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 "access/relation.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "port/pg_numa.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "utils/rel.h"
 
 
#define NUM_BUFFERCACHE_PAGES_MIN_ELEM  8
#define NUM_BUFFERCACHE_PAGES_ELEM  9
#define NUM_BUFFERCACHE_SUMMARY_ELEM 5
#define NUM_BUFFERCACHE_USAGE_COUNTS_ELEM 4
#define NUM_BUFFERCACHE_EVICT_ELEM 2
#define NUM_BUFFERCACHE_EVICT_RELATION_ELEM 3
#define NUM_BUFFERCACHE_EVICT_ALL_ELEM 3
 
#define NUM_BUFFERCACHE_OS_PAGES_ELEM   3
 
PG_MODULE_MAGIC_EXT(
                    .name = "pg_buffercache",
                    .version = PG_VERSION
);
 
/*
 * 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;
 
/*
 * Record structure holding the to be exposed cache data for OS pages.  This
 * structure is used by pg_buffercache_os_pages(), where NUMA information may
 * or may not be included.
 */
typedef struct
{
    uint32      bufferid;
    int64       page_num;
    int32       numa_node;
} BufferCacheOsPagesRec;
 
/*
 * Function context for data persisting over repeated calls.
 */
typedef struct
{
    TupleDesc   tupdesc;
    bool        include_numa;
    BufferCacheOsPagesRec *record;
} BufferCacheOsPagesContext;
 
 
/*
 * 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_os_pages);
PG_FUNCTION_INFO_V1(pg_buffercache_numa_pages);
PG_FUNCTION_INFO_V1(pg_buffercache_summary);
PG_FUNCTION_INFO_V1(pg_buffercache_usage_counts);
PG_FUNCTION_INFO_V1(pg_buffercache_evict);
PG_FUNCTION_INFO_V1(pg_buffercache_evict_relation);
PG_FUNCTION_INFO_V1(pg_buffercache_evict_all);
 
 
/* Only need to touch memory once per backend process lifetime */
static bool firstNumaTouch = true;
 
 
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;
 
            CHECK_FOR_INTERRUPTS();
 
            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);
        }
    }
 
    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] = Int16GetDatum(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);
}
 
/*
 * Inquire about OS pages mappings for shared buffers, with NUMA information,
 * optionally.
 *
 * When "include_numa" is false, this routines ignores everything related
 * to NUMA (returned as NULL values), returning mapping information between
 * shared buffers and OS pages.
 *
 * When "include_numa" is true, NUMA is initialized and numa_node values
 * are generated.  In order to get reliable results we also need to touch
 * memory pages, so that the inquiry about NUMA memory node does not return
 * -2, indicating unmapped/unallocated pages.
 *
 * Buffers may be smaller or larger than OS memory pages. For each buffer we
 * return one entry for each memory page used by the buffer (if the buffer is
 * smaller, it only uses a part of one memory page).
 *
 * We expect both sizes (for buffers and memory pages) to be a power-of-2, so
 * one is always a multiple of the other.
 *
 */
static Datum
pg_buffercache_os_pages_internal(FunctionCallInfo fcinfo, bool include_numa)
{
    FuncCallContext *funcctx;
    MemoryContext oldcontext;
    BufferCacheOsPagesContext *fctx;    /* User function context. */
    TupleDesc   tupledesc;
    TupleDesc   expected_tupledesc;
    HeapTuple   tuple;
    Datum       result;
 
    if (SRF_IS_FIRSTCALL())
    {
        int         i,
                    idx;
        Size        os_page_size;
        int         pages_per_buffer;
        int        *os_page_status = NULL;
        uint64      os_page_count = 0;
        int         max_entries;
        char       *startptr,
                   *endptr;
 
        /* If NUMA information is requested, initialize NUMA support. */
        if (include_numa && pg_numa_init() == -1)
            elog(ERROR, "libnuma initialization failed or NUMA is not supported on this platform");
 
        /*
         * The database block size and OS memory page size are unlikely to be
         * the same. The block size is 1-32KB, the memory page size depends on
         * platform. On x86 it's usually 4KB, on ARM it's 4KB or 64KB, but
         * there are also features like THP etc. Moreover, we don't quite know
         * how the pages and buffers "align" in memory - the buffers may be
         * shifted in some way, using more memory pages than necessary.
         *
         * So we need to be careful about mapping buffers to memory pages. We
         * calculate the maximum number of pages a buffer might use, so that
         * we allocate enough space for the entries. And then we count the
         * actual number of entries as we scan the buffers.
         *
         * This information is needed before calling move_pages() for NUMA
         * node id inquiry.
         */
        os_page_size = pg_get_shmem_pagesize();
 
        /*
         * The pages and block size is expected to be 2^k, so one divides the
         * other (we don't know in which direction). This does not say
         * anything about relative alignment of pages/buffers.
         */
        Assert((os_page_size % BLCKSZ == 0) || (BLCKSZ % os_page_size == 0));
 
        if (include_numa)
        {
            void      **os_page_ptrs = NULL;
 
            /*
             * How many addresses we are going to query?  Simply get the page
             * for the first buffer, and first page after the last buffer, and
             * count the pages from that.
             */
            startptr = (char *) TYPEALIGN_DOWN(os_page_size,
                                               BufferGetBlock(1));
            endptr = (char *) TYPEALIGN(os_page_size,
                                        (char *) BufferGetBlock(NBuffers) + BLCKSZ);
            os_page_count = (endptr - startptr) / os_page_size;
 
            /* Used to determine the NUMA node for all OS pages at once */
            os_page_ptrs = palloc0(sizeof(void *) * os_page_count);
            os_page_status = palloc(sizeof(uint64) * os_page_count);
 
            /*
             * Fill pointers for all the memory pages.  This loop stores and
             * touches (if needed) addresses into os_page_ptrs[] as input to
             * one big move_pages(2) inquiry system call, as done in
             * pg_numa_query_pages().
             */
            idx = 0;
            for (char *ptr = startptr; ptr < endptr; ptr += os_page_size)
            {
                os_page_ptrs[idx++] = ptr;
 
                /* Only need to touch memory once per backend process lifetime */
                if (firstNumaTouch)
                    pg_numa_touch_mem_if_required(ptr);
            }
 
            Assert(idx == os_page_count);
 
            elog(DEBUG1, "NUMA: NBuffers=%d os_page_count=" UINT64_FORMAT " "
                 "os_page_size=%zu", NBuffers, os_page_count, os_page_size);
 
            /*
             * If we ever get 0xff back from kernel inquiry, then we probably
             * have bug in our buffers to OS page mapping code here.
             */
            memset(os_page_status, 0xff, sizeof(int) * os_page_count);
 
            /* Query NUMA status for all the pointers */
            if (pg_numa_query_pages(0, os_page_count, os_page_ptrs, os_page_status) == -1)
                elog(ERROR, "failed NUMA pages inquiry: %m");
        }
 
        /* Initialize the multi-call context, load entries about buffers */
 
        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 = (BufferCacheOsPagesContext *) palloc(sizeof(BufferCacheOsPagesContext));
 
        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_OS_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, "os_page_num",
                           INT8OID, -1, 0);
        TupleDescInitEntry(tupledesc, (AttrNumber) 3, "numa_node",
                           INT4OID, -1, 0);
 
        fctx->tupdesc = BlessTupleDesc(tupledesc);
        fctx->include_numa = include_numa;
 
        /*
         * Each buffer needs at least one entry, but it might be offset in
         * some way, and use one extra entry. So we allocate space for the
         * maximum number of entries we might need, and then count the exact
         * number as we're walking buffers. That way we can do it in one pass,
         * without reallocating memory.
         */
        pages_per_buffer = Max(1, BLCKSZ / os_page_size) + 1;
        max_entries = NBuffers * pages_per_buffer;
 
        /* Allocate entries for BufferCacheOsPagesRec records. */
        fctx->record = (BufferCacheOsPagesRec *)
            MemoryContextAllocHuge(CurrentMemoryContext,
                                   sizeof(BufferCacheOsPagesRec) * max_entries);
 
        /* Return to original context when allocating transient memory */
        MemoryContextSwitchTo(oldcontext);
 
        if (include_numa && firstNumaTouch)
            elog(DEBUG1, "NUMA: page-faulting the buffercache for proper NUMA readouts");
 
        /*
         * 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.
         */
        startptr = (char *) TYPEALIGN_DOWN(os_page_size, (char *) BufferGetBlock(1));
        idx = 0;
        for (i = 0; i < NBuffers; i++)
        {
            char       *buffptr = (char *) BufferGetBlock(i + 1);
            BufferDesc *bufHdr;
            uint32      bufferid;
            int32       page_num;
            char       *startptr_buff,
                       *endptr_buff;
 
            CHECK_FOR_INTERRUPTS();
 
            bufHdr = GetBufferDescriptor(i);
 
            /* Lock each buffer header before inspecting. */
            LockBufHdr(bufHdr);
            bufferid = BufferDescriptorGetBuffer(bufHdr);
            UnlockBufHdr(bufHdr);
 
            /* start of the first page of this buffer */
            startptr_buff = (char *) TYPEALIGN_DOWN(os_page_size, buffptr);
 
            /* end of the buffer (no need to align to memory page) */
            endptr_buff = buffptr + BLCKSZ;
 
            Assert(startptr_buff < endptr_buff);
 
            /* calculate ID of the first page for this buffer */
            page_num = (startptr_buff - startptr) / os_page_size;
 
            /* Add an entry for each OS page overlapping with this buffer. */
            for (char *ptr = startptr_buff; ptr < endptr_buff; ptr += os_page_size)
            {
                fctx->record[idx].bufferid = bufferid;
                fctx->record[idx].page_num = page_num;
                fctx->record[idx].numa_node = include_numa ? os_page_status[page_num] : -1;
 
                /* advance to the next entry/page */
                ++idx;
                ++page_num;
            }
        }
 
        Assert(idx <= max_entries);
 
        if (include_numa)
            Assert(idx >= os_page_count);
 
        /* Set max calls and remember the user function context. */
        funcctx->max_calls = idx;
        funcctx->user_fctx = fctx;
 
        /* Remember this backend touched the pages (only relevant for NUMA) */
        if (include_numa)
            firstNumaTouch = false;
    }
 
    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_OS_PAGES_ELEM];
        bool        nulls[NUM_BUFFERCACHE_OS_PAGES_ELEM];
 
        values[0] = Int32GetDatum(fctx->record[i].bufferid);
        nulls[0] = false;
 
        values[1] = Int64GetDatum(fctx->record[i].page_num);
        nulls[1] = false;
 
        if (fctx->include_numa)
        {
            values[2] = Int32GetDatum(fctx->record[i].numa_node);
            nulls[2] = false;
        }
        else
        {
            values[2] = (Datum) 0;
            nulls[2] = true;
        }
 
        /* 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);
}
 
/*
 * pg_buffercache_os_pages
 *
 * Retrieve information about OS pages, with or without NUMA information.
 */
Datum
pg_buffercache_os_pages(PG_FUNCTION_ARGS)
{
    bool        include_numa;
 
    /* Get the boolean parameter that controls the NUMA behavior. */
    include_numa = PG_GETARG_BOOL(0);
 
    return pg_buffercache_os_pages_internal(fcinfo, include_numa);
}
 
/* Backward-compatible wrapper for v1.6. */
Datum
pg_buffercache_numa_pages(PG_FUNCTION_ARGS)
{
    /* Call internal function with include_numa=true */
    return pg_buffercache_os_pages_internal(fcinfo, true);
}
 
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;
 
        CHECK_FOR_INTERRUPTS();
 
        /*
         * 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);
}
 
Datum
pg_buffercache_usage_counts(PG_FUNCTION_ARGS)
{
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    int         usage_counts[BM_MAX_USAGE_COUNT + 1] = {0};
    int         dirty[BM_MAX_USAGE_COUNT + 1] = {0};
    int         pinned[BM_MAX_USAGE_COUNT + 1] = {0};
    Datum       values[NUM_BUFFERCACHE_USAGE_COUNTS_ELEM];
    bool        nulls[NUM_BUFFERCACHE_USAGE_COUNTS_ELEM] = {0};
 
    InitMaterializedSRF(fcinfo, 0);
 
    for (int i = 0; i < NBuffers; i++)
    {
        BufferDesc *bufHdr = GetBufferDescriptor(i);
        uint32      buf_state = pg_atomic_read_u32(&bufHdr->state);
        int         usage_count;
 
        CHECK_FOR_INTERRUPTS();
 
        usage_count = BUF_STATE_GET_USAGECOUNT(buf_state);
        usage_counts[usage_count]++;
 
        if (buf_state & BM_DIRTY)
            dirty[usage_count]++;
 
        if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
            pinned[usage_count]++;
    }
 
    for (int i = 0; i < BM_MAX_USAGE_COUNT + 1; i++)
    {
        values[0] = Int32GetDatum(i);
        values[1] = Int32GetDatum(usage_counts[i]);
        values[2] = Int32GetDatum(dirty[i]);
        values[3] = Int32GetDatum(pinned[i]);
 
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
    }
 
    return (Datum) 0;
}
 
/*
 * Helper function to check if the user has superuser privileges.
 */
static void
pg_buffercache_superuser_check(char *func_name)
{
    if (!superuser())
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                 errmsg("must be superuser to use %s()",
                        func_name)));
}
 
/*
 * Try to evict a shared buffer.
 */
Datum
pg_buffercache_evict(PG_FUNCTION_ARGS)
{
    Datum       result;
    TupleDesc   tupledesc;
    HeapTuple   tuple;
    Datum       values[NUM_BUFFERCACHE_EVICT_ELEM];
    bool        nulls[NUM_BUFFERCACHE_EVICT_ELEM] = {0};
 
    Buffer      buf = PG_GETARG_INT32(0);
    bool        buffer_flushed;
 
    if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");
 
    pg_buffercache_superuser_check("pg_buffercache_evict");
 
    if (buf < 1 || buf > NBuffers)
        elog(ERROR, "bad buffer ID: %d", buf);
 
    values[0] = BoolGetDatum(EvictUnpinnedBuffer(buf, &buffer_flushed));
    values[1] = BoolGetDatum(buffer_flushed);
 
    tuple = heap_form_tuple(tupledesc, values, nulls);
    result = HeapTupleGetDatum(tuple);
 
    PG_RETURN_DATUM(result);
}
 
/*
 * Try to evict specified relation.
 */
Datum
pg_buffercache_evict_relation(PG_FUNCTION_ARGS)
{
    Datum       result;
    TupleDesc   tupledesc;
    HeapTuple   tuple;
    Datum       values[NUM_BUFFERCACHE_EVICT_RELATION_ELEM];
    bool        nulls[NUM_BUFFERCACHE_EVICT_RELATION_ELEM] = {0};
 
    Oid         relOid;
    Relation    rel;
 
    int32       buffers_evicted = 0;
    int32       buffers_flushed = 0;
    int32       buffers_skipped = 0;
 
    if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");
 
    pg_buffercache_superuser_check("pg_buffercache_evict_relation");
 
    relOid = PG_GETARG_OID(0);
 
    rel = relation_open(relOid, AccessShareLock);
 
    if (RelationUsesLocalBuffers(rel))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("relation uses local buffers, %s() is intended to be used for shared buffers only",
                        "pg_buffercache_evict_relation")));
 
    EvictRelUnpinnedBuffers(rel, &buffers_evicted, &buffers_flushed,
                            &buffers_skipped);
 
    relation_close(rel, AccessShareLock);
 
    values[0] = Int32GetDatum(buffers_evicted);
    values[1] = Int32GetDatum(buffers_flushed);
    values[2] = Int32GetDatum(buffers_skipped);
 
    tuple = heap_form_tuple(tupledesc, values, nulls);
    result = HeapTupleGetDatum(tuple);
 
    PG_RETURN_DATUM(result);
}
 
 
/*
 * Try to evict all shared buffers.
 */
Datum
pg_buffercache_evict_all(PG_FUNCTION_ARGS)
{
    Datum       result;
    TupleDesc   tupledesc;
    HeapTuple   tuple;
    Datum       values[NUM_BUFFERCACHE_EVICT_ALL_ELEM];
    bool        nulls[NUM_BUFFERCACHE_EVICT_ALL_ELEM] = {0};
 
    int32       buffers_evicted = 0;
    int32       buffers_flushed = 0;
    int32       buffers_skipped = 0;
 
    if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");
 
    pg_buffercache_superuser_check("pg_buffercache_evict_all");
 
    EvictAllUnpinnedBuffers(&buffers_evicted, &buffers_flushed,
                            &buffers_skipped);
 
    values[0] = Int32GetDatum(buffers_evicted);
    values[1] = Int32GetDatum(buffers_flushed);
    values[2] = Int32GetDatum(buffers_skipped);
 
    tuple = heap_form_tuple(tupledesc, values, nulls);
    result = HeapTupleGetDatum(tuple);
 
    PG_RETURN_DATUM(result);
}