PostgreSQL使用进程模式，对于每一个客户端会Fork一个后台进程响应客户端的请求。本节介绍了每个后台进程在共享内存中都存在一个的数据结构：PGPROC。
一、数据结构
宏定义
/*
* Note: MAX_BACKENDS is limited to 2^18-1 because that's the width reserved
* for buffer references in buf_internals.h. This limitation could be lifted
* by using a 64bit state; but it's unlikely to be worthwhile as 2^18-1
* backends exceed currently realistic configurations. Even if that limitation
* were removed, we still could not a) exceed 2^23-1 because inval.c stores
* the backend ID as a 3-byte signed integer, b) INT_MAX/4 because some places
* compute 4*MaxBackends without any overflow check. This is rechecked in the
* relevant GUC check hooks and in RegisterBackgroundWorker().
* 注意:MAX_BACKENDS限制为2^18-1,
* 这是因为该值为buf_internals.h中定义的缓存依赖的最大宽度.
* 该限制可以通过使用64bit的状态来提升,但它看起来并不值当.
* 如果去掉该限制,我们仍然不能够超过:
* a) 2^23-1,因为inval.c使用3个字节的有符号整数存储后台进程ID
* b) INT_MAX/4 ,因为某些地方没有任何的溢出检查,直接计算4*MaxBackends的值.
* 该值会在相关的GUC检查钩子和RegisterBackgroundWorker()函数中检查.
*/
#define MAX_BACKENDS 0x3FFFF
/* shmqueue.c */
typedef struct SHM_QUEUE
{
struct SHM_QUEUE *prev;
struct SHM_QUEUE *next;
} SHM_QUEUE;
/*
* An invalid pgprocno. Must be larger than the maximum number of PGPROC
* structures we could possibly have. See comments for MAX_BACKENDS.
* 无效的pg进程号.
* 必须大于我们可能拥有的最大的PGPROC数目.
* 详细解释见MAX_BACKENDS
*/
#define INVALID_PGPROCNO PG_INT32_MAX
LWLock
lwlock.c外的代码不应直接操作这个结构的内容,但我们必须声明该结构体以便将LWLocks合并到其他数据结构中。
/*
* Code outside of lwlock.c should not manipulate the contents of this
* structure directly, but we have to declare it here to allow LWLocks to be
* incorporated into other data structures.
* lwlock.c外的代码不应直接操作这个结构的内容,
* 但我们必须声明该结构体以便将LWLocks合并到其他数据结构中。
*/
typedef struct LWLock
{
uint16 tranche; /* tranche ID */
//独占/非独占locker的状态
pg_atomic_uint32 state; /* state of exclusive/nonexclusive lockers */
//正在等待的PGPROCs链表
proclist_head waiters; /* list of waiting PGPROCs */
#ifdef LOCK_DEBUG//用于DEBUG
//waiters的数量
pg_atomic_uint32 nwaiters; /* number of waiters */
//锁的最后独占者
struct PGPROC *owner; /* last exclusive owner of the lock */
#endif
} LWLock;
PGPROC
每个后台进程在共享内存中都有一个PGPROC结构体.
全局上也存在未使用的PGPROC结构体链表,用于重用以便为新的后台进程进行分配.
该数据结构的作用是:
PostgreSQL backend processes can’t see each other’s memory directly, nor can the postmaster see into PostgreSQL backend process memory. Yet they need some way to communicate and co-ordinate, and the postmaster needs a way to keep track of them.
简单来说作用是为了进程间协同和通讯以及postmaster的跟踪.
/*
* Each backend has a PGPROC struct in shared memory. There is also a list of
* currently-unused PGPROC structs that will be reallocated to new backends.
* 每个后台进程在共享内存中都有一个PGPROC结构体.
* 存在未使用的PGPROC结构体链表,用于为新的后台进程重新进行分配.
*
* links: list link for any list the PGPROC is in. When waiting for a lock,
* the PGPROC is linked into that lock's waitProcs queue. A recycled PGPROC
* is linked into ProcGlobal's freeProcs list.
* links: PGPROC所在的链表的链接.
* 在等待锁时,PGPROC链接到该锁的waiProc队列中.
* 回收的PGPROC链接到ProcGlobal的freeProcs链表中.
*
* Note: twophase.c also sets up a dummy PGPROC struct for each currently
* prepared transaction. These PGPROCs appear in the ProcArray data structure
* so that the prepared transactions appear to be still running and are
* correctly shown as holding locks. A prepared transaction PGPROC can be
* distinguished from a real one at need by the fact that it has pid == 0.
* The semaphore and lock-activity fields in a prepared-xact PGPROC are unused,
* but its myProcLocks[] lists are valid.
* 注意:twophase.c也会为每一个当前已准备妥当的事务配置一个虚拟的PGPROC结构.
* 这些PGPROCs在数组ProcArray数据结构中出现,以便已准备的事务看起来仍在运行,
* 并正确的显示为持有锁.
* 已准备妥当的事务PGPROC与一个真正的PGPROC事实上的区别是pid == 0.
* 在prepared-xact PGPROC中的信号量和活动锁域字段没有使用,但myProcLocks[]链表是有效的.
*/
struct PGPROC
{
/* proc->links MUST BE FIRST IN STRUCT (see ProcSleep,ProcWakeup,etc) */
//proc->links必须是结构体的第一个域(参考ProcSleep,ProcWakeup...等)
//如进程在链表中,这是链表的链接
SHM_QUEUE links; /* list link if process is in a list */
//持有该PGPROC的procglobal链表数组
PGPROC **procgloballist; /* procglobal list that owns this PGPROC */
//可以休眠的信号量
PGSemaphore sem; /* ONE semaphore to sleep on */
//状态为:STATUS_WAITING, STATUS_OK or STATUS_ERROR
int waitStatus; /* STATUS_WAITING, STATUS_OK or STATUS_ERROR */
//进程通用的latch
Latch procLatch; /* generic latch for process */
//运行中的进程正在执行的最高层的事务本地ID,如无运行则为InvalidLocalTransactionId
LocalTransactionId lxid; /* local id of top-level transaction currently
* being executed by this proc, if running;
* else InvalidLocalTransactionId */
//后台进程的ID,如为虚拟事务则为0
int pid; /* Backend's process ID; 0 if prepared xact */
int pgprocno;
/* These fields are zero while a backend is still starting up: */
//------------ 这些域在进程正在启动时为0
//已分配的后台进程的backend ID
BackendId backendId; /* This backend's backend ID (if assigned) */
//该进程使用的数据库ID
Oid databaseId; /* OID of database this backend is using */
//使用该进程的角色ID
Oid roleId; /* OID of role using this backend */
//该进程使用的临时schema OID
Oid tempNamespaceId; /* OID of temp schema this backend is
* using */
//如后台进程,则为T
bool isBackgroundWorker; /* true if background worker. */
/*
* While in hot standby mode, shows that a conflict signal has been sent
* for the current transaction. Set/cleared while holding ProcArrayLock,
* though not required. Accessed without lock, if needed.
* 如在hot standby模式,显示已为当前事务发送冲突信号.
* 尽管不需要,设置/清除持有的ProcArrayLock.
* 如需要,则在没有持有锁的情况下访问.
*/
bool recoveryConflictPending;
/* Info about LWLock the process is currently waiting for, if any. */
//-------------- 进程正在等待的LWLock相关信息
//等待LW lock,为T
bool lwWaiting; /* true if waiting for an LW lock */
//正在等的LWLock锁模式
uint8 lwWaitMode; /* lwlock mode being waited for */
//等待链表中的位置
proclist_node lwWaitLink; /* position in LW lock wait list */
/* Support for condition variables. */
//-------------- 支持条件变量
//CV等待链表中的位置
proclist_node cvWaitLink; /* position in CV wait list */
/* Info about lock the process is currently waiting for, if any. */
//-------------- 进程正在等待的锁信息
/* waitLock and waitProcLock are NULL if not currently waiting. */
//如没有在等待,则waitLock和waitProcLock为NULL
//休眠...等待的锁对象
LOCK *waitLock; /* Lock object we're sleeping on ... */
//等待锁的每个持锁人信息
PROCLOCK *waitProcLock; /* Per-holder info for awaited lock */
//等待的所类型
LOCKMODE waitLockMode; /* type of lock we're waiting for */
//该进程已持有锁的类型位掩码
LOCKMASK heldLocks; /* bitmask for lock types already held on this
* lock object by this backend */
/*
* Info to allow us to wait for synchronous replication, if needed.
* waitLSN is InvalidXLogRecPtr if not waiting; set only by user backend.
* syncRepState must not be touched except by owning process or WALSender.
* syncRepLinks used only while holding SyncRepLock.
* 允许我们等待同步复制的相关信息.
* 如无需等待,则waitLSN为InvalidXLogRecPtr;仅允许由用户后台设置。
* 除非拥有process或WALSender，否则不能修改syncRepState。
* 仅在持有SyncRepLock时使用的syncrepink。
*/
//---------------------
//等待该LSN或者更高的LSN
XLogRecPtr waitLSN; /* waiting for this LSN or higher */
//同步复制的等待状态
int syncRepState; /* wait state for sync rep */
//如进程处于syncrep队列中,则该值保存链表链接
SHM_QUEUE syncRepLinks; /* list link if process is in syncrep queue */
/*
* All PROCLOCK objects for locks held or awaited by this backend are
* linked into one of these lists, according to the partition number of
* their lock.
* 该后台进程持有或等待的锁相关的所有PROCLOCK对象链接在这些链表的末尾,
* 根据棣属于这些锁的分区号进行区分.
*/
SHM_QUEUE myProcLocks[NUM_LOCK_PARTITIONS];
//子事务的XIDs
struct XidCache subxids; /* cache for subtransaction XIDs */
/* Support for group XID clearing. */
/* true, if member of ProcArray group waiting for XID clear */
//支持XID分组清除
//如属于等待XID清理的ProcArray组,则为T
bool procArrayGroupMember;
/* next ProcArray group member waiting for XID clear */
//等待XID清理的下一个ProcArray组编号
pg_atomic_uint32 procArrayGroupNext;
/*
* latest transaction id among the transaction's main XID and
* subtransactions
* 在事务主XID和子事务之间的最后的事务ID
*/
TransactionId procArrayGroupMemberXid;
//进程的等待信息
uint32 wait_event_info; /* proc's wait information */
/* Support for group transaction status update. */
//--------------- 支持组事务状态更新
//clog组成员,则为T
bool clogGroupMember; /* true, if member of clog group */
//下一个clog组成员
pg_atomic_uint32 clogGroupNext; /* next clog group member */
//clog组成员事务ID
TransactionId clogGroupMemberXid; /* transaction id of clog group member */
//clog组成员的事务状态
XidStatus clogGroupMemberXidStatus; /* transaction status of clog
* group member */
//属于clog组成员的事务ID的clog page
int clogGroupMemberPage; /* clog page corresponding to
* transaction id of clog group member */
//clog组成员已提交记录的WAL位置
XLogRecPtr clogGroupMemberLsn; /* WAL location of commit record for clog
* group member */
/* Per-backend LWLock. Protects fields below (but not group fields). */
//每一个后台进程一个LWLock.保护下面的域字段(非组字段)
LWLock backendLock;
/* Lock manager data, recording fast-path locks taken by this backend. */
//---------- 锁管理数据,记录该后台进程以最快路径获得的锁
//每一个fast-path slot的锁模式
uint64 fpLockBits; /* lock modes held for each fast-path slot */
//rel oids的slots
Oid fpRelId[FP_LOCK_SLOTS_PER_BACKEND]; /* slots for rel oids */
//是否持有fast-path VXID锁
bool fpVXIDLock; /* are we holding a fast-path VXID lock? */
//fast-path VXID锁的lxid
LocalTransactionId fpLocalTransactionId; /* lxid for fast-path VXID
* lock */
/*
* Support for lock groups. Use LockHashPartitionLockByProc on the group
* leader to get the LWLock protecting these fields.
*/
//--------- 支持锁组.
// 在组leader中使用LockHashPartitionLockByProc获取LWLock保护这些域
//锁组的leader,如果"我"是其中一员
PGPROC *lockGroupLeader; /* lock group leader, if I'm a member */
//如果"我"是leader,这是成员的链表
dlist_head lockGroupMembers; /* list of members, if I'm a leader */
//成员连接,如果"我"是其中一员
dlist_node lockGroupLink; /* my member link, if I'm a member */
};
MyProc
每个进程都有一个全局变量:MyProc
extern PGDLLIMPORT PGPROC *MyProc;
extern PGDLLIMPORT struct PGXACT *MyPgXact;
二、源码解读
N/A
三、跟踪分析
启动两个Session,执行同样的SQL语句:
insert into t_wal_partition(c1,c2,c3) VALUES(0,'HASH0','HAHS0');
Session 1
启动gdb,开启跟踪
(gdb) b XLogInsertRecord
Breakpoint 1 at 0x54d122: file xlog.c, line 970.
(gdb) c
Continuing.
Breakpoint 1, XLogInsertRecord (rdata=0xf9cc70 <hdr_rdt>, fpw_lsn=0, flags=1 '\001') at xlog.c:970
970 XLogCtlInsert *Insert = &XLogCtl->Insert;
查看内存中的数据结构
(gdb) p *MyProc
$3 = {links = {prev = 0x0, next = 0x0}, procgloballist = 0x7fa79c087c98, sem = 0x7fa779fc81b8, waitStatus = 0, procLatch = {
is_set = 0, is_shared = true, owner_pid = 1398}, lxid = 3, pid = 1398, pgprocno = 99, backendId = 3,
databaseId = 16402, roleId = 10, tempNamespaceId = 0, isBackgroundWorker = false, recoveryConflictPending = false,
lwWaiting = false, lwWaitMode = 0 '\000', lwWaitLink = {next = 0, prev = 0}, cvWaitLink = {next = 0, prev = 0},
waitLock = 0x0, waitProcLock = 0x0, waitLockMode = 0, heldLocks = 0, waitLSN = 0, syncRepState = 0, syncRepLinks = {
prev = 0x0, next = 0x0}, myProcLocks = {{prev = 0x7fa79c09c588, next = 0x7fa79c09c588}, {prev = 0x7fa79c09c598,
next = 0x7fa79c09c598}, {prev = 0x7fa79c09c5a8, next = 0x7fa79c09c5a8}, {prev = 0x7fa79c09c5b8,
next = 0x7fa79c09c5b8}, {prev = 0x7fa79c09c5c8, next = 0x7fa79c09c5c8}, {prev = 0x7fa79c09c5d8,
next = 0x7fa79c09c5d8}, {prev = 0x7fa79c09c5e8, next = 0x7fa79c09c5e8}, {prev = 0x7fa79c09c5f8,
next = 0x7fa79c09c5f8}, {prev = 0x7fa79c09c608, next = 0x7fa79c09c608}, {prev = 0x7fa79c09c618,
next = 0x7fa79c09c618}, {prev = 0x7fa79c09c628, next = 0x7fa79c09c628}, {prev = 0x7fa79c09c638,
next = 0x7fa79c09c638}, {prev = 0x7fa79c09c648, next = 0x7fa79c09c648}, {prev = 0x7fa79c09c658,
next = 0x7fa79c09c658}, {prev = 0x7fa79c09c668, next = 0x7fa79c09c668}, {prev = 0x7fa79be25e70,
next = 0x7fa79be25e70}}, subxids = {xids = {0 <repeats 64 times>}}, procArrayGroupMember = false,
procArrayGroupNext = {value = 2147483647}, procArrayGroupMemberXid = 0, wait_event_info = 0, clogGroupMember = false,
clogGroupNext = {value = 2147483647}, clogGroupMemberXid = 0, clogGroupMemberXidStatus = 0, clogGroupMemberPage = -1,
clogGroupMemberLsn = 0, backendLock = {tranche = 58, state = {value = 536870912}, waiters = {head = 2147483647,
tail = 2147483647}}, fpLockBits = 196027139227648, fpRelId = {0, 0, 0, 0, 0, 2679, 2610, 2680, 2611, 17043, 17040,
17037, 17034, 17031, 17028, 17025}, fpVXIDLock = true, fpLocalTransactionId = 3, lockGroupLeader = 0x0,
lockGroupMembers = {head = {prev = 0x7fa79c09c820, next = 0x7fa79c09c820}}, lockGroupLink = {prev = 0x0, next = 0x0}}
注意:lwWaiting值为false,表示没有在等待LW Lock
Session 2
启动gdb,开启跟踪
(gdb) b heap_insert
Breakpoint 2 at 0x4df4d1: file heapam.c, line 2449.
(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x00007fa7a7ee7a0b in futex_abstimed_wait (cancel=true, private=<optimized out>, abstime=0x0, expected=0,
futex=0x7fa779fc8138) at ../nptl/sysdeps/unix/sysv/linux/sem_waitcommon.c:43
43 err = lll_futex_wait (futex, expected, private);
暂无法进入heap_insert
查看内存中的数据结构
(gdb) p *MyProc
$36 = {links = {prev = 0x0, next = 0x0}, procgloballist = 0x7fa79c087c98, sem = 0x7fa779fc8138, waitStatus = 0,
procLatch = {is_set = 1, is_shared = true, owner_pid = 1449}, lxid = 13, pid = 1449, pgprocno = 98, backendId = 4,
databaseId = 16402, roleId = 10, tempNamespaceId = 0, isBackgroundWorker = false, recoveryConflictPending = false,
lwWaiting = true, lwWaitMode = 0 '\000', lwWaitLink = {next = 114, prev = 2147483647}, cvWaitLink = {next = 0, prev = 0},
waitLock = 0x0, waitProcLock = 0x0, waitLockMode = 0, heldLocks = 0, waitLSN = 0, syncRepState = 0, syncRepLinks = {
prev = 0x0, next = 0x0}, myProcLocks = {{prev = 0x7fa79c09c238, next = 0x7fa79c09c238}, {prev = 0x7fa79c09c248,
next = 0x7fa79c09c248}, {prev = 0x7fa79c09c258, next = 0x7fa79c09c258}, {prev = 0x7fa79c09c268,
next = 0x7fa79c09c268}, {prev = 0x7fa79c09c278, next = 0x7fa79c09c278}, {prev = 0x7fa79c09c288,
next = 0x7fa79c09c288}, {prev = 0x7fa79c09c298, next = 0x7fa79c09c298}, {prev = 0x7fa79c09c2a8,
next = 0x7fa79c09c2a8}, {prev = 0x7fa79c09c2b8, next = 0x7fa79c09c2b8}, {prev = 0x7fa79c09c2c8,
next = 0x7fa79c09c2c8}, {prev = 0x7fa79c09c2d8, next = 0x7fa79c09c2d8}, {prev = 0x7fa79c09c2e8,
next = 0x7fa79c09c2e8}, {prev = 0x7fa79c09c2f8, next = 0x7fa79c09c2f8}, {prev = 0x7fa79c09c308,
next = 0x7fa79c09c308}, {prev = 0x7fa79be21870, next = 0x7fa79be21870}, {prev = 0x7fa79c09c328,
next = 0x7fa79c09c328}}, subxids = {xids = {0 <repeats 64 times>}}, procArrayGroupMember = false,
procArrayGroupNext = {value = 2147483647}, procArrayGroupMemberXid = 0, wait_event_info = 16777270,
clogGroupMember = false, clogGroupNext = {value = 2147483647}, clogGroupMemberXid = 0, clogGroupMemberXidStatus = 0,
clogGroupMemberPage = -1, clogGroupMemberLsn = 0, backendLock = {tranche = 58, state = {value = 536870912}, waiters = {
head = 2147483647, tail = 2147483647}}, fpLockBits = 196027139227648, fpRelId = {0, 0, 0, 0, 0, 2655, 2603, 2680,
2611, 17043, 17040, 17037, 17034, 17031, 17028, 17025}, fpVXIDLock = true, fpLocalTransactionId = 13,
lockGroupLeader = 0x0, lockGroupMembers = {head = {prev = 0x7fa79c09c4d0, next = 0x7fa79c09c4d0}}, lockGroupLink = {
prev = 0x0, next = 0x0}}
注意:
lwWaiting值为true,正在等待Session 1的LWLock.
lwWaitLink = {next = 114, prev = 2147483647},其中next = 114,这里的114是指全局变量ProcGlobal(类型为PROC_HDR)->allProcs数组下标为114的ITEM.
(gdb) p ProcGlobal->allProcs[114]
$41 = {links = {prev = 0x0, next = 0x0}, procgloballist = 0x0, sem = 0x7fa779fc8938, waitStatus = 0, procLatch = {
is_set = 0, is_shared = true, owner_pid = 1351}, lxid = 0, pid = 1351, pgprocno = 114, backendId = -1, databaseId = 0,
roleId = 0, tempNamespaceId = 0, isBackgroundWorker = false, recoveryConflictPending = false, lwWaiting = true,
lwWaitMode = 1 '\001', lwWaitLink = {next = 2147483647, prev = 98}, cvWaitLink = {next = 0, prev = 0}, waitLock = 0x0,
waitProcLock = 0x0, waitLockMode = 0, heldLocks = 0, waitLSN = 0, syncRepState = 0, syncRepLinks = {prev = 0x0,
next = 0x0}, myProcLocks = {{prev = 0x7fa79c09f738, next = 0x7fa79c09f738}, {prev = 0x7fa79c09f748,
next = 0x7fa79c09f748}, {prev = 0x7fa79c09f758, next = 0x7fa79c09f758}, {prev = 0x7fa79c09f768,
next = 0x7fa79c09f768}, {prev = 0x7fa79c09f778, next = 0x7fa79c09f778}, {prev = 0x7fa79c09f788,
next = 0x7fa79c09f788}, {prev = 0x7fa79c09f798, next = 0x7fa79c09f798}, {prev = 0x7fa79c09f7a8,
next = 0x7fa79c09f7a8}, {prev = 0x7fa79c09f7b8, next = 0x7fa79c09f7b8}, {prev = 0x7fa79c09f7c8,
next = 0x7fa79c09f7c8}, {prev = 0x7fa79c09f7d8, next = 0x7fa79c09f7d8}, {prev = 0x7fa79c09f7e8,
next = 0x7fa79c09f7e8}, {prev = 0x7fa79c09f7f8, next = 0x7fa79c09f7f8}, {prev = 0x7fa79c09f808,
next = 0x7fa79c09f808}, {prev = 0x7fa79c09f818, next = 0x7fa79c09f818}, {prev = 0x7fa79c09f828,
next = 0x7fa79c09f828}}, subxids = {xids = {0 <repeats 64 times>}}, procArrayGroupMember = false,
procArrayGroupNext = {value = 0}, procArrayGroupMemberXid = 0, wait_event_info = 16777270, clogGroupMember = false,
clogGroupNext = {value = 0}, clogGroupMemberXid = 0, clogGroupMemberXidStatus = 0, clogGroupMemberPage = 0,
clogGroupMemberLsn = 0, backendLock = {tranche = 58, state = {value = 536870912}, waiters = {head = 2147483647,
tail = 2147483647}}, fpLockBits = 0, fpRelId = {0 <repeats 16 times>}, fpVXIDLock = false, fpLocalTransactionId = 0,
lockGroupLeader = 0x0, lockGroupMembers = {head = {prev = 0x7fa79c09f9d0, next = 0x7fa79c09f9d0}}, lockGroupLink = {
prev = 0x0, next = 0x0}}
ProcGlobal下节再行介绍
