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1 .WIREDTIGER实现探秘
Presented by 许鹏
2 . 引擎特点
- 最小化线程间的竞争
- 无锁算法 如hazard pinters
- 减少并发访问中的互斥操作
- 充分挖掘硬件能力
- 多核
- 大内存
- 缓存和文件压缩
7 . 初始化写入 – 数据没有落入磁盘
p *((WT_CURSOR_BTREE*)cursor).btree.root.page.u.intl.__index.index[0].page p ((WT_CURSOR_BTREE*)cursor).btree.root.page.u.intl.__index.index[0].page.modify.u2
$97 = {
u={ $99 = {
intl = { intl = {
parent_ref = 0x0, root_split = 0x693080
split_gen = 0, },
column_leaf = {
__index = 0x0
}, append = 0x693080,
row = 0x0, update = 0x0,
fix_bitf = 0x0, split_recno = 0
col_var = { },
row_leaf = {
col_var = 0x0,
repeats = 0x0 insert = 0x693080,
} update = 0x0
}, }
entries = 0, }
type = 7 '\a',
flags_atomic = 0 '\000',
unused = "\000",
read_gen = 201,
memory_footprint = 746,
dsk = 0x0,
modify = 0x6933a0,
cache_create_gen = 1,
evict_pass_gen = 0
}
8 . ROW LEAF -- 叶子结点的内存信息
$61 = {
u={
- 从磁盘加载数据 intl = {
- 修改已经存在的key parent_ref = 0x69fe30,
split_gen = 0,
- 关注如下两个成员变量 __index = 0x0
- dsk },
row = 0x69fe30,
- Modify fix_bitf = 0x69fe30 "\277",
- row表示从dsk位置开始的偏移量 col_var = {
col_var = 0x69fe30,
repeats = 0x0
}
GDB调试指令 },
entries = 2,
p *((WT_CURSOR_BTREE*)cursor).btree.root.page.u.intl.__index.index[0].page type = 7 '\a',
flags_atomic = 2 '\002',
unused = "\000",
read_gen = 201,
memory_footprint = 555,
dsk = 0x69edd0,
modify = 0x69fe50,
cache_create_gen = 1,
evict_pass_gen = 0
}
9 .页面释放 – HAZARD POINTER
1.建立一个全局数组，数组容量为线程数目，每个线程只能修
改自己的数组元素，而不允许修改其他的数组元素，但可以读
别的数组元素。
2.当线程尝试访问一个关键数据节点时，先把该节点指针赋给
自己的数组元素（即不要释放这个节点）。
3.每个线程自己维护一个私有链表，当线程准备释放掉某个节
点时，将该节点放入到链表中。当链表内的数目达到一个设定
的数目后，遍历该链表用于释放链表内所有节点。
4.当释放节点时，需要检查全局数组，确定没有任何一个线程
的数组元素与当前指针相同时，就释放该节点。否则仍然滞留
在自己的链表中。
10 .数据持久化
- Checkpoint
- Occur at an interval of
60 seconds
- Journal log
- Sync to disk every 50
milliseconds
11 . 文件压缩
- 默认使用Snappy压缩算法
- 缓存中是解压后的格式, 占用内存空间远大于磁盘
12 . 并发访问控制 -- TICKET LOCK
struct __wt_rwlock { /* Read/write lock */
volatile union {
uint64_t v; /* Full 64-bit value */
struct {
uint8_t current; /* Current ticket */
uint8_t next; /* Next available ticket */
uint8_t reader; /* Read queue ticket */
uint8_t readers_queued; /* Count of queued readers */
uint32_t readers_active;/* Count of active readers */
} s;
} u;
int16_t stat_read_count_off; /* read acquisitions offset */
int16_t stat_write_count_off; /* write acquisitions offset */
int16_t stat_app_usecs_off; /* waiting application threads offset */
int16_t stat_int_usecs_off; /* waiting server threads offset */
WT_CONDVAR *cond_readers; /* Blocking readers */
WT_CONDVAR *cond_writers; /* Blocking writers */
};
db.serverStatus().wiredTiger.concurrentTransactions
13 . 性能监控指标
• db.server.status().wiredtiger
• db.coll.stats()
• db.currentop()
14 . 参数设置
db.createCollection(
"users",
{ storageEngine: { wiredTiger: { configString:
"leaf_page_max=64kb, leaf_value_max=64MB" } } }
)
15 . DEMO APPLIC ATION
#include "wiredtiger.h“
#include <stdio.h>
int main(int argc, char** argv)
{
char* home = "/tmp/wt";
WT_CONNECTION *conn;
WT_SESSION *session;
WT_CURSOR *cursor;
const char* key, *value;
int ret;
wiredtiger_open(home, NULL, "create", &conn);
conn->open_session(conn, NULL, NULL, &session);
//create a table
session->create(session, "table:demo", "key_format=S, value_format=S");
//open the table
session->open_cursor(session, "table:demo", NULL, NULL, &cursor);
cursor->set_key(cursor, "name");
cursor->set_value(cursor, "peng");
cursor->insert(cursor);
cursor->set_key(cursor, "iris");
cursor->set_value(cursor, "teacher");
cursor->insert(cursor);
cursor->reset(cursor);
while ( (ret = cursor->next(cursor) ) == 0 ) {
cursor->get_key(cursor, &key);
cursor->get_value(cursor, &value);
printf("Got Record: %s, %s\n", key, value);
}
conn->close(conn, NULL);
}
16 . DEMO APPLICATION – 编译和运行
编译
gcc -g -I. -o /tmp/wt_demo.o -c ~/working/wt_demo.c
gcc -L .libs/ $PWD/.libs/libwiredtiger-3.1.0.so /tmp/wt_demo.o -o /tmp/wt_demo
LD_LIBRARY_PATH=$PWD/.libs /tmp/wt_demo
调试
set print pretty
br wt_demo.c:37
r
set scheduler-locking on
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