PostgreSQL中StrategyGetBuffer函数有什么作用
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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)
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