对PostgreSQL中后台进程的内存结构建立的初步跟踪

开始

基本上：

AuxiliaryProcessMain --> BaseInit --> InitCommunication --> CreateSharedMemoryAndSemaphores

AuxiliaryProcessMain 是各个后台进程(bgwriter等)的调用起始点

[作者：技术者高健@博客园 mail: ]luckyjackgao@gmail.com ]

/*
*     AuxiliaryProcessMain
*
*     The main entry point for auxiliary processes, such as the bgwriter,
*     walwriter, walreceiver, bootstrapper and the shared memory checker code.
*
*     This code is here just because of historical reasons.
*/
void
AuxiliaryProcessMain(int argc, char *argv[])
{
……
/*
* Fire up essential subsystems: error and memory management
*
* If we are running under the postmaster, this is done already.
*/
if (!IsUnderPostmaster)
MemoryContextInit();

……
/*
* Identify myself via ps
*/
if (IsUnderPostmaster)
{
const char *statmsg;

switch (MyAuxProcType)
{
case StartupProcess:
statmsg = "startup process";
break;
case BgWriterProcess:
statmsg = "writer process";
break;
case CheckpointerProcess:
statmsg = "checkpointer process";
break;
case WalWriterProcess:
statmsg = "wal writer process";
break;
case WalReceiverProcess:
statmsg = "wal receiver process";
break;
default:
statmsg = "??? process";
break;
}
init_ps_display(statmsg, "", "", "");
}

……
BaseInit();

/*
* When we are an auxiliary process, we aren't going to do the full
* InitPostgres pushups, but there are a couple of things that need to get
* lit up even in an auxiliary process.
*/
if (IsUnderPostmaster)
{
/*
* Create a PGPROC so we can use LWLocks.  In the EXEC_BACKEND case,
* this was already done by SubPostmasterMain().
*/
#ifndef EXEC_BACKEND
InitAuxiliaryProcess();
#endif
……
}

/*
* XLOG operations
*/
SetProcessingMode(NormalProcessing);

switch (MyAuxProcType)
{
……
case BgWriterProcess:
/* don't set signals, bgwriter has its own agenda */
BackgroundWriterMain();
proc_exit(1);        /* should never return */
……
}
}

而其中的 BaseInit 要完成如下几件事：

* Early initialization of a backend (either standalone or under postmaster).
* This happens even before InitPostgres.
*
* This is separate from InitPostgres because it is also called by auxiliary
* processes, such as the background writer process, which may not call
* InitPostgres at all.
*/
void
BaseInit(void)
{
/*
* Attach to shared memory and semaphores, and initialize our
* input/output/debugging file descriptors.
*/
InitCommunication();
DebugFileOpen();

/* Do local initialization of file, storage and buffer managers */
InitFileAccess();
smgrinit();
InitBufferPoolAccess();
}

对于 InitCommunication ，是这样的：

/* --------------------------------
*        InitCommunication
*
*        This routine initializes stuff needed for ipc, locking, etc.
*        it should be called something more informative.
* --------------------------------
*/
static void
InitCommunication(void)
{
/*
* initialize shared memory and semaphores appropriately.
*/
if (!IsUnderPostmaster)        /* postmaster already did this */
{
/*
* We're running a postgres bootstrap process or a standalone backend.
* Create private "shmem" and semaphores.
*/
CreateSharedMemoryAndSemaphores(true, 0);
}
}

真正建立为每个后台进程建立内存结构的，就是这个 CreateSharedMemoryAndSemaphores

[作者：技术者高健@博客园 mail: ]luckyjackgao@gmail.com ]

结束