diff options
Diffstat (limited to 'src/backend/storage/buffer')
| -rw-r--r-- | src/backend/storage/buffer/README | 44 | ||||
| -rw-r--r-- | src/backend/storage/buffer/buf_init.c | 9 | ||||
| -rw-r--r-- | src/backend/storage/buffer/bufmgr.c | 35 | ||||
| -rw-r--r-- | src/backend/storage/buffer/freelist.c | 127 | ||||
| -rw-r--r-- | src/backend/storage/buffer/localbuf.c | 2 |
5 files changed, 26 insertions, 191 deletions
diff --git a/src/backend/storage/buffer/README b/src/backend/storage/buffer/README index a182fcd660c..119f31b5d65 100644 --- a/src/backend/storage/buffer/README +++ b/src/backend/storage/buffer/README @@ -128,11 +128,11 @@ independently. If it is necessary to lock more than one partition at a time, they must be locked in partition-number order to avoid risk of deadlock. * A separate system-wide spinlock, buffer_strategy_lock, provides mutual -exclusion for operations that access the buffer free list or select -buffers for replacement. A spinlock is used here rather than a lightweight -lock for efficiency; no other locks of any sort should be acquired while -buffer_strategy_lock is held. This is essential to allow buffer replacement -to happen in multiple backends with reasonable concurrency. +exclusion for operations that select buffers for replacement. A spinlock is +used here rather than a lightweight lock for efficiency; no other locks of any +sort should be acquired while buffer_strategy_lock is held. This is essential +to allow buffer replacement to happen in multiple backends with reasonable +concurrency. * Each buffer header contains a spinlock that must be taken when examining or changing fields of that buffer header. This allows operations such as @@ -158,18 +158,8 @@ unset by sleeping on the buffer's condition variable. Normal Buffer Replacement Strategy ---------------------------------- -There is a "free list" of buffers that are prime candidates for replacement. -In particular, buffers that are completely free (contain no valid page) are -always in this list. We could also throw buffers into this list if we -consider their pages unlikely to be needed soon; however, the current -algorithm never does that. The list is singly-linked using fields in the -buffer headers; we maintain head and tail pointers in global variables. -(Note: although the list links are in the buffer headers, they are -considered to be protected by the buffer_strategy_lock, not the buffer-header -spinlocks.) To choose a victim buffer to recycle when there are no free -buffers available, we use a simple clock-sweep algorithm, which avoids the -need to take system-wide locks during common operations. It works like -this: +To choose a victim buffer to recycle we use a simple clock-sweep algorithm. It +works like this: Each buffer header contains a usage counter, which is incremented (up to a small limit value) whenever the buffer is pinned. (This requires only the @@ -184,20 +174,14 @@ The algorithm for a process that needs to obtain a victim buffer is: 1. Obtain buffer_strategy_lock. -2. If buffer free list is nonempty, remove its head buffer. Release -buffer_strategy_lock. If the buffer is pinned or has a nonzero usage count, -it cannot be used; ignore it go back to step 1. Otherwise, pin the buffer, -and return it. +2. Select the buffer pointed to by nextVictimBuffer, and circularly advance +nextVictimBuffer for next time. Release buffer_strategy_lock. -3. Otherwise, the buffer free list is empty. Select the buffer pointed to by -nextVictimBuffer, and circularly advance nextVictimBuffer for next time. -Release buffer_strategy_lock. - -4. If the selected buffer is pinned or has a nonzero usage count, it cannot +3. If the selected buffer is pinned or has a nonzero usage count, it cannot be used. Decrement its usage count (if nonzero), reacquire buffer_strategy_lock, and return to step 3 to examine the next buffer. -5. Pin the selected buffer, and return. +4. Pin the selected buffer, and return. (Note that if the selected buffer is dirty, we will have to write it out before we can recycle it; if someone else pins the buffer meanwhile we will @@ -211,9 +195,9 @@ Buffer Ring Replacement Strategy When running a query that needs to access a large number of pages just once, such as VACUUM or a large sequential scan, a different strategy is used. A page that has been touched only by such a scan is unlikely to be needed -again soon, so instead of running the normal clock sweep algorithm and +again soon, so instead of running the normal clock-sweep algorithm and blowing out the entire buffer cache, a small ring of buffers is allocated -using the normal clock sweep algorithm and those buffers are reused for the +using the normal clock-sweep algorithm and those buffers are reused for the whole scan. This also implies that much of the write traffic caused by such a statement will be done by the backend itself and not pushed off onto other processes. @@ -234,7 +218,7 @@ the ring strategy effectively degrades to the normal strategy. VACUUM uses a ring like sequential scans, however, the size of this ring is controlled by the vacuum_buffer_usage_limit GUC. Dirty pages are not removed -from the ring. Instead, WAL is flushed if needed to allow reuse of the +from the ring. Instead, the WAL is flushed if needed to allow reuse of the buffers. Before introducing the buffer ring strategy in 8.3, VACUUM's buffers were sent to the freelist, which was effectively a buffer ring of 1 buffer, resulting in excessive WAL flushing. diff --git a/src/backend/storage/buffer/buf_init.c b/src/backend/storage/buffer/buf_init.c index ed1dc488a42..6fd3a6bbac5 100644 --- a/src/backend/storage/buffer/buf_init.c +++ b/src/backend/storage/buffer/buf_init.c @@ -128,20 +128,11 @@ BufferManagerShmemInit(void) pgaio_wref_clear(&buf->io_wref); - /* - * 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); ConditionVariableInit(BufferDescriptorGetIOCV(buf)); } - - /* Correct last entry of linked list */ - GetBufferDescriptor(NBuffers - 1)->freeNext = FREENEXT_END_OF_LIST; } /* Init other shared buffer-management stuff */ diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index 350cc0402aa..fe470de63f2 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -2094,12 +2094,6 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum, */ UnpinBuffer(victim_buf_hdr); - /* - * The victim buffer we acquired previously is clean and unused, let - * it be found again quickly - */ - StrategyFreeBuffer(victim_buf_hdr); - /* remaining code should match code at top of routine */ existing_buf_hdr = GetBufferDescriptor(existing_buf_id); @@ -2158,8 +2152,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum, } /* - * InvalidateBuffer -- mark a shared buffer invalid and return it to the - * freelist. + * InvalidateBuffer -- mark a shared buffer invalid. * * The buffer header spinlock must be held at entry. We drop it before * returning. (This is sane because the caller must have locked the @@ -2257,11 +2250,6 @@ retry: * Done with mapping lock. */ LWLockRelease(oldPartitionLock); - - /* - * Insert the buffer at the head of the list of free buffers. - */ - StrategyFreeBuffer(buf); } /* @@ -2679,11 +2667,6 @@ ExtendBufferedRelShared(BufferManagerRelation bmr, { BufferDesc *buf_hdr = GetBufferDescriptor(buffers[i] - 1); - /* - * The victim buffer we acquired previously is clean and unused, - * let it be found again quickly - */ - StrategyFreeBuffer(buf_hdr); UnpinBuffer(buf_hdr); } @@ -2756,12 +2739,6 @@ ExtendBufferedRelShared(BufferManagerRelation bmr, valid = PinBuffer(existing_hdr, strategy); LWLockRelease(partition_lock); - - /* - * The victim buffer we acquired previously is clean and unused, - * let it be found again quickly - */ - StrategyFreeBuffer(victim_buf_hdr); UnpinBuffer(victim_buf_hdr); buffers[i] = BufferDescriptorGetBuffer(existing_hdr); @@ -3608,7 +3585,7 @@ BufferSync(int flags) * This is called periodically by the background writer process. * * Returns true if it's appropriate for the bgwriter process to go into - * low-power hibernation mode. (This happens if the strategy clock sweep + * low-power hibernation mode. (This happens if the strategy clock-sweep * has been "lapped" and no buffer allocations have occurred recently, * or if the bgwriter has been effectively disabled by setting * bgwriter_lru_maxpages to 0.) @@ -3658,8 +3635,8 @@ BgBufferSync(WritebackContext *wb_context) uint32 new_recent_alloc; /* - * Find out where the freelist clock sweep currently is, and how many - * buffer allocations have happened since our last call. + * Find out where the clock-sweep currently is, and how many buffer + * allocations have happened since our last call. */ strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc); @@ -3679,8 +3656,8 @@ BgBufferSync(WritebackContext *wb_context) /* * Compute strategy_delta = how many buffers have been scanned by the - * clock sweep since last time. If first time through, assume none. Then - * see if we are still ahead of the clock sweep, and if so, how many + * clock-sweep since last time. If first time through, assume none. Then + * see if we are still ahead of the clock-sweep, and if so, how many * buffers we could scan before we'd catch up with it and "lap" it. Note: * weird-looking coding of xxx_passes comparisons are to avoid bogus * behavior when the passes counts wrap around. diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c index 01909be0272..7d59a92bd1a 100644 --- a/src/backend/storage/buffer/freelist.c +++ b/src/backend/storage/buffer/freelist.c @@ -33,25 +33,17 @@ typedef struct slock_t buffer_strategy_lock; /* - * Clock sweep hand: index of next buffer to consider grabbing. Note that + * clock-sweep hand: index of next buffer to consider grabbing. Note that * this isn't a concrete buffer - we only ever increase the value. So, to * get an actual buffer, it needs to be used modulo NBuffers. */ pg_atomic_uint32 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 completePasses; /* Complete cycles of the clock-sweep */ pg_atomic_uint32 numBufferAllocs; /* Buffers allocated since last reset */ /* @@ -164,23 +156,6 @@ ClockSweepTick(void) } /* - * have_free_buffer -- a lockless check to see if there is a free buffer in - * buffer pool. - * - * If the result is true that will become stale once free buffers are moved out - * by other operations, so the caller who strictly want to use a free buffer - * should not call this. - */ -bool -have_free_buffer(void) -{ - if (StrategyControl->firstFreeBuffer >= 0) - return true; - else - return false; -} - -/* * StrategyGetBuffer * * Called by the bufmgr to get the next candidate buffer to use in @@ -249,69 +224,7 @@ StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state, bool *from_r */ 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.) - */ - local_buf_state = LockBufHdr(buf); - if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0 - && BUF_STATE_GET_USAGECOUNT(local_buf_state) == 0) - { - if (strategy != NULL) - AddBufferToRing(strategy, buf); - *buf_state = local_buf_state; - return buf; - } - UnlockBufHdr(buf, local_buf_state); - } - } - - /* Nothing on the freelist, so run the "clock sweep" algorithm */ + /* Use the "clock sweep" algorithm to find a free buffer */ trycounter = NBuffers; for (;;) { @@ -357,29 +270,6 @@ StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state, bool *from_r } /* - * StrategyFreeBuffer: put a buffer on the freelist - */ -void -StrategyFreeBuffer(BufferDesc *buf) -{ - SpinLockAcquire(&StrategyControl->buffer_strategy_lock); - - /* - * 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; - } - - SpinLockRelease(&StrategyControl->buffer_strategy_lock); -} - -/* * StrategySyncStart -- tell BgBufferSync where to start syncing * * The result is the buffer index of the best buffer to sync first. @@ -504,14 +394,7 @@ StrategyInitialize(bool init) SpinLockInit(&StrategyControl->buffer_strategy_lock); - /* - * Grab the whole linked list of free buffers for our strategy. We - * assume it was previously set up by BufferManagerShmemInit(). - */ - StrategyControl->firstFreeBuffer = 0; - StrategyControl->lastFreeBuffer = NBuffers - 1; - - /* Initialize the clock sweep pointer */ + /* Initialize the clock-sweep pointer */ pg_atomic_init_u32(&StrategyControl->nextVictimBuffer, 0); /* Clear statistics */ @@ -759,7 +642,7 @@ GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state) * * 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 + * 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. */ diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c index 3c0d20f4659..04fef13409b 100644 --- a/src/backend/storage/buffer/localbuf.c +++ b/src/backend/storage/buffer/localbuf.c @@ -229,7 +229,7 @@ GetLocalVictimBuffer(void) ResourceOwnerEnlarge(CurrentResourceOwner); /* - * Need to get a new buffer. We use a clock sweep algorithm (essentially + * Need to get a new buffer. We use a clock-sweep algorithm (essentially * the same as what freelist.c does now...) */ trycounter = NLocBuffer; |