buf_init.c

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/*-------------------------------------------------------------------------
 *
 * buf_init.c
 *    buffer manager initialization routines
 *
 * Portions Copyright (c) 1996-2013, 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/bufmgr.h"
#include "storage/buf_internals.h"


BufferDesc *BufferDescriptors;
char       *BufferBlocks;
int32      *PrivateRefCount;


/*
 * 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.
 *
 * PrivateRefCount -- Each buffer also has a private refcount that keeps
 *      track of the number of times the buffer is pinned in the current
 *      process.    This is used for two purposes: first, if we pin a
 *      a buffer more than once, we only need to change the shared refcount
 *      once, thus only lock the shared state once; second, when a transaction
 *      aborts, it should only unpin the buffers exactly the number of times it
 *      has pinned them, so that it will not blow away buffers of another
 *      backend.
 */


/*
 * Initialize shared buffer pool
 *
 * This is called once during shared-memory initialization (either in the
 * postmaster, or in a standalone backend).
 */
void
InitBufferPool(void)
{
    bool        foundBufs,
                foundDescs;

    BufferDescriptors = (BufferDesc *)
        ShmemInitStruct("Buffer Descriptors",
                        NBuffers * sizeof(BufferDesc), &foundDescs);

    BufferBlocks = (char *)
        ShmemInitStruct("Buffer Blocks",
                        NBuffers * (Size) BLCKSZ, &foundBufs);

    if (foundDescs || foundBufs)
    {
        /* both should be present or neither */
        Assert(foundDescs && foundBufs);
        /* note: this path is only taken in EXEC_BACKEND case */
    }
    else
    {
        BufferDesc *buf;
        int         i;

        buf = BufferDescriptors;

        /*
         * Initialize all the buffer headers.
         */
        for (i = 0; i < NBuffers; buf++, i++)
        {
            CLEAR_BUFFERTAG(buf->tag);
            buf->flags = 0;
            buf->usage_count = 0;
            buf->refcount = 0;
            buf->wait_backend_pid = 0;

            SpinLockInit(&buf->buf_hdr_lock);

            buf->buf_id = i;

            /*
             * Initially link all the buffers together as unused. Subsequent
             * management of this list is done by freelist.c.
             */
            buf->freeNext = i + 1;

            buf->io_in_progress_lock = LWLockAssign();
            buf->content_lock = LWLockAssign();
        }

        /* Correct last entry of linked list */
        BufferDescriptors[NBuffers - 1].freeNext = FREENEXT_END_OF_LIST;
    }

    /* Init other shared buffer-management stuff */
    StrategyInitialize(!foundDescs);
}

/*
 * Initialize access to shared buffer pool
 *
 * This is called during backend startup (whether standalone or under the
 * postmaster).  It sets up for this backend's access to the already-existing
 * buffer pool.
 *
 * NB: this is called before InitProcess(), so we do not have a PGPROC and
 * cannot do LWLockAcquire; hence we can't actually access stuff in
 * shared memory yet.  We are only initializing local data here.
 * (See also InitBufferPoolBackend, over in bufmgr.c.)
 */
void
InitBufferPoolAccess(void)
{
    /*
     * Allocate and zero local arrays of per-buffer info.
     */
    PrivateRefCount = (int32 *) calloc(NBuffers, sizeof(int32));
    if (!PrivateRefCount)
        ereport(FATAL,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory")));
}

/*
 * BufferShmemSize
 *
 * compute the size of shared memory for the buffer pool including
 * data pages, buffer descriptors, hash tables, etc.
 */
Size
BufferShmemSize(void)
{
    Size        size = 0;

    /* size of buffer descriptors */
    size = add_size(size, mul_size(NBuffers, sizeof(BufferDesc)));

    /* size of data pages */
    size = add_size(size, mul_size(NBuffers, BLCKSZ));

    /* size of stuff controlled by freelist.c */
    size = add_size(size, StrategyShmemSize());

    return size;
}