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9a20a9b21b
This patch replaces WALInsertLock with a number of WAL insertion slots, allowing multiple backends to insert WAL records to the WAL buffers concurrently. This is particularly useful for parallel loading large amounts of data on a system with many CPUs. This has one user-visible change: switching to a new WAL segment with pg_switch_xlog() now fills the remaining unused portion of the segment with zeros. This potentially adds some overhead, but it has been a very common practice by DBA's to clear the "tail" of the segment with an external pg_clearxlogtail utility anyway, to make the WAL files compress better. With this patch, it's no longer necessary to do that. This patch adds a new GUC, xloginsert_slots, to tune the number of WAL insertion slots. Performance testing suggests that the default, 8, works pretty well for all kinds of worklods, but I left the GUC in place to allow others with different hardware to test that easily. We might want to remove that before release. Reviewed by Andres Freund.
105 lines
2.4 KiB
C
105 lines
2.4 KiB
C
/*-------------------------------------------------------------------------
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*
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* spin.c
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* Hardware-independent implementation of spinlocks.
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*
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*
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* For machines that have test-and-set (TAS) instructions, s_lock.h/.c
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* define the spinlock implementation. This file contains only a stub
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* implementation for spinlocks using PGSemaphores. Unless semaphores
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* are implemented in a way that doesn't involve a kernel call, this
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* is too slow to be very useful :-(
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*
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*
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* Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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*
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* IDENTIFICATION
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* src/backend/storage/lmgr/spin.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include "access/xlog.h"
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#include "miscadmin.h"
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#include "replication/walsender.h"
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#include "storage/lwlock.h"
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#include "storage/spin.h"
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#ifdef HAVE_SPINLOCKS
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/*
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* Report number of semaphores needed to support spinlocks.
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*/
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int
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SpinlockSemas(void)
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{
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return 0;
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}
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#else /* !HAVE_SPINLOCKS */
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/*
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* No TAS, so spinlocks are implemented as PGSemaphores.
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*/
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/*
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* Report number of semaphores needed to support spinlocks.
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*/
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int
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SpinlockSemas(void)
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{
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int nsemas;
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/*
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* It would be cleaner to distribute this logic into the affected modules,
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* similar to the way shmem space estimation is handled.
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*
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* For now, though, there are few enough users of spinlocks that we just
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* keep the knowledge here.
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*/
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nsemas = NumLWLocks(); /* one for each lwlock */
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nsemas += NBuffers; /* one for each buffer header */
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nsemas += max_wal_senders; /* one for each wal sender process */
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nsemas += num_xloginsert_slots; /* one for each WAL insertion slot */
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nsemas += 30; /* plus a bunch for other small-scale use */
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return nsemas;
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}
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/*
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* s_lock.h hardware-spinlock emulation
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*/
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void
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s_init_lock_sema(volatile slock_t *lock)
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{
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PGSemaphoreCreate((PGSemaphore) lock);
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}
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void
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s_unlock_sema(volatile slock_t *lock)
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{
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PGSemaphoreUnlock((PGSemaphore) lock);
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}
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bool
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s_lock_free_sema(volatile slock_t *lock)
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{
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/* We don't currently use S_LOCK_FREE anyway */
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elog(ERROR, "spin.c does not support S_LOCK_FREE()");
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return false;
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}
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int
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tas_sema(volatile slock_t *lock)
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{
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/* Note that TAS macros return 0 if *success* */
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return !PGSemaphoreTryLock((PGSemaphore) lock);
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}
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#endif /* !HAVE_SPINLOCKS */
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