本节简单介绍了PostgreSQL缓存管理(Buffer Manager)中的实现函数ReadBuffer_common->BufferAlloc->StrategyGetBuffer,该函数在BufferAlloc()中,由bufmgr调用,用于获得下一个候选的buffer.。 ### 一、数据结构 **BufferDesc** 共享缓冲区的共享描述符(状态)数据 ``` /* * Flags for buffer descriptors * buffer描述器标记 * * Note: TAG_VALID essentially means that there is a buffer hashtable * entry associated with the buffer's tag. * 注意:TAG_VALID本质上意味着有一个与缓冲区的标记相关联的缓冲区散列表条目。 */ //buffer header锁定 #define BM_LOCKED (1U << 22) /* buffer header is locked */ //数据需要写入(标记为DIRTY) #define BM_DIRTY (1U << 23) /* data needs writing */ //数据是有效的 #define BM_VALID (1U << 24) /* data is valid */ //已分配buffer tag #define BM_TAG_VALID (1U << 25) /* tag is assigned */ //正在R/W #define BM_IO_IN_PROGRESS (1U << 26) /* read or write in progress */ //上一个I/O出现错误 #define BM_IO_ERROR (1U << 27) /* previous I/O failed */ //开始写则变DIRTY #define BM_JUST_DIRTIED (1U << 28) /* dirtied since write started */ //存在等待sole pin的其他进程 #define BM_PIN_COUNT_WAITER (1U << 29) /* have waiter for sole pin */ //checkpoint发生,必须刷到磁盘上 #define BM_CHECKPOINT_NEEDED (1U << 30) /* must write for checkpoint */ //持久化buffer(不是unlogged或者初始化fork) #define BM_PERMANENT (1U << 31) /* permanent buffer (not unlogged, * or init fork) */ /* * BufferDesc -- shared descriptor/state data for a single shared buffer. * BufferDesc -- 共享缓冲区的共享描述符(状态)数据 * * Note: Buffer header lock (BM_LOCKED flag) must be held to examine or change * the tag, state or wait_backend_pid fields. In general, buffer header lock * is a spinlock which is combined with flags, refcount and usagecount into * single atomic variable. This layout allow us to do some operations in a * single atomic operation, without actually acquiring and releasing spinlock; * for instance, increase or decrease refcount. buf_id field never changes * after initialization, so does not need locking. freeNext is protected by * the buffer_strategy_lock not buffer header lock. The LWLock can take care * of itself. The buffer header lock is *not* used to control access to the * data in the buffer! * 注意:必须持有Buffer header锁(BM_LOCKED标记)才能检查或修改tag/state/wait_backend_pid字段. * 通常来说,buffer header lock是spinlock,它与标记位/参考计数/使用计数组合到单个原子变量中. * 这个布局设计允许我们执行原子操作,而不需要实际获得或者释放spinlock(比如,增加或者减少参考计数). * buf_id字段在初始化后不会出现变化,因此不需要锁定. * freeNext通过buffer_strategy_lock锁而不是buffer header lock保护. * LWLock可以很好的处理自己的状态. * 务请注意的是:buffer header lock不用于控制buffer中的数据访问! * * It's assumed that nobody changes the state field while buffer header lock * is held. Thus buffer header lock holder can do complex updates of the * state variable in single write, simultaneously with lock release (cleaning * BM_LOCKED flag). On the other hand, updating of state without holding * buffer header lock is restricted to CAS, which insure that BM_LOCKED flag * is not set. Atomic increment/decrement, OR/AND etc. are not allowed. * 假定在持有buffer header lock的情况下,没有人改变状态字段. * 持有buffer header lock的进程可以执行在单个写操作中执行复杂的状态变量更新, * 同步的释放锁(清除BM_LOCKED标记). * 换句话说,如果没有持有buffer header lock的状态更新,会受限于CAS, * 这种情况下确保BM_LOCKED没有被设置. * 比如原子的增加/减少(AND/OR)等操作是不允许的. * * An exception is that if we have the buffer pinned, its tag can't change * underneath us, so we can examine the tag without locking the buffer header. * Also, in places we do one-time reads of the flags without bothering to * lock the buffer header; this is generally for situations where we don't * expect the flag bit being tested to be changing. * 一种例外情况是如果我们已有buffer pinned,该buffer的tag不能改变(在本进程之下), * 因此不需要锁定buffer header就可以检查tag了. * 同时,在执行一次性的flags读取时不需要锁定buffer header. * 这种情况通常用于我们不希望正在测试的flag bit将被改变. * * We can't physically remove items from a disk page if another backend has * the buffer pinned. Hence, a backend may need to wait for all other pins * to go away. This is signaled by storing its own PID into * wait_backend_pid and setting flag bit BM_PIN_COUNT_WAITER. At present, * there can be only one such waiter per buffer. * 如果其他进程有buffer pinned,那么进程不能物理的从磁盘页面中删除items. * 因此,后台进程需要等待其他pins清除.这可以通过存储它自己的PID到wait_backend_pid中, * 并设置标记位BM_PIN_COUNT_WAITER. * 目前,每个缓冲区只能由一个等待进程. * * We use this same struct for local buffer headers, but the locks are not * used and not all of the flag bits are useful either. To avoid unnecessary * overhead, manipulations of the state field should be done without actual * atomic operations (i.e. only pg_atomic_read_u32() and * pg_atomic_unlocked_write_u32()). * 本地缓冲头部使用同样的结构,但并不需要使用locks,而且并不是所有的标记位都使用. * 为了避免不必要的负载,状态域的维护不需要实际的原子操作 * (比如只有pg_atomic_read_u32() and pg_atomic_unlocked_write_u32()) * * Be careful to avoid increasing the size of the struct when adding or * reordering members. Keeping it below 64 bytes (the most common CPU * cache line size) is fairly important for performance. * 在增加或者记录成员变量时,小心避免增加结构体的大小. * 保持结构体大小在64字节内(通常的CPU缓存线大小)对于性能是非常重要的. */ typedef struct BufferDesc { //buffer tag BufferTag tag; /* ID of page contained in buffer */ //buffer索引编号(0开始) int buf_id; /* buffer's index number (from 0) */ /* state of the tag, containing flags, refcount and usagecount */ //tag状态,包括flags/refcount和usagecount pg_atomic_uint32 state; //pin-count等待进程ID int wait_backend_pid; /* backend PID of pin-count waiter */ //空闲链表链中下一个空闲的buffer int freeNext; /* link in freelist chain */ //缓冲区内容锁 LWLock content_lock; /* to lock access to buffer contents */ } BufferDesc; ``` **BufferTag** Buffer tag标记了buffer存储的是磁盘中哪个block ``` /* * Buffer tag identifies which disk block the buffer contains. * Buffer tag标记了buffer存储的是磁盘中哪个block * * Note: the BufferTag data must be sufficient to determine where to write the * block, without reference to pg_class or pg_tablespace entries. It's * possible that the backend flushing the buffer doesn't even believe the * relation is visible yet (its xact may have started before the xact that * created the rel). The storage manager must be able to cope anyway. * 注意:BufferTag必须足以确定如何写block而不需要参照pg_class或者pg_tablespace数据字典信息. * 有可能后台进程在刷新缓冲区的时候深圳不相信关系是可见的(事务可能在创建rel的事务之前). * 存储管理器必须可以处理这些事情. * * Note: if there's any pad bytes in the struct, INIT_BUFFERTAG will have * to be fixed to zero them, since this struct is used as a hash key. * 注意:如果在结构体中有填充的字节,INIT_BUFFERTAG必须将它们固定为零，因为这个结构体用作散列键. */ typedef struct buftag { //物理relation标识符 RelFileNode rnode; /* physical relation identifier */ ForkNumber forkNum; //相对于relation起始的块号 BlockNumber blockNum; /* blknum relative to begin of reln */ } BufferTag; ``` **SMgrRelation** smgr.c维护一个包含SMgrRelation对象的hash表,SMgrRelation对象本质上是缓存的文件句柄. ``` /* * smgr.c maintains a table of SMgrRelation objects, which are essentially * cached file handles. An SMgrRelation is created (if not already present) * by smgropen(), and destroyed by smgrclose(). Note that neither of these * operations imply I/O, they just create or destroy a hashtable entry. * (But smgrclose() may release associated resources, such as OS-level file * descriptors.) * smgr.c维护一个包含SMgrRelation对象的hash表,SMgrRelation对象本质上是缓存的文件句柄. * SMgrRelation对象(如非现成)通过smgropen()方法创建,通过smgrclose()方法销毁. * 注意:这些操作都不会执行I/O操作,只会创建或者销毁哈希表条目. * (但是smgrclose()方法可能会释放相关的资源,比如OS基本的文件描述符) * * An SMgrRelation may have an "owner", which is just a pointer to it from * somewhere else; smgr.c will clear this pointer if the SMgrRelation is * closed. We use this to avoid dangling pointers from relcache to smgr * without having to make the smgr explicitly aware of relcache. There * can't be more than one "owner" pointer per SMgrRelation, but that's * all we need. * SMgrRelation可能会有"宿主",这个宿主可能只是从某个地方指向它的指针而已; * 如SMgrRelationsmgr.c会清除该指针.这样做可以避免从relcache到smgr的悬空指针, * 而不必要让smgr显式的感知relcache(也就是隔离了smgr了relcache). * 每个SMgrRelation不能跟多个"owner"指针关联,但这就是我们所需要的. * * SMgrRelations that do not have an "owner" are considered to be transient, * and are deleted at end of transaction. * SMgrRelations如无owner指针,则被视为临时对象,在事务的最后被删除. */ typedef struct SMgrRelationData { /* rnode is the hashtable lookup key, so it must be first! */ //-------- rnode是哈希表的搜索键,因此在结构体的首位 //关系物理定义ID RelFileNodeBackend smgr_rnode; /* relation physical identifier */ /* pointer to owning pointer, or NULL if none */ //--------- 指向拥有的指针,如无则为NULL struct SMgrRelationData **smgr_owner; /* * These next three fields are not actually used or manipulated by smgr, * except that they are reset to InvalidBlockNumber upon a cache flush * event (in particular, upon truncation of the relation). Higher levels * store cached state here so that it will be reset when truncation * happens. In all three cases, InvalidBlockNumber means "unknown". * 接下来的3个字段实际上并不用于或者由smgr管理, * 除非这些表里在cache flush event发生时被重置为InvalidBlockNumber * (特别是在关系被截断时). * 在这里,更高层的存储缓存了状态因此在截断发生时会被重置. * 在这3种情况下,InvalidBlockNumber都意味着"unknown". */ //当前插入的目标bloc BlockNumber smgr_targblock; /* current insertion target block */ //最后已知的fsm fork大小 BlockNumber smgr_fsm_nblocks; /* last known size of fsm fork */ //最后已知的vm fork大小 BlockNumber smgr_vm_nblocks; /* last known size of vm fork */ /* additional public fields may someday exist here */ //------- 未来可能新增的公共域 /* * Fields below here are intended to be private to smgr.c and its * submodules. Do not touch them from elsewhere. * 下面的字段是smgr.c及其子模块私有的,不要从其他模块接触这些字段. */ //存储管理器选择器 int smgr_which; /* storage manager selector */ /* * for md.c; per-fork arrays of the number of open segments * (md_num_open_segs) and the segments themselves (md_seg_fds). * 用于md.c,打开段(md_num_open_segs)和段自身(md_seg_fds)的数组(每个fork一个) */ int md_num_open_segs[MAX_FORKNUM + 1]; struct _MdfdVec *md_seg_fds[MAX_FORKNUM + 1]; /* if unowned, list link in list of all unowned SMgrRelations */ //如没有宿主,未宿主的SMgrRelations链表的链表链接. struct SMgrRelationData *next_unowned_reln; } SMgrRelationData; typedef SMgrRelationData *SMgrRelation; ``` **RelFileNodeBackend** 组合relfilenode和后台进程ID,用于提供需要定位物理存储的所有信息. ``` /* * Augmenting a relfilenode with the backend ID provides all the information * we need to locate the physical storage. The backend ID is InvalidBackendId * for regular relations (those accessible to more than one backend), or the * owning backend's ID for backend-local relations. Backend-local relations * are always transient and removed in case of a database crash; they are * never WAL-logged or fsync'd. * 组合relfilenode和后台进程ID,用于提供需要定位物理存储的所有信息. * 对于普通的关系(可通过多个后台进程访问),后台进程ID是InvalidBackendId; * 如为临时表,则为自己的后台进程ID. * 临时表(backend-local relations)通常是临时存在的,在数据库崩溃时删除,无需WAL-logged或者fsync. */ typedef struct RelFileNodeBackend { RelFileNode node;//节点 BackendId backend;//后台进程 } RelFileNodeBackend; ``` **StrategyControl** 共享的空闲链表控制信息 ``` /* * The shared freelist control information. * 共享的空闲链表控制信息. */ typedef struct { /* Spinlock: protects the values below */ //自旋锁,用于保护下面的值 slock_t buffer_strategy_lock; /* * Clock sweep hand: index of next buffer to consider grabbing. Note that * this isn't a concrete buffer - we only ever increase the value. So, to * get an actual buffer, it needs to be used modulo NBuffers. * Clock sweep hand:下一个考虑交换出去的buffer索引. * 注意这并不是一个精确的buffer - 我们只是曾经增加值而已. * 因此,获得一个实际的buffer,需要取模(使用NBuffers). */ pg_atomic_uint32 nextVictimBuffer; //未使用的buffers链表头部 int firstFreeBuffer; /* Head of list of unused buffers */ //未使用的buffers链表尾部 int lastFreeBuffer; /* Tail of list of unused buffers */ /* * NOTE: lastFreeBuffer is undefined when firstFreeBuffer is -1 (that is, * when the list is empty) * 注意:如firstFreeBuffer是-1,则lastFreeBuffer是未定义的. * (这意味着,当链表是空的时候会出现这种情况) */ /* * Statistics. These counters should be wide enough that they can't * overflow during a single bgwriter cycle. * 统计信息.这些计数器需要足够大,以确保在单个bgwriter循环时不会溢出. */ //完成一轮clock sweep循环,进行计数 uint32 completePasses; /* Complete cycles of the clock sweep */ //自上次重置后分配的buffers pg_atomic_uint32 numBufferAllocs; /* Buffers allocated since last reset */ /* * Bgworker process to be notified upon activity or -1 if none. See * StrategyNotifyBgWriter. * 活动时通知Bgworker进程,否则该值为-1.详细参见StrategyNotifyBgWriter. */ int bgwprocno; } BufferStrategyControl; /* Pointers to shared state */ //指向BufferStrategyControl结构体的指针 static BufferStrategyControl *StrategyControl = NULL; ``` ### 二、源码解读 StrategyGetBuffer在BufferAlloc()中,由bufmgr调用,用于获得下一个候选的buffer. 其主要的处理逻辑如下: 1.初始化相关变量 2.如策略对象不为空,则从环形缓冲区中获取buffer,如成功则返回buf 3.如需要,则唤醒后台进程bgwriter,从共享内存中读取一次,然后根据该值设置latch 4.计算buffer分配请求,这样bgwriter可以估算buffer消耗的比例. 5.检查freelist中是否存在buffer 5.1如存在,则执行相关判断逻辑,如成功,则返回buf 5.2如不存在 5.2.1则使用clock sweep算法,选择buffer,执行相关判断,如成功,则返回buf 5.2.2如无法获取,在尝试过trycounter次后,报错 ``` /* * StrategyGetBuffer * * Called by the bufmgr to get the next candidate buffer to use in * BufferAlloc(). The only hard requirement BufferAlloc() has is that * the selected buffer must not currently be pinned by anyone. * 在BufferAlloc()中,由bufmgr调用,用于获得下一个候选的buffer. * BufferAlloc()中唯一稍微困难的需求是选择的buffer不能被其他后台进程pinned. * * strategy is a BufferAccessStrategy object, or NULL for default strategy. * strategy是BufferAccessStrategy对象,如为默认策略,则为NULL. * * To ensure that no one else can pin the buffer before we do, we must * return the buffer with the buffer header spinlock still held. * 为了确保没有其他后台进程在我们完成之前pin buffer,必须返回仍持有buffer header自旋锁的buffer. */ BufferDesc * StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state) { BufferDesc *buf;//buffer描述符 int bgwprocno; int trycounter;//尝试次数 //避免重复的依赖和解依赖 uint32 local_buf_state; /* to avoid repeated (de-)referencing */ /* * If given a strategy object, see whether it can select a buffer. We * assume strategy objects don't need buffer_strategy_lock. * 如果给定了一个策略对象,看看是否可以选择一个buffer. * 我们假定策略对象不需要buffer_strategy_lock锁. */ if (strategy != NULL) { //从环形缓冲区中获取buffer,如获取成功,则返回该buffer buf = GetBufferFromRing(strategy, buf_state); if (buf != NULL) return buf; } /* * If asked, we need to waken the bgwriter. Since we don't want to rely on * a spinlock for this we force a read from shared memory once, and then * set the latch based on that value. We need to go through that length * because otherwise bgprocno might be reset while/after we check because * the compiler might just reread from memory. * 如需要,则唤醒后台进程bgwriter. * 我们不希望依赖自旋锁实现这一点,所以强制从共享内存中读取一次,然后根据该值设置latch. * 我们需要走完这一步,否则的话bgprocno在检查期间或之后被重置,因为编译器可能重新从内存中读取数据. * * This can possibly set the latch of the wrong process if the bgwriter * dies in the wrong moment. But since PGPROC->procLatch is never * deallocated the worst consequence of that is that we set the latch of * some arbitrary process. * 如果bgwriter出现异常宕机,可能会出现latch被设置为错误的进程. * 但是由于PGPROC->procLatch从来没有被释放过，最坏的结果是我们设置了一些任意进程的latch。 */ bgwprocno = INT_ACCESS_ONCE(StrategyControl->bgwprocno); if (bgwprocno != -1) { //--- 如bgwprocno不是-1 /* reset bgwprocno first, before setting the latch */ //在设置latch前,首先重置bgwprocno为-1 StrategyControl->bgwprocno = -1; /* * Not acquiring ProcArrayLock here which is slightly icky. It's * actually fine because procLatch isn't ever freed, so we just can * potentially set the wrong process' (or no process') latch. * 在这里不需要请求"令人生厌"的ProcArrayLock. * 因为procLatch未曾释放过,因此实际上是没有问题的, * 所以我们可能会设置错误的进程(或没有进程)latch。 */ SetLatch(&ProcGlobal->allProcs[bgwprocno].procLatch); } /* * We count buffer allocation requests so that the bgwriter can estimate * the rate of buffer consumption. Note that buffers recycled by a * strategy object are intentionally not counted here. * 计算buffer分配请求,这样bgwriter可以估算buffer消耗的比例. * 注意通过策略对象进行的buffer回收不会在这里计算. */ pg_atomic_fetch_add_u32(&StrategyControl->numBufferAllocs, 1); /* * First check, without acquiring the lock, whether there's buffers in the * freelist. Since we otherwise don't require the spinlock in every * StrategyGetBuffer() invocation, it'd be sad to acquire it here - * uselessly in most cases. That obviously leaves a race where a buffer is * put on the freelist but we don't see the store yet - but that's pretty * harmless, it'll just get used during the next buffer acquisition. * 不需要请求锁,首次检查在freelist中是否存在buffer. * 因为我们不需要在每次StrategyGetBuffer()调用时都使用自旋锁, * 在这里请求自旋锁有点郁闷 -- 因为大多数情况下都没有用. * 这显然存在一个竞争,其中缓冲区被放在空闲列表中,但进程却看不到存储 * -- 但这是无害的,在下次buffer申请期间使用. * * If there's buffers on the freelist, acquire the spinlock to pop one * buffer of the freelist. Then check whether that buffer is usable and * repeat if not. * 如果在空闲列表中有buffer存在,请求自旋锁,从空闲列表中弹出一个可用的buffer. * 然后检查该buffer是否可用,如不可用则继续处理. * * Note that the freeNext fields are considered to be protected by the * buffer_strategy_lock not the individual buffer spinlocks, so it's OK to * manipulate them without holding the spinlock. * 注意freeNext字段通过buffer_strategy_lock锁来保护,而不是使用独立的缓冲区自旋锁保护, * 因此不需要持有自旋锁就可以维护这些字段. */ if (StrategyControl->firstFreeBuffer >= 0) { while (true) { /* Acquire the spinlock to remove element from the freelist */ //请求自旋锁,删除空闲链表中的元素 SpinLockAcquire(&StrategyControl->buffer_strategy_lock); if (StrategyControl->firstFreeBuffer < 0) { //如无空闲空间,则马上跳出循环 SpinLockRelease(&StrategyControl->buffer_strategy_lock); break; } //获取缓冲描述符 buf = GetBufferDescriptor(StrategyControl->firstFreeBuffer); Assert(buf->freeNext != FREENEXT_NOT_IN_LIST); /* Unconditionally remove buffer from freelist */ //无条件的清除空闲链表中的buffer StrategyControl->firstFreeBuffer = buf->freeNext; buf->freeNext = FREENEXT_NOT_IN_LIST; /* * Release the lock so someone else can access the freelist while * we check out this buffer. * 释放锁,这样其他进程在我们检查该缓冲的时候可以访问空闲链表 */ SpinLockRelease(&StrategyControl->buffer_strategy_lock); /* * If the buffer is pinned or has a nonzero usage_count, we cannot * use it; discard it and retry. (This can only happen if VACUUM * put a valid buffer in the freelist and then someone else used * it before we got to it. It's probably impossible altogether as * of 8.3, but we'd better check anyway.) * 如果缓冲pinned或者usage_count非0,则不能使用该buffer,丢弃并重试. * (这种情况发生在VACUUM把一个有效的buffer放在空闲链表中,然后其他进程提前获得了这个buffer. * 在8.3中是完全不可能的,但最好执行该检查) */ //锁定缓冲头部 local_buf_state = LockBufHdr(buf); if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0 && BUF_STATE_GET_USAGECOUNT(local_buf_state) == 0) { //refcount == 0 && usagecount == 0 if (strategy != NULL) //非默认策略,则添加到环形缓冲区中 AddBufferToRing(strategy, buf); //设置输出参数 *buf_state = local_buf_state; //返回buf return buf; } //不满足条件,解锁buffer header UnlockBufHdr(buf, local_buf_state); } } /* Nothing on the freelist, so run the "clock sweep" algorithm */ //空闲链表中找不到或者满足不了条件,则执行"clock sweep"算法 //int NBuffers = 1000; trycounter = NBuffers;//尝试次数 for (;;) { //------- 循环 //获取buffer描述符 buf = GetBufferDescriptor(ClockSweepTick()); /* * If the buffer is pinned or has a nonzero usage_count, we cannot use * it; decrement the usage_count (unless pinned) and keep scanning. * 如果buffer已pinned,或者有一个非零值的usage_count,不能使用这个buffer. * 减少usage_count(除非已pinned)继续扫描. */ local_buf_state = LockBufHdr(buf); if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0) { //----- refcount == 0 if (BUF_STATE_GET_USAGECOUNT(local_buf_state) != 0) { //usage_count <> 0 //usage_count - 1 local_buf_state -= BUF_USAGECOUNT_ONE; //重置尝试次数 trycounter = NBuffers; } else { //usage_count = 0 /* Found a usable buffer */ //发现一个可用的buffer if (strategy != NULL) //添加到该策略的环形缓冲区中 AddBufferToRing(strategy, buf); //输出参数赋值 *buf_state = local_buf_state; //返回buf return buf; } } else if (--trycounter == 0) { //----- refcount <> 0 && --trycounter == 0 /* * We've scanned all the buffers without making any state changes, * so all the buffers are pinned (or were when we looked at them). * We could hope that someone will free one eventually, but it's * probably better to fail than to risk getting stuck in an * infinite loop. * 在没有改变任何状态的情况,我们已经完成了所有buffers的遍历, * 因此所有的buffers已pinned(或者在搜索的时候pinned). * 我们希望某些进程会周期性的释放buffer,但如果实在拿不到,那报错总比傻傻的死循环要好. */ UnlockBufHdr(buf, local_buf_state); elog(ERROR, "no unpinned buffers available"); } //解锁buffer header UnlockBufHdr(buf, local_buf_state); } } ``` ### 三、跟踪分析 测试脚本,查询数据表: ``` 10:01:54 (xdb@[local]:5432)testdb=# select * from t1 limit 10; ``` 启动gdb,设置断点 ``` (gdb) Continuing. Breakpoint 1, StrategyGetBuffer (strategy=0x0, buf_state=0x7ffcc97fb4ec) at freelist.c:212 212 if (strategy != NULL) (gdb) ``` 输入参数 strategy=NULL,策略对象,使用默认策略 ``` (gdb) p *buf_state $1 = 0 ``` 1.初始化相关变量 2.如策略对象不为空,则从环形缓冲区中获取buffer,如成功则返回buf 3.如需要,则唤醒后台进程bgwriter,从共享内存中读取一次,然后根据该值设置latch ``` (gdb) n 231 bgwprocno = INT_ACCESS_ONCE(StrategyControl->bgwprocno); (gdb) 232 if (bgwprocno != -1) (gdb) 235 StrategyControl->bgwprocno = -1; (gdb) p bgwprocno $2 = 112 (gdb) p StrategyControl $3 = (BufferStrategyControl *) 0x7f8607b21700 (gdb) p *StrategyControl $4 = {buffer_strategy_lock = 0 '\000', nextVictimBuffer = {value = 0}, firstFreeBuffer = 134, lastFreeBuffer = 65535, completePasses = 0, numBufferAllocs = {value = 0}, bgwprocno = 112} (gdb) n 242 SetLatch(&ProcGlobal->allProcs[bgwprocno].procLatch); (gdb) ``` 4.计算buffer分配请求,这样bgwriter可以估算buffer消耗的比例. ``` (gdb) 250 pg_atomic_fetch_add_u32(&StrategyControl->numBufferAllocs, 1); ``` 5.检查freelist中是否存在buffer ``` (gdb) 268 if (StrategyControl->firstFreeBuffer >= 0) ``` 5.1如存在,则执行相关判断逻辑,如成功,则返回buf ``` (gdb) n 273 SpinLockAcquire(&StrategyControl->buffer_strategy_lock); (gdb) 275 if (StrategyControl->firstFreeBuffer < 0) (gdb) 281 buf = GetBufferDescriptor(StrategyControl->firstFreeBuffer); (gdb) 282 Assert(buf->freeNext != FREENEXT_NOT_IN_LIST); (gdb) p *buf $5 = {tag = {rnode = {spcNode = 0, dbNode = 0, relNode = 0}, forkNum = InvalidForkNumber, blockNum = 4294967295}, buf_id = 134, state = {value = 0}, wait_backend_pid = 0, freeNext = 135, content_lock = {tranche = 54, state = { value = 536870912}, waiters = {head = 2147483647, tail = 2147483647}}} (gdb) n 285 StrategyControl->firstFreeBuffer = buf->freeNext; (gdb) 286 buf->freeNext = FREENEXT_NOT_IN_LIST; (gdb) 292 SpinLockRelease(&StrategyControl->buffer_strategy_lock); (gdb) 301 local_buf_state = LockBufHdr(buf); (gdb) 302 if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0 (gdb) 303 && BUF_STATE_GET_USAGECOUNT(local_buf_state) == 0) (gdb) 305 if (strategy != NULL) (gdb) 307 *buf_state = local_buf_state; (gdb) 308 return buf; (gdb) p *buf_state $6 = 4194304 (gdb) p *buf $7 = {tag = {rnode = {spcNode = 0, dbNode = 0, relNode = 0}, forkNum = InvalidForkNumber, blockNum = 4294967295}, buf_id = 134, state = {value = 4194304}, wait_backend_pid = 0, freeNext = -2, content_lock = {tranche = 54, state = { value = 536870912}, waiters = {head = 2147483647, tail = 2147483647}}} (gdb) ``` 返回结果,回到BufferAlloc ``` (gdb) n 358 } (gdb) BufferAlloc (smgr=0x22a38a0, relpersistence=112 'p', forkNum=MAIN_FORKNUM, blockNum=0, strategy=0x0, foundPtr=0x7ffcc97fb5c3) at bufmgr.c:1073 1073 Assert(BUF_STATE_GET_REFCOUNT(buf_state) == 0); (gdb) ``` DONE! ### 四、参考资料 [PG Source Code](https://doxygen.postgresql.org)