;
- char *ptr;
- // 1有内存限制并且超过了限制 2 并且确认page指针空间失败 3 分配一个page的内存空间失败
- if ((mem_limit && mem_malloced + len > mem_limit && p->slabs > 0) ||
- (grow_slab_list(id) == 0) ||
- ((ptr = memory_allocate((size_t)len)) == 0)) {
- MEMCACHED_SLABS_SLABCLASS_ALLOCATE_FAILED(id);
- return 0;
- }
- memset(ptr, 0, (size_t)len);
- p->end_page_ptr = ptr;
- p->end_page_free = p->perslab;
- p->slab_list[p->slabs++] = ptr;
- mem_malloced += len;
- MEMCACHED_SLABS_SLABCLASS_ALLOCATE(id);
- return 1;
- }
3.6 do_slabs_alloc
- //根据id,返回对应类别的一个item空间
- //按照下面优先级进行分配: 1 首先从空闲的空间(回收的) 2 从最后的page中获取 3
- //新分配一个page
- static void *do_slabs_alloc(const size_t size, unsigned int id) {
- slabclass_t *p;
- void *ret = NULL;
- if (id < POWER_SMALLEST || id > power_largest) {
- MEMCACHED_SLABS_ALLOCATE_FAILED(size, 0);
- return NULL;
- }
- p = &slabclass[id];
- assert(p->sl_curr == 0 || ((item *)p->slots[p->sl_curr - 1])->slabs_clsid == 0);
- #ifdef USE_SYSTEM_MALLOC
- if (mem_limit && mem_malloced + size > mem_limit) {
- MEMCACHED_SLABS_ALLOCATE_FAILED(size, id);
- return 0;
- }
- mem_malloced += size;
- ret = malloc(size);
- MEMCACHED_SLABS_ALLOCATE(size, id, 0, ret);
- return ret;
- #endif
- /* fail unless we have space at the end of a recently allocated page,
- we have something on our freelist, or we could allocate a new page */
- // 1 最后面的页面有空间 2 空闲的地方有空间 3 分配一个新的页面
- if (! (p->end_page_ptr != 0 || p->sl_curr != 0 ||
- do_slabs_newslab(id) != 0)) {
- /* We don't have more memory available */
- ret = NULL;
- } else if (p->sl_curr != 0) {
- /* return off our freelist */
- //从空闲(回收)地方分配
- ret = p->slots[--p->sl_curr];
- } else {
- /* if we recently allocated a whole page, return from that */
- assert(p->end_page_ptr != NULL);
- ret = p->end_page_ptr;
- if (--p->end_page_free != 0) {
- p->end_page_ptr = ((caddr_t)p->end_page_ptr) + p->size;
- } else {
- p->end_page_ptr = 0;
- }
- }
- if (ret) {
- p->requested += size;
- MEMCACHED_SLABS_ALLOCATE(size, id, p->size, ret);
- } else {
- MEMCACHED_SLABS_ALLOCATE_FAILED(size, id);
- }
- return ret;
- }
3.7 do_slabs_free
- //释放一个item空间
- //把释放的空间放到空闲的slot中,不过有可能出现空闲slot不够的情况,对slot进行扩大
- static void do_slabs_free(void *ptr, const size_t size, unsigned int id) {
- slabclass_t *p;
- assert(((item *)ptr)->slabs_clsid == 0);
- assert(id >= POWER_SMALLEST && id <= power_largest);
- if (id < POWER_SMALLEST || id > power_largest)
- return;
- MEMCACHED_SLABS_FREE(size, id, ptr);
- p = &slabclass[id];
- #ifdef USE_SYSTEM_MALLOC
- mem_malloced -= size;
- free(ptr);
- return;
- #endif
- //空闲slot空间不够,进行扩大
- if (p->sl_curr == p->sl_total) { /* need more space on the free list */
- int new_size = (p->sl_total != 0) ? p->sl_total * 2 : 16; /* 16 is arbitrary */
- void **new_slots = realloc(p->slots, new_size * sizeof(void *));
- if (new_slots == 0)
- return;
- p->slots = new_slots;
- p->sl_total = new_size;
- }
- p->slots[p->sl_curr++] = ptr;
- p->requested -= size;
- return;
- }
四 总结
优点: 通过不同层次的item,以及空闲item维护,可以降低请求过程中内存的分配和释放,提高效率
缺点: 1 每次使用的item可能大于时间需求空间量,导致内存浪费。
五 参考
http://blogold.chinaunix.net/u1/56406/showart_2331017.html memcache探索之slabs
http://tech.idv2.com/2008/07/11/memcached-002/ 理解memcache内存机制
- memcache slabs
- memcache slabs .
- memcache源码分析之items,slabs
- Memcached Slabs
- slabs 内存分配原理
- memcached内存管理(1) ----------------slabs
- memcached学习之slabs部分
- 自己写的内存池Slabs
- Memcache
- MemCache
- Memcache
- Memcache
- Memcache
- memcache
- Memcache
- Memcache
- memcache
- memcache
- Linux下安装mldonkey及配置
- 指针排序(数组)
- xxb xbx
- Javasript&Jquary网上文摘
- Ext.Slider进度条使用
- memcache slabs .
- Html学习总结一
- 程序员的奋斗史(十)——谈项目管理
- Android之getSystemService
- 第二章 linux下安装MQ
- java网络编程--socket服务端客户端(1:1)
- APU(Accelerated Processing Unit)中文名字叫加速处理器
- 作为一个程序员,你学会包容了吗?
- memcache 线程模型 .