#include "access/clog.h"
#include "access/slru.h"
#include "access/transam.h"
+#include "miscadmin.h"
#include "pg_trace.h"
/*
return status;
}
+/*
+ * Number of shared CLOG buffers.
+ *
+ * Testing during the PostgreSQL 9.2 development cycle revealed that on a
+ * large multi-processor system, it was possible to have more CLOG page
+ * requests in flight at one time than the numebr of CLOG buffers which existed
+ * at that time, which was hardcoded to 8. Further testing revealed that
+ * performance dropped off with more than 32 CLOG buffers, possibly because
+ * the linear buffer search algorithm doesn't scale well.
+ *
+ * Unconditionally increasing the number of CLOG buffers to 32 did not seem
+ * like a good idea, because it would increase the minimum amount of shared
+ * memory required to start, which could be a problem for people running very
+ * small configurations. The following formula seems to represent a reasonable
+ * compromise: people with very low values for shared_buffers will get fewer
+ * CLOG buffers as well, and everyone else will get 32.
+ *
+ * It is likely that some further work will be needed here in future releases;
+ * for example, on a 64-core server, the maximum number of CLOG requests that
+ * can be simultaneously in flight will be even larger. But that will
+ * apparently require more than just changing the formula, so for now we take
+ * the easy way out.
+ */
+Size
+CLOGShmemBuffers(void)
+{
+ return Min(32, Max(4, NBuffers / 512));
+}
/*
* Initialization of shared memory for CLOG
Size
CLOGShmemSize(void)
{
- return SimpleLruShmemSize(NUM_CLOG_BUFFERS, CLOG_LSNS_PER_PAGE);
+ return SimpleLruShmemSize(CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE);
}
void
CLOGShmemInit(void)
{
ClogCtl->PagePrecedes = CLOGPagePrecedes;
- SimpleLruInit(ClogCtl, "CLOG Ctl", NUM_CLOG_BUFFERS, CLOG_LSNS_PER_PAGE,
+ SimpleLruInit(ClogCtl, "CLOG Ctl", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
CLogControlLock, "pg_clog");
}
#define TRANSACTION_STATUS_SUB_COMMITTED 0x03
-/* Number of SLRU buffers to use for clog */
-#define NUM_CLOG_BUFFERS 8
-
-
extern void TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
TransactionId *subxids, XidStatus status, XLogRecPtr lsn);
extern XidStatus TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn);
+extern Size CLOGShmemBuffers(void);
extern Size CLOGShmemSize(void);
extern void CLOGShmemInit(void);
extern void BootStrapCLOG(void);