freelist.c

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/*-------------------------------------------------------------------------
 *
 * freelist.c
 *    routines for managing the buffer pool's replacement strategy.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/buffer/freelist.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "storage/buf_internals.h"
#include "storage/bufmgr.h"


/*
 * The shared freelist control information.
 */
typedef struct
{
    /* Clock sweep hand: index of next buffer to consider grabbing */
    int         nextVictimBuffer;

    int         firstFreeBuffer;    /* Head of list of unused buffers */
    int         lastFreeBuffer; /* Tail of list of unused buffers */

    /*
     * NOTE: lastFreeBuffer is undefined when firstFreeBuffer is -1 (that is,
     * when the list is empty)
     */

    /*
     * Statistics.  These counters should be wide enough that they can't
     * overflow during a single bgwriter cycle.
     */
    uint32      completePasses; /* Complete cycles of the clock sweep */
    uint32      numBufferAllocs;    /* Buffers allocated since last reset */

    /*
     * Notification latch, or NULL if none.  See StrategyNotifyBgWriter.
     */
    Latch      *bgwriterLatch;
} BufferStrategyControl;

/* Pointers to shared state */
static BufferStrategyControl *StrategyControl = NULL;

/*
 * Private (non-shared) state for managing a ring of shared buffers to re-use.
 * This is currently the only kind of BufferAccessStrategy object, but someday
 * we might have more kinds.
 */
typedef struct BufferAccessStrategyData
{
    /* Overall strategy type */
    BufferAccessStrategyType btype;
    /* Number of elements in buffers[] array */
    int         ring_size;

    /*
     * Index of the "current" slot in the ring, ie, the one most recently
     * returned by GetBufferFromRing.
     */
    int         current;

    /*
     * True if the buffer just returned by StrategyGetBuffer had been in the
     * ring already.
     */
    bool        current_was_in_ring;

    /*
     * Array of buffer numbers.  InvalidBuffer (that is, zero) indicates we
     * have not yet selected a buffer for this ring slot.  For allocation
     * simplicity this is palloc'd together with the fixed fields of the
     * struct.
     */
    Buffer      buffers[1];     /* VARIABLE SIZE ARRAY */
}   BufferAccessStrategyData;


/* Prototypes for internal functions */
static volatile BufferDesc *GetBufferFromRing(BufferAccessStrategy strategy);
static void AddBufferToRing(BufferAccessStrategy strategy,
                volatile BufferDesc *buf);


/*
 * 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.
 *
 *  strategy is a BufferAccessStrategy object, or NULL for default strategy.
 *
 *  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.  If
 *  *lock_held is set on exit, we have returned with the BufFreelistLock
 *  still held, as well; the caller must release that lock once the spinlock
 *  is dropped.  We do it that way because releasing the BufFreelistLock
 *  might awaken other processes, and it would be bad to do the associated
 *  kernel calls while holding the buffer header spinlock.
 */
volatile BufferDesc *
StrategyGetBuffer(BufferAccessStrategy strategy, bool *lock_held)
{
    volatile BufferDesc *buf;
    Latch      *bgwriterLatch;
    int         trycounter;

    /*
     * If given a strategy object, see whether it can select a buffer. We
     * assume strategy objects don't need the BufFreelistLock.
     */
    if (strategy != NULL)
    {
        buf = GetBufferFromRing(strategy);
        if (buf != NULL)
        {
            *lock_held = false;
            return buf;
        }
    }

    /* Nope, so lock the freelist */
    *lock_held = true;
    LWLockAcquire(BufFreelistLock, LW_EXCLUSIVE);

    /*
     * 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.
     */
    StrategyControl->numBufferAllocs++;

    /*
     * If bgwriterLatch is set, we need to waken the bgwriter, but we should
     * not do so while holding BufFreelistLock; so release and re-grab.  This
     * is annoyingly tedious, but it happens at most once per bgwriter cycle,
     * so the performance hit is minimal.
     */
    bgwriterLatch = StrategyControl->bgwriterLatch;
    if (bgwriterLatch)
    {
        StrategyControl->bgwriterLatch = NULL;
        LWLockRelease(BufFreelistLock);
        SetLatch(bgwriterLatch);
        LWLockAcquire(BufFreelistLock, LW_EXCLUSIVE);
    }

    /*
     * Try to get a buffer from the freelist.  Note that the freeNext fields
     * are considered to be protected by the BufFreelistLock not the
     * individual buffer spinlocks, so it's OK to manipulate them without
     * holding the spinlock.
     */
    while (StrategyControl->firstFreeBuffer >= 0)
    {
        buf = &BufferDescriptors[StrategyControl->firstFreeBuffer];
        Assert(buf->freeNext != FREENEXT_NOT_IN_LIST);

        /* Unconditionally remove buffer from freelist */
        StrategyControl->firstFreeBuffer = buf->freeNext;
        buf->freeNext = FREENEXT_NOT_IN_LIST;

        /*
         * 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.)
         */
        LockBufHdr(buf);
        if (buf->refcount == 0 && buf->usage_count == 0)
        {
            if (strategy != NULL)
                AddBufferToRing(strategy, buf);
            return buf;
        }
        UnlockBufHdr(buf);
    }

    /* Nothing on the freelist, so run the "clock sweep" algorithm */
    trycounter = NBuffers;
    for (;;)
    {
        buf = &BufferDescriptors[StrategyControl->nextVictimBuffer];

        if (++StrategyControl->nextVictimBuffer >= NBuffers)
        {
            StrategyControl->nextVictimBuffer = 0;
            StrategyControl->completePasses++;
        }

        /*
         * If the buffer is pinned or has a nonzero usage_count, we cannot use
         * it; decrement the usage_count (unless pinned) and keep scanning.
         */
        LockBufHdr(buf);
        if (buf->refcount == 0)
        {
            if (buf->usage_count > 0)
            {
                buf->usage_count--;
                trycounter = NBuffers;
            }
            else
            {
                /* Found a usable buffer */
                if (strategy != NULL)
                    AddBufferToRing(strategy, buf);
                return buf;
            }
        }
        else if (--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.
             */
            UnlockBufHdr(buf);
            elog(ERROR, "no unpinned buffers available");
        }
        UnlockBufHdr(buf);
    }
}

/*
 * StrategyFreeBuffer: put a buffer on the freelist
 */
void
StrategyFreeBuffer(volatile BufferDesc *buf)
{
    LWLockAcquire(BufFreelistLock, LW_EXCLUSIVE);

    /*
     * It is possible that we are told to put something in the freelist that
     * is already in it; don't screw up the list if so.
     */
    if (buf->freeNext == FREENEXT_NOT_IN_LIST)
    {
        buf->freeNext = StrategyControl->firstFreeBuffer;
        if (buf->freeNext < 0)
            StrategyControl->lastFreeBuffer = buf->buf_id;
        StrategyControl->firstFreeBuffer = buf->buf_id;
    }

    LWLockRelease(BufFreelistLock);
}

/*
 * StrategySyncStart -- tell BufferSync where to start syncing
 *
 * The result is the buffer index of the best buffer to sync first.
 * BufferSync() will proceed circularly around the buffer array from there.
 *
 * In addition, we return the completed-pass count (which is effectively
 * the higher-order bits of nextVictimBuffer) and the count of recent buffer
 * allocs if non-NULL pointers are passed.  The alloc count is reset after
 * being read.
 */
int
StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc)
{
    int         result;

    LWLockAcquire(BufFreelistLock, LW_EXCLUSIVE);
    result = StrategyControl->nextVictimBuffer;
    if (complete_passes)
        *complete_passes = StrategyControl->completePasses;
    if (num_buf_alloc)
    {
        *num_buf_alloc = StrategyControl->numBufferAllocs;
        StrategyControl->numBufferAllocs = 0;
    }
    LWLockRelease(BufFreelistLock);
    return result;
}

/*
 * StrategyNotifyBgWriter -- set or clear allocation notification latch
 *
 * If bgwriterLatch isn't NULL, the next invocation of StrategyGetBuffer will
 * set that latch.  Pass NULL to clear the pending notification before it
 * happens.  This feature is used by the bgwriter process to wake itself up
 * from hibernation, and is not meant for anybody else to use.
 */
void
StrategyNotifyBgWriter(Latch *bgwriterLatch)
{
    /*
     * We acquire the BufFreelistLock just to ensure that the store appears
     * atomic to StrategyGetBuffer.  The bgwriter should call this rather
     * infrequently, so there's no performance penalty from being safe.
     */
    LWLockAcquire(BufFreelistLock, LW_EXCLUSIVE);
    StrategyControl->bgwriterLatch = bgwriterLatch;
    LWLockRelease(BufFreelistLock);
}


/*
 * StrategyShmemSize
 *
 * estimate the size of shared memory used by the freelist-related structures.
 *
 * Note: for somewhat historical reasons, the buffer lookup hashtable size
 * is also determined here.
 */
Size
StrategyShmemSize(void)
{
    Size        size = 0;

    /* size of lookup hash table ... see comment in StrategyInitialize */
    size = add_size(size, BufTableShmemSize(NBuffers + NUM_BUFFER_PARTITIONS));

    /* size of the shared replacement strategy control block */
    size = add_size(size, MAXALIGN(sizeof(BufferStrategyControl)));

    return size;
}

/*
 * StrategyInitialize -- initialize the buffer cache replacement
 *      strategy.
 *
 * Assumes: All of the buffers are already built into a linked list.
 *      Only called by postmaster and only during initialization.
 */
void
StrategyInitialize(bool init)
{
    bool        found;

    /*
     * Initialize the shared buffer lookup hashtable.
     *
     * Since we can't tolerate running out of lookup table entries, we must be
     * sure to specify an adequate table size here.  The maximum steady-state
     * usage is of course NBuffers entries, but BufferAlloc() tries to insert
     * a new entry before deleting the old.  In principle this could be
     * happening in each partition concurrently, so we could need as many as
     * NBuffers + NUM_BUFFER_PARTITIONS entries.
     */
    InitBufTable(NBuffers + NUM_BUFFER_PARTITIONS);

    /*
     * Get or create the shared strategy control block
     */
    StrategyControl = (BufferStrategyControl *)
        ShmemInitStruct("Buffer Strategy Status",
                        sizeof(BufferStrategyControl),
                        &found);

    if (!found)
    {
        /*
         * Only done once, usually in postmaster
         */
        Assert(init);

        /*
         * Grab the whole linked list of free buffers for our strategy. We
         * assume it was previously set up by InitBufferPool().
         */
        StrategyControl->firstFreeBuffer = 0;
        StrategyControl->lastFreeBuffer = NBuffers - 1;

        /* Initialize the clock sweep pointer */
        StrategyControl->nextVictimBuffer = 0;

        /* Clear statistics */
        StrategyControl->completePasses = 0;
        StrategyControl->numBufferAllocs = 0;

        /* No pending notification */
        StrategyControl->bgwriterLatch = NULL;
    }
    else
        Assert(!init);
}


/* ----------------------------------------------------------------
 *              Backend-private buffer ring management
 * ----------------------------------------------------------------
 */


/*
 * GetAccessStrategy -- create a BufferAccessStrategy object
 *
 * The object is allocated in the current memory context.
 */
BufferAccessStrategy
GetAccessStrategy(BufferAccessStrategyType btype)
{
    BufferAccessStrategy strategy;
    int         ring_size;

    /*
     * Select ring size to use.  See buffer/README for rationales.
     *
     * Note: if you change the ring size for BAS_BULKREAD, see also
     * SYNC_SCAN_REPORT_INTERVAL in access/heap/syncscan.c.
     */
    switch (btype)
    {
        case BAS_NORMAL:
            /* if someone asks for NORMAL, just give 'em a "default" object */
            return NULL;

        case BAS_BULKREAD:
            ring_size = 256 * 1024 / BLCKSZ;
            break;
        case BAS_BULKWRITE:
            ring_size = 16 * 1024 * 1024 / BLCKSZ;
            break;
        case BAS_VACUUM:
            ring_size = 256 * 1024 / BLCKSZ;
            break;

        default:
            elog(ERROR, "unrecognized buffer access strategy: %d",
                 (int) btype);
            return NULL;        /* keep compiler quiet */
    }

    /* Make sure ring isn't an undue fraction of shared buffers */
    ring_size = Min(NBuffers / 8, ring_size);

    /* Allocate the object and initialize all elements to zeroes */
    strategy = (BufferAccessStrategy)
        palloc0(offsetof(BufferAccessStrategyData, buffers) +
                ring_size * sizeof(Buffer));

    /* Set fields that don't start out zero */
    strategy->btype = btype;
    strategy->ring_size = ring_size;

    return strategy;
}

/*
 * FreeAccessStrategy -- release a BufferAccessStrategy object
 *
 * A simple pfree would do at the moment, but we would prefer that callers
 * don't assume that much about the representation of BufferAccessStrategy.
 */
void
FreeAccessStrategy(BufferAccessStrategy strategy)
{
    /* don't crash if called on a "default" strategy */
    if (strategy != NULL)
        pfree(strategy);
}

/*
 * GetBufferFromRing -- returns a buffer from the ring, or NULL if the
 *      ring is empty.
 *
 * The bufhdr spin lock is held on the returned buffer.
 */
static volatile BufferDesc *
GetBufferFromRing(BufferAccessStrategy strategy)
{
    volatile BufferDesc *buf;
    Buffer      bufnum;

    /* Advance to next ring slot */
    if (++strategy->current >= strategy->ring_size)
        strategy->current = 0;

    /*
     * If the slot hasn't been filled yet, tell the caller to allocate a new
     * buffer with the normal allocation strategy.  He will then fill this
     * slot by calling AddBufferToRing with the new buffer.
     */
    bufnum = strategy->buffers[strategy->current];
    if (bufnum == InvalidBuffer)
    {
        strategy->current_was_in_ring = false;
        return NULL;
    }

    /*
     * If the buffer is pinned we cannot use it under any circumstances.
     *
     * If usage_count is 0 or 1 then the buffer is fair game (we expect 1,
     * since our own previous usage of the ring element would have left it
     * there, but it might've been decremented by clock sweep since then). A
     * higher usage_count indicates someone else has touched the buffer, so we
     * shouldn't re-use it.
     */
    buf = &BufferDescriptors[bufnum - 1];
    LockBufHdr(buf);
    if (buf->refcount == 0 && buf->usage_count <= 1)
    {
        strategy->current_was_in_ring = true;
        return buf;
    }
    UnlockBufHdr(buf);

    /*
     * Tell caller to allocate a new buffer with the normal allocation
     * strategy.  He'll then replace this ring element via AddBufferToRing.
     */
    strategy->current_was_in_ring = false;
    return NULL;
}

/*
 * AddBufferToRing -- add a buffer to the buffer ring
 *
 * Caller must hold the buffer header spinlock on the buffer.  Since this
 * is called with the spinlock held, it had better be quite cheap.
 */
static void
AddBufferToRing(BufferAccessStrategy strategy, volatile BufferDesc *buf)
{
    strategy->buffers[strategy->current] = BufferDescriptorGetBuffer(buf);
}

/*
 * StrategyRejectBuffer -- consider rejecting a dirty buffer
 *
 * When a nondefault strategy is used, the buffer manager calls this function
 * when it turns out that the buffer selected by StrategyGetBuffer needs to
 * be written out and doing so would require flushing WAL too.  This gives us
 * a chance to choose a different victim.
 *
 * Returns true if buffer manager should ask for a new victim, and false
 * if this buffer should be written and re-used.
 */
bool
StrategyRejectBuffer(BufferAccessStrategy strategy, volatile BufferDesc *buf)
{
    /* We only do this in bulkread mode */
    if (strategy->btype != BAS_BULKREAD)
        return false;

    /* Don't muck with behavior of normal buffer-replacement strategy */
    if (!strategy->current_was_in_ring ||
      strategy->buffers[strategy->current] != BufferDescriptorGetBuffer(buf))
        return false;

    /*
     * Remove the dirty buffer from the ring; necessary to prevent infinite
     * loop if all ring members are dirty.
     */
    strategy->buffers[strategy->current] = InvalidBuffer;

    return true;
}