pg_buffercache_pages.c

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/*-------------------------------------------------------------------------
 *
 * pg_buffercache_pages.c
 *    display some contents of the buffer cache
 *
 *    contrib/pg_buffercache/pg_buffercache_pages.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/htup_details.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
 
 
#define NUM_BUFFERCACHE_PAGES_MIN_ELEM  8
#define NUM_BUFFERCACHE_PAGES_ELEM  9
#define NUM_BUFFERCACHE_SUMMARY_ELEM 5
 
PG_MODULE_MAGIC;
 
/*
 * Record structure holding the to be exposed cache data.
 */
typedef struct
{
    uint32      bufferid;
    RelFileNumber relfilenumber;
    Oid         reltablespace;
    Oid         reldatabase;
    ForkNumber  forknum;
    BlockNumber blocknum;
    bool        isvalid;
    bool        isdirty;
    uint16      usagecount;
 
    /*
     * An int32 is sufficiently large, as MAX_BACKENDS prevents a buffer from
     * being pinned by too many backends and each backend will only pin once
     * because of bufmgr.c's PrivateRefCount infrastructure.
     */
    int32       pinning_backends;
} BufferCachePagesRec;
 
 
/*
 * Function context for data persisting over repeated calls.
 */
typedef struct
{
    TupleDesc   tupdesc;
    BufferCachePagesRec *record;
} BufferCachePagesContext;
 
 
/*
 * Function returning data from the shared buffer cache - buffer number,
 * relation node/tablespace/database/blocknum and dirty indicator.
 */
PG_FUNCTION_INFO_V1(pg_buffercache_pages);
PG_FUNCTION_INFO_V1(pg_buffercache_summary);
 
Datum
pg_buffercache_pages(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    Datum       result;
    MemoryContext oldcontext;
    BufferCachePagesContext *fctx;  /* User function context. */
    TupleDesc   tupledesc;
    TupleDesc   expected_tupledesc;
    HeapTuple   tuple;
 
    if (SRF_IS_FIRSTCALL())
    {
        int         i;
 
        funcctx = SRF_FIRSTCALL_INIT();
 
        /* Switch context when allocating stuff to be used in later calls */
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
 
        /* Create a user function context for cross-call persistence */
        fctx = (BufferCachePagesContext *) palloc(sizeof(BufferCachePagesContext));
 
        /*
         * To smoothly support upgrades from version 1.0 of this extension
         * transparently handle the (non-)existence of the pinning_backends
         * column. We unfortunately have to get the result type for that... -
         * we can't use the result type determined by the function definition
         * without potentially crashing when somebody uses the old (or even
         * wrong) function definition though.
         */
        if (get_call_result_type(fcinfo, NULL, &expected_tupledesc) != TYPEFUNC_COMPOSITE)
            elog(ERROR, "return type must be a row type");
 
        if (expected_tupledesc->natts < NUM_BUFFERCACHE_PAGES_MIN_ELEM ||
            expected_tupledesc->natts > NUM_BUFFERCACHE_PAGES_ELEM)
            elog(ERROR, "incorrect number of output arguments");
 
        /* Construct a tuple descriptor for the result rows. */
        tupledesc = CreateTemplateTupleDesc(expected_tupledesc->natts);
        TupleDescInitEntry(tupledesc, (AttrNumber) 1, "bufferid",
                           INT4OID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 2, "relfilenode",
                           OIDOID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 3, "reltablespace",
                           OIDOID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 4, "reldatabase",
                           OIDOID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 5, "relforknumber",
                           INT2OID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 6, "relblocknumber",
                           INT8OID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 7, "isdirty",
                           BOOLOID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 8, "usage_count",
                           INT2OID, -1, 0);
 
        if (expected_tupledesc->natts == NUM_BUFFERCACHE_PAGES_ELEM)
            TupleDescInitEntry(tupledesc, (AttrNumber) 9, "pinning_backends",
                               INT4OID, -1, 0);
 
        fctx->tupdesc = BlessTupleDesc(tupledesc);
 
        /* Allocate NBuffers worth of BufferCachePagesRec records. */
        fctx->record = (BufferCachePagesRec *)
            MemoryContextAllocHuge(CurrentMemoryContext,
                                   sizeof(BufferCachePagesRec) * NBuffers);
 
        /* Set max calls and remember the user function context. */
        funcctx->max_calls = NBuffers;
        funcctx->user_fctx = fctx;
 
        /* Return to original context when allocating transient memory */
        MemoryContextSwitchTo(oldcontext);
 
        /*
         * Scan through all the buffers, saving the relevant fields in the
         * fctx->record structure.
         *
         * We don't hold the partition locks, so we don't get a consistent
         * snapshot across all buffers, but we do grab the buffer header
         * locks, so the information of each buffer is self-consistent.
         */
        for (i = 0; i < NBuffers; i++)
        {
            BufferDesc *bufHdr;
            uint32      buf_state;
 
            bufHdr = GetBufferDescriptor(i);
            /* Lock each buffer header before inspecting. */
            buf_state = LockBufHdr(bufHdr);
 
            fctx->record[i].bufferid = BufferDescriptorGetBuffer(bufHdr);
            fctx->record[i].relfilenumber = BufTagGetRelNumber(&bufHdr->tag);
            fctx->record[i].reltablespace = bufHdr->tag.spcOid;
            fctx->record[i].reldatabase = bufHdr->tag.dbOid;
            fctx->record[i].forknum = BufTagGetForkNum(&bufHdr->tag);
            fctx->record[i].blocknum = bufHdr->tag.blockNum;
            fctx->record[i].usagecount = BUF_STATE_GET_USAGECOUNT(buf_state);
            fctx->record[i].pinning_backends = BUF_STATE_GET_REFCOUNT(buf_state);
 
            if (buf_state & BM_DIRTY)
                fctx->record[i].isdirty = true;
            else
                fctx->record[i].isdirty = false;
 
            /* Note if the buffer is valid, and has storage created */
            if ((buf_state & BM_VALID) && (buf_state & BM_TAG_VALID))
                fctx->record[i].isvalid = true;
            else
                fctx->record[i].isvalid = false;
 
            UnlockBufHdr(bufHdr, buf_state);
        }
    }
 
    funcctx = SRF_PERCALL_SETUP();
 
    /* Get the saved state */
    fctx = funcctx->user_fctx;
 
    if (funcctx->call_cntr < funcctx->max_calls)
    {
        uint32      i = funcctx->call_cntr;
        Datum       values[NUM_BUFFERCACHE_PAGES_ELEM];
        bool        nulls[NUM_BUFFERCACHE_PAGES_ELEM];
 
        values[0] = Int32GetDatum(fctx->record[i].bufferid);
        nulls[0] = false;
 
        /*
         * Set all fields except the bufferid to null if the buffer is unused
         * or not valid.
         */
        if (fctx->record[i].blocknum == InvalidBlockNumber ||
            fctx->record[i].isvalid == false)
        {
            nulls[1] = true;
            nulls[2] = true;
            nulls[3] = true;
            nulls[4] = true;
            nulls[5] = true;
            nulls[6] = true;
            nulls[7] = true;
            /* unused for v1.0 callers, but the array is always long enough */
            nulls[8] = true;
        }
        else
        {
            values[1] = ObjectIdGetDatum(fctx->record[i].relfilenumber);
            nulls[1] = false;
            values[2] = ObjectIdGetDatum(fctx->record[i].reltablespace);
            nulls[2] = false;
            values[3] = ObjectIdGetDatum(fctx->record[i].reldatabase);
            nulls[3] = false;
            values[4] = ObjectIdGetDatum(fctx->record[i].forknum);
            nulls[4] = false;
            values[5] = Int64GetDatum((int64) fctx->record[i].blocknum);
            nulls[5] = false;
            values[6] = BoolGetDatum(fctx->record[i].isdirty);
            nulls[6] = false;
            values[7] = Int16GetDatum(fctx->record[i].usagecount);
            nulls[7] = false;
            /* unused for v1.0 callers, but the array is always long enough */
            values[8] = Int32GetDatum(fctx->record[i].pinning_backends);
            nulls[8] = false;
        }
 
        /* Build and return the tuple. */
        tuple = heap_form_tuple(fctx->tupdesc, values, nulls);
        result = HeapTupleGetDatum(tuple);
 
        SRF_RETURN_NEXT(funcctx, result);
    }
    else
        SRF_RETURN_DONE(funcctx);
}
 
Datum
pg_buffercache_summary(PG_FUNCTION_ARGS)
{
    Datum       result;
    TupleDesc   tupledesc;
    HeapTuple   tuple;
    Datum       values[NUM_BUFFERCACHE_SUMMARY_ELEM];
    bool        nulls[NUM_BUFFERCACHE_SUMMARY_ELEM];
 
    int32       buffers_used = 0;
    int32       buffers_unused = 0;
    int32       buffers_dirty = 0;
    int32       buffers_pinned = 0;
    int64       usagecount_total = 0;
 
    if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");
 
    for (int i = 0; i < NBuffers; i++)
    {
        BufferDesc *bufHdr;
        uint32      buf_state;
 
        /*
         * This function summarizes the state of all headers. Locking the
         * buffer headers wouldn't provide an improved result as the state of
         * the buffer can still change after we release the lock and it'd
         * noticeably increase the cost of the function.
         */
        bufHdr = GetBufferDescriptor(i);
        buf_state = pg_atomic_read_u32(&bufHdr->state);
 
        if (buf_state & BM_VALID)
        {
            buffers_used++;
            usagecount_total += BUF_STATE_GET_USAGECOUNT(buf_state);
 
            if (buf_state & BM_DIRTY)
                buffers_dirty++;
        }
        else
            buffers_unused++;
 
        if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
            buffers_pinned++;
    }
 
    memset(nulls, 0, sizeof(nulls));
    values[0] = Int32GetDatum(buffers_used);
    values[1] = Int32GetDatum(buffers_unused);
    values[2] = Int32GetDatum(buffers_dirty);
    values[3] = Int32GetDatum(buffers_pinned);
 
    if (buffers_used != 0)
        values[4] = Float8GetDatum((double) usagecount_total / buffers_used);
    else
        nulls[4] = true;
 
    /* Build and return the tuple. */
    tuple = heap_form_tuple(tupledesc, values, nulls);
    result = HeapTupleGetDatum(tuple);
 
    PG_RETURN_DATUM(result);
}