items是memcache用来管理item的封装,采用的hash表和LRU链的形式,关于hash表的操作见我前几天的文章 memcache源码分析之assoc
关于item内容的存储机制简介
item的内容存储是在slab中管理的,为了对内存进行有效的管理,slab采用的是分桶的大小来存储item的内容的,简单举例解释一下,初始化时会有不同块大小的桶,比如桶1里面的
内存块都是80b的,专门用来存储item内容大小接近80b的。桶2的内存块是100b的,专门用来存储内容大小接近100b的item,桶3是120b的,用来存储大小接近120b的item,等等。所以，如果有一个item的内容大小是90b,那它只能存储在100b的桶内,不能存储在其他里面的,120b的也不可以。具体详细介绍请见我后续关于slab的文章。
问题:当100b的桶存储满的时候,memcache怎么办呢?
这个问题的答案就在本文介绍的内容里面。
为一个item分配存储空间的时候,具体的操作是这样的:
1、首先,计算该item占用的空间大小,只有知道了它的大小,才能知道它需要存储在哪个桶中。一个item的大小包括它的item结构体大小部分、名字长度部分、状态标识部分、内容大小部分等的总和。具体计算方法请看下面的代码分析中 item_make_header 函数。
2、然后寻找合适的slab用于存储,这一部分主要是比较item 和各slab桶的大小,寻找最合适的slab,此部分代码是文件 slabs.c 中的 slabs_clsid 函数,具体内容我后续关于slab的文章会详细分析。
3、从对应slab的tail队列中寻找是否存在过期的item,如果有,清除掉,此处操作最多尝试50次。
4、如果第3步操作失败,并且在对应slab中分配空间失败,那么从slab对应的tail队列中删除没有被引用的item,且最多也是尝试50次。
5、尝试从slab中分配空间。
6、如果第5步失败,会从slab对应的tail队列中删除3个小时(默认)之前的正在引用的item。
7、然后尝试从slab中分配空间。如果失败,返回NULL,成功则会设置item对应的一些信息,返回成功标识。
item的删除过程:
1、设置已被删除状态。并从hash表中删除,次部分代码调用的是 memcache源码分析之assoc 中介绍到的函数assoc_delete
2、从LRU链中删除。函数item_unlink_q。
3、如果要清除item占用的资源,则调用函数do_item_remove和item_free,释放占用内存空间。
另外还提供了一些其他操作,分别包括,获取某个item(会判断是否过期),获取某个item(不判断是否过期),客户端通过flush_all操作清空所有过期item,item的新值替换,访问时间更新等。
当然,有item的删除操作,就要有相应的加入hash表和LRU链的操作。
另外,还提供了一些item和slab状态函数。
想了解详细代码的同学可以看一下下面的简要分析。有错误之处请指正。
items.h
/* See items.c */ uint64_t get_cas_id(void); /*@null@*/ item *do_item_alloc(char *key, const size_t nkey, const int flags, const rel_time_t exptime, const int nbytes); void item_free(item *it); bool item_size_ok(const size_t nkey, const int flags, const int nbytes); int do_item_link(item *it); /** may fail if transgresses limits */ void do_item_unlink(item *it); void do_item_remove(item *it); void do_item_update(item *it); /** update LRU time to current and reposition */ int do_item_replace(item *it, item *new_it); /*@null@*/ char *do_item_cachedump(const unsigned int slabs_clsid, const unsigned int limit, unsigned int *bytes); void do_item_stats(ADD_STAT add_stats, void *c); /*@null@*/ void do_item_stats_sizes(ADD_STAT add_stats, void *c); void do_item_flush_expired(void); item *do_item_get(const char *key, const size_t nkey); item *do_item_get_nocheck(const char *key, const size_t nkey); void item_stats_reset(void); extern pthread_mutex_t cache_lock;
items.c
/* -*- Mode: C; tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*- */
#include "memcached.h"
#include <sys/stat.h>
#include <sys/socket.h>
#include <sys/signal.h>
#include <sys/resource.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <assert.h>
/* Forward Declarations */
static void item_link_q(item *it);
static void item_unlink_q(item *it);
/*
* We only reposition items in the LRU queue if they haven't been repositioned
* in this many seconds. That saves us from churning on frequently-accessed
* items.
*/
#define ITEM_UPDATE_INTERVAL 60
#define LARGEST_ID POWER_LARGEST
//item状态信息结构体
typedef struct {
unsigned int evicted;
unsigned int evicted_nonzero;
rel_time_t evicted_time;
unsigned int reclaimed;
unsigned int outofmemory;
unsigned int tailrepairs;
} itemstats_t;
static item *heads[LARGEST_ID];
static item *tails[LARGEST_ID];
static itemstats_t itemstats[LARGEST_ID];
static unsigned int sizes[LARGEST_ID];//记录每个slab的元素个数
void item_stats_reset(void) {
pthread_mutex_lock(&cache_lock);
memset(itemstats, 0, sizeof(itemstats));
pthread_mutex_unlock(&cache_lock);
}
//获取新的CAS值
uint64_t get_cas_id(void) {
static uint64_t cas_id = 0;
return ++cas_id;
}
/* Enable this for reference-count debugging. */
#if 0
# define DEBUG_REFCNT(it,op) \
fprintf(stderr, "item %x refcnt(%c) %d %c%c%c\n", \
it, op, it->refcount, \
(it->it_flags & ITEM_LINKED) ? 'L' : ' ', \
(it->it_flags & ITEM_SLABBED) ? 'S' : ' ')
#else
# define DEBUG_REFCNT(it,op) while(0)
#endif
/**
* Generates the variable-sized part of the header for an object.
*
* key - The key
* nkey - The length of the key
* flags - key flags
* nbytes - Number of bytes to hold value and addition CRLF terminator
* suffix - Buffer for the "VALUE" line suffix (flags, size).
* nsuffix - The length of the suffix is stored here.
*
* Returns the total size of the header.
*/
//计算item占用空间大小
static size_t item_make_header(const uint8_t nkey, const int flags, const int nbytes,char *suffix, uint8_t *nsuffix) {
/* suffix is defined at 40 chars elsewhere.. */
*nsuffix = (uint8_t) snprintf(suffix, 40, " %d %d\r\n", flags, nbytes - 2);
return sizeof(item) + nkey + *nsuffix + nbytes;
}
//分配一个item空间
item *do_item_alloc(char *key, const size_t nkey, const int flags, const rel_time_t exptime, const int nbytes) {
uint8_t nsuffix;
item *it = NULL;
char suffix[40];
size_t ntotal = item_make_header(nkey + 1, flags, nbytes, suffix, &nsuffix);//获取item占用空间大小
if (settings.use_cas) {
ntotal += sizeof(uint64_t);
}
unsigned int id = slabs_clsid(ntotal);//寻找合适的slab
if (id == 0)
return 0;
/* do a quick check if we have any expired items in the tail.. */
int tries = 50;
item *search;
for (search = tails[id];tries > 0 && search != NULL;tries--, search=search->prev) {
if (search->refcount == 0 && (search->exptime != 0 && search->exptime < current_time)) {//过期
it = search;
/* I don't want to actually free the object, just steal
* the item to avoid to grab the slab mutex twice ;-)
*/
STATS_LOCK();
stats.reclaimed++;
STATS_UNLOCK();
itemstats[id].reclaimed++;
it->refcount = 1;
do_item_unlink(it);//从hash表删除
/* Initialize the item block: */
it->slabs_clsid = 0;
it->refcount = 0;
break;
}
}
if (it == NULL && (it = slabs_alloc(ntotal, id)) == NULL) {//没有过期元素且加入相应slab失败
tries = 50;
/* If requested to not push old items out of cache when memory runs out,
* we're out of luck at this point...
*/
if (settings.evict_to_free == 0) {
itemstats[id].outofmemory++;
return NULL;
}
/*
* try to get one off the right LRU
* don't necessariuly unlink the tail because it may be locked: refcount>0
* search up from tail an item with refcount==0 and unlink it; give up after 50
* tries
*/
if (tails[id] == 0) {
itemstats[id].outofmemory++;
return NULL;
}
for (search = tails[id]; tries > 0 && search != NULL; tries--, search=search->prev) {
if (search->refcount == 0) {//没有被引用的情况下删除之
if (search->exptime == 0 || search->exptime > current_time) {
itemstats[id].evicted++;
itemstats[id].evicted_time = current_time - search->time;
if (search->exptime != 0)
itemstats[id].evicted_nonzero++;
STATS_LOCK();
stats.evictions++;
STATS_UNLOCK();
} else {
itemstats[id].reclaimed++;
STATS_LOCK();
stats.reclaimed++;
STATS_UNLOCK();
}
do_item_unlink(search);
break;
}
}
it = slabs_alloc(ntotal, id);
if (it == 0) {
itemstats[id].outofmemory++;
/* Last ditch effort. There is a very rare bug which causes
* refcount leaks. We've fixed most of them, but it still happens,
* and it may happen in the future.
* We can reasonably assume no item can stay locked for more than
* three hours, so if we find one in the tail which is that old,
* free it anyway.
*/
tries = 50;
for (search = tails[id]; tries > 0 && search != NULL; tries--, search=search->prev) {
if (search->refcount != 0 && search->time + TAIL_REPAIR_TIME < current_time) {//没有被引用并且是3小时之前的item
itemstats[id].tailrepairs++;
search->refcount = 0;
do_item_unlink(search);
break;
}
}
it = slabs_alloc(ntotal, id);
if (it == 0) {
return NULL;
}
}
}
assert(it->slabs_clsid == 0);
it->slabs_clsid = id;
assert(it != heads[it->slabs_clsid]);
it->next = it->prev = it->h_next = 0;
it->refcount = 1; /* the caller will have a reference */
DEBUG_REFCNT(it, '*');
it->it_flags = settings.use_cas ? ITEM_CAS : 0;
it->nkey = nkey;
it->nbytes = nbytes;
memcpy(ITEM_key(it), key, nkey);
it->exptime = exptime;
memcpy(ITEM_suffix(it), suffix, (size_t)nsuffix);
it->nsuffix = nsuffix;
return it;
}
//释放item
void item_free(item *it) {
size_t ntotal = ITEM_ntotal(it);
unsigned int clsid;
assert((it->it_flags & ITEM_LINKED) == 0);//没有在hash表和LRU链中
assert(it != heads[it->slabs_clsid]);
assert(it != tails[it->slabs_clsid]);
assert(it->refcount == 0);
/* so slab size changer can tell later if item is already free or not */
clsid = it->slabs_clsid;
it->slabs_clsid = 0;
it->it_flags |= ITEM_SLABBED;//内存空闲交给slab
DEBUG_REFCNT(it, 'F');
slabs_free(it, ntotal, clsid);
}
//检验某item是否有适合的slab来存储
bool item_size_ok(const size_t nkey, const int flags, const int nbytes) {
char prefix[40];
uint8_t nsuffix;
return slabs_clsid(item_make_header(nkey + 1, flags, nbytes,prefix, &nsuffix)) != 0;
}
//加入LRU队列,成为新的head
static void item_link_q(item *it) { /* item is the new head */
item **head, **tail;
assert(it->slabs_clsid < LARGEST_ID);//判断所设置slab是否有效
assert((it->it_flags & ITEM_SLABBED) == 0);//判断状态
head = &heads[it->slabs_clsid];
tail = &tails[it->slabs_clsid];
assert(it != *head);
assert((*head && *tail) || (*head == 0 && *tail == 0));
it->prev = 0;
it->next = *head;
if (it->next) it->next->prev = it;
*head = it;
if (*tail == 0) *tail = it;//只有tail为空时才加入？
sizes[it->slabs_clsid]++;
return;
}
//从对应的slab的LRU链上删除
static void item_unlink_q(item *it) {
item **head, **tail;
assert(it->slabs_clsid < LARGEST_ID);
head = &heads[it->slabs_clsid];
tail = &tails[it->slabs_clsid];
if (*head == it) {
assert(it->prev == 0);
*head = it->next;
}
if (*tail == it) {
assert(it->next == 0);
*tail = it->prev;
}
assert(it->next != it);
assert(it->prev != it);
if (it->next) it->next->prev = it->prev;
if (it->prev) it->prev->next = it->next;
sizes[it->slabs_clsid]--;
return;
}
//将item加入到hashtable和LRU链中
int do_item_link(item *it) {
MEMCACHED_ITEM_LINK(ITEM_key(it), it->nkey, it->nbytes);//ITEM_key在memcached.h中定义
assert((it->it_flags & (ITEM_LINKED|ITEM_SLABBED)) == 0);//判断状态,既没有在hash表LRU链中或被释放
it->it_flags |= ITEM_LINKED;//设置linked状态
it->time = current_time;//设置最近访问时间
assoc_insert(it);//插入hashtable assoc.c
STATS_LOCK();
stats.curr_bytes += ITEM_ntotal(it);//增加每个item所需要的字节大小,包括item结构体和item内容大小
stats.curr_items += 1;
stats.total_items += 1;
STATS_UNLOCK();
/* Allocate a new CAS ID on link. */
ITEM_set_cas(it, (settings.use_cas) ? get_cas_id() : 0);//设置新CAS,CAS是memcache用来处理并发请求的一种机制
item_link_q(it);//加入LRU链
return 1;
}
//从hash表和LRU链中删除item
void do_item_unlink(item *it) {
MEMCACHED_ITEM_UNLINK(ITEM_key(it), it->nkey, it->nbytes);
if ((it->it_flags & ITEM_LINKED) != 0) {
it->it_flags &= ~ITEM_LINKED;//设置为非linked
STATS_LOCK();
stats.curr_bytes -= ITEM_ntotal(it);
stats.curr_items -= 1;
STATS_UNLOCK();
assoc_delete(ITEM_key(it), it->nkey);//从hash表中删除
item_unlink_q(it);//从LRU链中删除
if (it->refcount == 0) item_free(it);
}
}
//remove item
void do_item_remove(item *it) {
MEMCACHED_ITEM_REMOVE(ITEM_key(it), it->nkey, it->nbytes);
assert((it->it_flags & ITEM_SLABBED) == 0);
if (it->refcount != 0) {
it->refcount--;
DEBUG_REFCNT(it, '-');
}
if (it->refcount == 0 && (it->it_flags & ITEM_LINKED) == 0) {//没有人在引用并且没有在hash表和LEU链中
item_free(it);
}
}
//更新item最后访问时间
void do_item_update(item *it) {
MEMCACHED_ITEM_UPDATE(ITEM_key(it), it->nkey, it->nbytes);
if (it->time < current_time - ITEM_UPDATE_INTERVAL) {
assert((it->it_flags & ITEM_SLABBED) == 0);//没有被释放
if ((it->it_flags & ITEM_LINKED) != 0) {
item_unlink_q(it);
it->time = current_time;
item_link_q(it);
}
}
}
//item替换
int do_item_replace(item *it, item *new_it) {
MEMCACHED_ITEM_REPLACE(ITEM_key(it), it->nkey, it->nbytes,ITEM_key(new_it), new_it->nkey, new_it->nbytes);
assert((it->it_flags & ITEM_SLABBED) == 0);//确保没有被释放
do_item_unlink(it);
return do_item_link(new_it);
}
/*@null@*/
char *do_item_cachedump(const unsigned int slabs_clsid, const unsigned int limit, unsigned int *bytes) {
unsigned int memlimit = 2 * 1024 * 1024; /* 2MB max response size */
char *buffer;
unsigned int bufcurr;
item *it;
unsigned int len;
unsigned int shown = 0;
char key_temp[KEY_MAX_LENGTH + 1];
char temp[512];
it = heads[slabs_clsid];
buffer = malloc((size_t)memlimit);
if (buffer == 0) return NULL;
bufcurr = 0;
while (it != NULL && (limit == 0 || shown < limit)) {
assert(it->nkey <= KEY_MAX_LENGTH);
/* Copy the key since it may not be null-terminated in the struct */
strncpy(key_temp, ITEM_key(it), it->nkey);
key_temp[it->nkey] = 0x00; /* terminate */
len = snprintf(temp, sizeof(temp), "ITEM %s [%d b; %lu s]\r\n",key_temp, it->nbytes - 2,(unsigned long)it->exptime + process_started);
if (bufcurr + len + 6 > memlimit) /* 6 is END\r\n\0 */
break;
memcpy(buffer + bufcurr, temp, len);
bufcurr += len;
shown++;
it = it->next;
}
memcpy(buffer + bufcurr, "END\r\n", 6);
bufcurr += 5;
*bytes = bufcurr;
return buffer;
}
//slab状态信息
void do_item_stats(ADD_STAT add_stats, void *c) {
int i;
for (i = 0; i < LARGEST_ID; i++) {
if (tails[i] != NULL) {
const char *fmt = "items:%d:%s";
char key_str[STAT_KEY_LEN];
char val_str[STAT_VAL_LEN];
int klen = 0, vlen = 0;
APPEND_NUM_FMT_STAT(fmt, i, "number", "%u", sizes[i]);
APPEND_NUM_FMT_STAT(fmt, i, "age", "%u", tails[i]->time);
APPEND_NUM_FMT_STAT(fmt, i, "evicted","%u", itemstats[i].evicted);
APPEND_NUM_FMT_STAT(fmt, i, "evicted_nonzero","%u", itemstats[i].evicted_nonzero);
APPEND_NUM_FMT_STAT(fmt, i, "evicted_time","%u", itemstats[i].evicted_time);
APPEND_NUM_FMT_STAT(fmt, i, "outofmemory","%u", itemstats[i].outofmemory);
APPEND_NUM_FMT_STAT(fmt, i, "tailrepairs","%u", itemstats[i].tailrepairs);;
APPEND_NUM_FMT_STAT(fmt, i, "reclaimed","%u", itemstats[i].reclaimed);;
}
}
/* getting here means both ascii and binary terminators fit */
add_stats(NULL, 0, NULL, 0, c);
}
/** dumps out a list of objects of each size, with granularity of 32 bytes */
/*@null@*/
void do_item_stats_sizes(ADD_STAT add_stats, void *c) {
/* max 1MB object, divided into 32 bytes size buckets */
const int num_buckets = 32768;
unsigned int *histogram = calloc(num_buckets, sizeof(int));
if (histogram != NULL) {
int i;
/* build the histogram */
for (i = 0; i < LARGEST_ID; i++) {
item *iter = heads[i];
while (iter) {
int ntotal = ITEM_ntotal(iter);
int bucket = ntotal / 32;
if ((ntotal % 32) != 0) bucket++;
if (bucket < num_buckets) histogram[bucket]++;
iter = iter->next;
}
}
/* write the buffer */
for (i = 0; i < num_buckets; i++) {
if (histogram[i] != 0) {
char key[8];
int klen = 0;
klen = snprintf(key, sizeof(key), "%d", i * 32);
assert(klen < sizeof(key));
APPEND_STAT(key, "%u", histogram[i]);
}
}
free(histogram);
}
add_stats(NULL, 0, NULL, 0, c);
}
//获取item
item *do_item_get(const char *key, const size_t nkey) {
item *it = assoc_find(key, nkey);
int was_found = 0;
if (settings.verbose > 2) {//输出调试信息
if (it == NULL) {
fprintf(stderr, "> NOT FOUND %s", key);
} else {
fprintf(stderr, "> FOUND KEY %s", ITEM_key(it));
was_found++;
}
}
//忽略比设置日期早的item
if (it != NULL && settings.oldest_live != 0 && settings.oldest_live <= current_time && it->time <= settings.oldest_live) {
do_item_unlink(it); /* MTSAFE - cache_lock held */
it = NULL;
}
if (it == NULL && was_found) {
fprintf(stderr, " -nuked by flush");//被忽略错误信息
was_found--;
}
if (it != NULL && it->exptime != 0 && it->exptime <= current_time) {//过期
do_item_unlink(it); /* MTSAFE - cache_lock held */
it = NULL;
}
if (it == NULL && was_found) {
fprintf(stderr, " -nuked by expire");//过期错误
was_found--;
}
if (it != NULL) {
it->refcount++;
DEBUG_REFCNT(it, '+');
}
if (settings.verbose > 2)
fprintf(stderr, "\n");
return it;
}
//获取一个item,不论过期与否
item *do_item_get_nocheck(const char *key, const size_t nkey) {
item *it = assoc_find(key, nkey);
if (it) {
it->refcount++;
DEBUG_REFCNT(it, '+');
}
return it;
}
//flush all items
void do_item_flush_expired(void) {
int i;
item *iter, *next;
if (settings.oldest_live == 0)
return;
for (i = 0; i < LARGEST_ID; i++) {
/* The LRU is sorted in decreasing time order, and an item's timestamp
* is never newer than its last access time, so we only need to walk
* back until we hit an item older than the oldest_live time.
* The oldest_live checking will auto-expire the remaining items.
*/
for (iter = heads[i]; iter != NULL; iter = next) {
if (iter->time >= settings.oldest_live) {
next = iter->next;
if ((iter->it_flags & ITEM_SLABBED) == 0) {//没有被释放,unlink
do_item_unlink(iter);
}
} else {
break;
}
}
}
}
