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memcache源码分析之items

Posted on 2011-01-18 00:14 先贝 阅读(...) 评论(...) 编辑 收藏

  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;
            }
        }
    }
}