buf_init.c

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/*-------------------------------------------------------------------------
 *
 * buf_init.c
 *    buffer manager initialization routines
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/buffer/buf_init.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "storage/aio.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
 
BufferDescPadded *BufferDescriptors;
char       *BufferBlocks;
ConditionVariableMinimallyPadded *BufferIOCVArray;
WritebackContext BackendWritebackContext;
CkptSortItem *CkptBufferIds;
 
 
/*
 * Data Structures:
 *      buffers live in a freelist and a lookup data structure.
 *
 *
 * Buffer Lookup:
 *      Two important notes.  First, the buffer has to be
 *      available for lookup BEFORE an IO begins.  Otherwise
 *      a second process trying to read the buffer will
 *      allocate its own copy and the buffer pool will
 *      become inconsistent.
 *
 * Buffer Replacement:
 *      see freelist.c.  A buffer cannot be replaced while in
 *      use either by data manager or during IO.
 *
 *
 * Synchronization/Locking:
 *
 * IO_IN_PROGRESS -- this is a flag in the buffer descriptor.
 *      It must be set when an IO is initiated and cleared at
 *      the end of the IO.  It is there to make sure that one
 *      process doesn't start to use a buffer while another is
 *      faulting it in.  see WaitIO and related routines.
 *
 * refcount --  Counts the number of processes holding pins on a buffer.
 *      A buffer is pinned during IO and immediately after a BufferAlloc().
 *      Pins must be released before end of transaction.  For efficiency the
 *      shared refcount isn't increased if an individual backend pins a buffer
 *      multiple times. Check the PrivateRefCount infrastructure in bufmgr.c.
 */
 
 
/*
 * Initialize shared buffer pool
 *
 * This is called once during shared-memory initialization (either in the
 * postmaster, or in a standalone backend).
 */
void
BufferManagerShmemInit(void)
{
    bool        foundBufs,
                foundDescs,
                foundIOCV,
                foundBufCkpt;
 
    /* Align descriptors to a cacheline boundary. */
    BufferDescriptors = (BufferDescPadded *)
        ShmemInitStruct("Buffer Descriptors",
                        NBuffers * sizeof(BufferDescPadded),
                        &foundDescs);
 
    /* Align buffer pool on IO page size boundary. */
    BufferBlocks = (char *)
        TYPEALIGN(PG_IO_ALIGN_SIZE,
                  ShmemInitStruct("Buffer Blocks",
                                  NBuffers * (Size) BLCKSZ + PG_IO_ALIGN_SIZE,
                                  &foundBufs));
 
    /* Align condition variables to cacheline boundary. */
    BufferIOCVArray = (ConditionVariableMinimallyPadded *)
        ShmemInitStruct("Buffer IO Condition Variables",
                        NBuffers * sizeof(ConditionVariableMinimallyPadded),
                        &foundIOCV);
 
    /*
     * The array used to sort to-be-checkpointed buffer ids is located in
     * shared memory, to avoid having to allocate significant amounts of
     * memory at runtime. As that'd be in the middle of a checkpoint, or when
     * the checkpointer is restarted, memory allocation failures would be
     * painful.
     */
    CkptBufferIds = (CkptSortItem *)
        ShmemInitStruct("Checkpoint BufferIds",
                        NBuffers * sizeof(CkptSortItem), &foundBufCkpt);
 
    if (foundDescs || foundBufs || foundIOCV || foundBufCkpt)
    {
        /* should find all of these, or none of them */
        Assert(foundDescs && foundBufs && foundIOCV && foundBufCkpt);
        /* note: this path is only taken in EXEC_BACKEND case */
    }
    else
    {
        int         i;
 
        /*
         * Initialize all the buffer headers.
         */
        for (i = 0; i < NBuffers; i++)
        {
            BufferDesc *buf = GetBufferDescriptor(i);
 
            ClearBufferTag(&buf->tag);
 
            pg_atomic_init_u32(&buf->state, 0);
            buf->wait_backend_pgprocno = INVALID_PROC_NUMBER;
 
            buf->buf_id = i;
 
            pgaio_wref_clear(&buf->io_wref);
 
            /*
             * Initially link all the buffers together as unused. Subsequent
             * management of this list is done by freelist.c.
             */
            buf->freeNext = i + 1;
 
            LWLockInitialize(BufferDescriptorGetContentLock(buf),
                             LWTRANCHE_BUFFER_CONTENT);
 
            ConditionVariableInit(BufferDescriptorGetIOCV(buf));
        }
 
        /* Correct last entry of linked list */
        GetBufferDescriptor(NBuffers - 1)->freeNext = FREENEXT_END_OF_LIST;
    }
 
    /* Init other shared buffer-management stuff */
    StrategyInitialize(!foundDescs);
 
    /* Initialize per-backend file flush context */
    WritebackContextInit(&BackendWritebackContext,
                         &backend_flush_after);
}
 
/*
 * BufferManagerShmemSize
 *
 * compute the size of shared memory for the buffer pool including
 * data pages, buffer descriptors, hash tables, etc.
 */
Size
BufferManagerShmemSize(void)
{
    Size        size = 0;
 
    /* size of buffer descriptors */
    size = add_size(size, mul_size(NBuffers, sizeof(BufferDescPadded)));
    /* to allow aligning buffer descriptors */
    size = add_size(size, PG_CACHE_LINE_SIZE);
 
    /* size of data pages, plus alignment padding */
    size = add_size(size, PG_IO_ALIGN_SIZE);
    size = add_size(size, mul_size(NBuffers, BLCKSZ));
 
    /* size of stuff controlled by freelist.c */
    size = add_size(size, StrategyShmemSize());
 
    /* size of I/O condition variables */
    size = add_size(size, mul_size(NBuffers,
                                   sizeof(ConditionVariableMinimallyPadded)));
    /* to allow aligning the above */
    size = add_size(size, PG_CACHE_LINE_SIZE);
 
    /* size of checkpoint sort array in bufmgr.c */
    size = add_size(size, mul_size(NBuffers, sizeof(CkptSortItem)));
 
    return size;
}