/*
* 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 = GetBufferDescriptor(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;
}
/*
* 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,
foundIOLocks,
foundBufCkpt;
/* Align descriptors to a cacheline boundary. */
BufferDescriptors = (BufferDescPadded *)
ShmemInitStruct("Buffer Descriptors",
NBuffers * sizeof(BufferDescPadded),
&foundDescs);
BufferBlocks = (char *)
ShmemInitStruct("Buffer Blocks",
NBuffers * (Size) BLCKSZ, &foundBufs);
/* Align lwlocks to cacheline boundary */
BufferIOLWLockArray = (LWLockMinimallyPadded *)
ShmemInitStruct("Buffer IO Locks",
NBuffers * (Size) sizeof(LWLockMinimallyPadded),
&foundIOLocks);
LWLockRegisterTranche(LWTRANCHE_BUFFER_IO_IN_PROGRESS, "buffer_io");
LWLockRegisterTranche(LWTRANCHE_BUFFER_CONTENT, "buffer_content");
/*
* 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 || foundIOLocks || foundBufCkpt)
{
/* should find all of these, or none of them */
Assert(foundDescs && foundBufs && foundIOLocks && 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);
CLEAR_BUFFERTAG(buf->tag);
pg_atomic_init_u32(&buf->state, 0);
buf->wait_backend_pid = 0;
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;
LWLockInitialize(BufferDescriptorGetContentLock(buf),
LWTRANCHE_BUFFER_CONTENT);
LWLockInitialize(BufferDescriptorGetIOLock(buf),
LWTRANCHE_BUFFER_IO_IN_PROGRESS);
}
/* 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);
}
/*
* 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,
foundIOLocks;
/* Align descriptors to a cacheline boundary. */
BufferDescriptors = (BufferDescPadded *)
CACHELINEALIGN(
ShmemInitStruct("Buffer Descriptors",
NBuffers * sizeof(BufferDescPadded)
+ PG_CACHE_LINE_SIZE,
&foundDescs));
BufferBlocks = (char *)
ShmemInitStruct("Buffer Blocks",
NBuffers * (Size) BLCKSZ, &foundBufs);
/* Align lwlocks to cacheline boundary */
BufferIOLWLockArray = (LWLockMinimallyPadded *)
CACHELINEALIGN(ShmemInitStruct("Buffer IO Locks",
NBuffers * (Size) sizeof(LWLockMinimallyPadded)
+ PG_CACHE_LINE_SIZE,
&foundIOLocks));
BufferIOLWLockTranche.name = "buffer_io";
BufferIOLWLockTranche.array_base = BufferIOLWLockArray;
BufferIOLWLockTranche.array_stride = sizeof(LWLockMinimallyPadded);
LWLockRegisterTranche(LWTRANCHE_BUFFER_IO_IN_PROGRESS,
&BufferIOLWLockTranche);
BufferContentLWLockTranche.name = "buffer_content";
BufferContentLWLockTranche.array_base =
((char *) BufferDescriptors) + offsetof(BufferDesc, content_lock);
BufferContentLWLockTranche.array_stride = sizeof(BufferDescPadded);
LWLockRegisterTranche(LWTRANCHE_BUFFER_CONTENT,
&BufferContentLWLockTranche);
if (foundDescs || foundBufs || foundIOLocks)
{
/* should find all of these, or none of them */
Assert(foundDescs && foundBufs && foundIOLocks);
/* 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);
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;
LWLockInitialize(BufferDescriptorGetContentLock(buf),
LWTRANCHE_BUFFER_CONTENT);
LWLockInitialize(BufferDescriptorGetIOLock(buf),
LWTRANCHE_BUFFER_IO_IN_PROGRESS);
}
/* Correct last entry of linked list */
GetBufferDescriptor(NBuffers - 1)->freeNext = FREENEXT_END_OF_LIST;
}
/* Init other shared buffer-management stuff */
StrategyInitialize(!foundDescs);
}
/*
* 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.
*/
volatile BufferDesc *
StrategyGetBuffer(BufferAccessStrategy strategy)
{
volatile BufferDesc *buf;
int bgwprocno;
int trycounter;
/*
* If given a strategy object, see whether it can select a buffer. We
* assume strategy objects don't need buffer_strategy_lock.
*/
if (strategy != NULL)
{
buf = GetBufferFromRing(strategy);
if (buf != NULL)
return buf;
}
/*
* If asked, we need to waken the bgwriter. Since we don't want to rely on
* a spinlock for this we force a read from shared memory once, and then
* set the latch based on that value. We need to go through that length
* because otherwise bgprocno might be reset while/after we check because
* the compiler might just reread from memory.
*
* This can possibly set the latch of the wrong process if the bgwriter
* dies in the wrong moment. But since PGPROC->procLatch is never
* deallocated the worst consequence of that is that we set the latch of
* some arbitrary process.
*/
bgwprocno = INT_ACCESS_ONCE(StrategyControl->bgwprocno);
if (bgwprocno != -1)
{
/* reset bgwprocno first, before setting the latch */
StrategyControl->bgwprocno = -1;
/*
* Not acquiring ProcArrayLock here which is slightly icky. It's
* actually fine because procLatch isn't ever freed, so we just can
* potentially set the wrong process' (or no process') latch.
*/
SetLatch(&ProcGlobal->allProcs[bgwprocno].procLatch);
}
/*
* 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.
*/
pg_atomic_fetch_add_u32(&StrategyControl->numBufferAllocs, 1);
/*
* First check, without acquiring the lock, whether there's buffers in the
* freelist. Since we otherwise don't require the spinlock in every
* StrategyGetBuffer() invocation, it'd be sad to acquire it here -
* uselessly in most cases. That obviously leaves a race where a buffer is
* put on the freelist but we don't see the store yet - but that's pretty
* harmless, it'll just get used during the next buffer acquisition.
*
* If there's buffers on the freelist, acquire the spinlock to pop one
* buffer of the freelist. Then check whether that buffer is usable and
* repeat if not.
*
* Note that the freeNext fields are considered to be protected by the
* buffer_strategy_lock not the individual buffer spinlocks, so it's OK to
* manipulate them without holding the spinlock.
*/
if (StrategyControl->firstFreeBuffer >= 0)
{
while (true)
{
/* Acquire the spinlock to remove element from the freelist */
SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
if (StrategyControl->firstFreeBuffer < 0)
{
SpinLockRelease(&StrategyControl->buffer_strategy_lock);
break;
}
buf = GetBufferDescriptor(StrategyControl->firstFreeBuffer);
Assert(buf->freeNext != FREENEXT_NOT_IN_LIST);
/* Unconditionally remove buffer from freelist */
StrategyControl->firstFreeBuffer = buf->freeNext;
buf->freeNext = FREENEXT_NOT_IN_LIST;
/*
* Release the lock so someone else can access the freelist while
* we check out this buffer.
*/
SpinLockRelease(&StrategyControl->buffer_strategy_lock);
/*
* 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 = GetBufferDescriptor(ClockSweepTick());
/*
* 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);
}
}
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, false);
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].relfilenode = bufHdr->tag.rnode.relNode;
fctx->record[i].reltablespace = bufHdr->tag.rnode.spcNode;
fctx->record[i].reldatabase = bufHdr->tag.rnode.dbNode;
fctx->record[i].forknum = bufHdr->tag.forkNum;
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].relfilenode);
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);
}
