Improve performance of subsystems on top of SLRU

More precisely, what we do here is make the SLRU cache sizes
configurable with new GUCs, so that sites with high concurrency and big
ranges of transactions in flight (resp. multixacts/subtransactions) can
benefit from bigger caches.  In order for this to work with good
performance, two additional changes are made:

1. the cache is divided in "banks" (to borrow terminology from CPU
   caches), and algorithms such as eviction buffer search only affect
   one specific bank.  This forestalls the problem that linear searching
   for a specific buffer across the whole cache takes too long: we only
   have to search the specific bank, whose size is small.  This work is
   authored by Andrey Borodin.

2. Change the locking regime for the SLRU banks, so that each bank uses
   a separate LWLock.  This allows for increased scalability.  This work
   is authored by Dilip Kumar.  (A part of this was previously committed as
   d172b717c6f4.)

Special care is taken so that the algorithms that can potentially
traverse more than one bank release one bank's lock before acquiring the
next.  This should happen rarely, but particularly clog.c's group commit
feature needed code adjustment to cope with this.  I (Álvaro) also added
lots of comments to make sure the design is sound.

The new GUCs match the names introduced by bcdfa5f2e2f2 in the
pg_stat_slru view.

The default values for these parameters are similar to the previous
sizes of each SLRU.  commit_ts, clog and subtrans accept value 0, which
means to adjust by dividing shared_buffers by 512 (so 2MB for every 1GB
of shared_buffers), with a cap of 8MB.  (A new slru.c function
SimpleLruAutotuneBuffers() was added to support this.)  The cap was
previously 1MB for clog, so for sites with more than 512MB of shared
memory the total memory used increases, which is likely a good tradeoff.
However, other SLRUs (notably multixact ones) retain smaller sizes and
don't support a configured value of 0.  These values based on
shared_buffers may need to be revisited, but that's an easy change.

There was some resistance to adding these new GUCs: it would be better
to adjust to memory pressure automatically somehow, for example by
stealing memory from shared_buffers (where the caches can grow and
shrink naturally).  However, doing that seems to be a much larger
project and one which has made virtually no progress in several years,
and because this is such a pain point for so many users, here we take
the pragmatic approach.

Author: Andrey Borodin <x4mmm@yandex-team.ru>
Author: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Amul Sul, Gilles Darold, Anastasia Lubennikova,
	Ivan Lazarev, Robert Haas, Thomas Munro, Tomas Vondra,
	Yura Sokolov, Васильев Дмитрий (Dmitry Vasiliev).
Discussion: https://postgr.es/m/2BEC2B3F-9B61-4C1D-9FB5-5FAB0F05EF86@yandex-team.ru
Discussion: https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com

1 files changed, 94 insertions, 16 deletions

diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 6aa47af43e2..dc9566fb51b 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -31,7 +31,9 @@
 #include "access/slru.h"
 #include "access/subtrans.h"
 #include "access/transam.h"
+#include "miscadmin.h"
 #include "pg_trace.h"
+#include "utils/guc_hooks.h"
 #include "utils/snapmgr.h"
 
 
@@ -85,12 +87,14 @@ SubTransSetParent(TransactionId xid, TransactionId parent)
 	int64		pageno = TransactionIdToPage(xid);
 	int			entryno = TransactionIdToEntry(xid);
 	int			slotno;
+	LWLock	   *lock;
 	TransactionId *ptr;
 
 	Assert(TransactionIdIsValid(parent));
 	Assert(TransactionIdFollows(xid, parent));
 
-	LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
+	lock = SimpleLruGetBankLock(SubTransCtl, pageno);
+	LWLockAcquire(lock, LW_EXCLUSIVE);
 
 	slotno = SimpleLruReadPage(SubTransCtl, pageno, true, xid);
 	ptr = (TransactionId *) SubTransCtl->shared->page_buffer[slotno];
@@ -108,7 +112,7 @@ SubTransSetParent(TransactionId xid, TransactionId parent)
 		SubTransCtl->shared->page_dirty[slotno] = true;
 	}
 
-	LWLockRelease(SubtransSLRULock);
+	LWLockRelease(lock);
 }
 
 /*
@@ -138,7 +142,7 @@ SubTransGetParent(TransactionId xid)
 
 	parent = *ptr;
 
-	LWLockRelease(SubtransSLRULock);
+	LWLockRelease(SimpleLruGetBankLock(SubTransCtl, pageno));
 
 	return parent;
 }
@@ -186,6 +190,22 @@ SubTransGetTopmostTransaction(TransactionId xid)
 	return previousXid;
 }
 
+/*
+ * Number of shared SUBTRANS buffers.
+ *
+ * If asked to autotune, use 2MB for every 1GB of shared buffers, up to 8MB.
+ * Otherwise just cap the configured amount to be between 16 and the maximum
+ * allowed.
+ */
+static int
+SUBTRANSShmemBuffers(void)
+{
+	/* auto-tune based on shared buffers */
+	if (subtransaction_buffers == 0)
+		return SimpleLruAutotuneBuffers(512, 1024);
+
+	return Min(Max(16, subtransaction_buffers), SLRU_MAX_ALLOWED_BUFFERS);
+}
 
 /*
  * Initialization of shared memory for SUBTRANS
@@ -193,21 +213,50 @@ SubTransGetTopmostTransaction(TransactionId xid)
 Size
 SUBTRANSShmemSize(void)
 {
-	return SimpleLruShmemSize(NUM_SUBTRANS_BUFFERS, 0);
+	return SimpleLruShmemSize(SUBTRANSShmemBuffers(), 0);
 }
 
 void
 SUBTRANSShmemInit(void)
 {
+	/* If auto-tuning is requested, now is the time to do it */
+	if (subtransaction_buffers == 0)
+	{
+		char		buf[32];
+
+		snprintf(buf, sizeof(buf), "%d", SUBTRANSShmemBuffers());
+		SetConfigOption("subtransaction_buffers", buf, PGC_POSTMASTER,
+						PGC_S_DYNAMIC_DEFAULT);
+
+		/*
+		 * We prefer to report this value's source as PGC_S_DYNAMIC_DEFAULT.
+		 * However, if the DBA explicitly set subtransaction_buffers = 0 in
+		 * the config file, then PGC_S_DYNAMIC_DEFAULT will fail to override
+		 * that and we must force the matter with PGC_S_OVERRIDE.
+		 */
+		if (subtransaction_buffers == 0)	/* failed to apply it? */
+			SetConfigOption("subtransaction_buffers", buf, PGC_POSTMASTER,
+							PGC_S_OVERRIDE);
+	}
+	Assert(subtransaction_buffers != 0);
+
 	SubTransCtl->PagePrecedes = SubTransPagePrecedes;
-	SimpleLruInit(SubTransCtl, "subtransaction", NUM_SUBTRANS_BUFFERS, 0,
-				  SubtransSLRULock, "pg_subtrans",
-				  LWTRANCHE_SUBTRANS_BUFFER, SYNC_HANDLER_NONE,
-				  false);
+	SimpleLruInit(SubTransCtl, "subtransaction", SUBTRANSShmemBuffers(), 0,
+				  "pg_subtrans", LWTRANCHE_SUBTRANS_BUFFER,
+				  LWTRANCHE_SUBTRANS_SLRU, SYNC_HANDLER_NONE, false);
 	SlruPagePrecedesUnitTests(SubTransCtl, SUBTRANS_XACTS_PER_PAGE);
 }
 
 /*
+ * GUC check_hook for subtransaction_buffers
+ */
+bool
+check_subtrans_buffers(int *newval, void **extra, GucSource source)
+{
+	return check_slru_buffers("subtransaction_buffers", newval);
+}
+
+/*
  * This func must be called ONCE on system install.  It creates
  * the initial SUBTRANS segment.  (The SUBTRANS directory is assumed to
  * have been created by the initdb shell script, and SUBTRANSShmemInit
@@ -221,8 +270,9 @@ void
 BootStrapSUBTRANS(void)
 {
 	int			slotno;
+	LWLock	   *lock = SimpleLruGetBankLock(SubTransCtl, 0);
 
-	LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
+	LWLockAcquire(lock, LW_EXCLUSIVE);
 
 	/* Create and zero the first page of the subtrans log */
 	slotno = ZeroSUBTRANSPage(0);
@@ -231,7 +281,7 @@ BootStrapSUBTRANS(void)
 	SimpleLruWritePage(SubTransCtl, slotno);
 	Assert(!SubTransCtl->shared->page_dirty[slotno]);
 
-	LWLockRelease(SubtransSLRULock);
+	LWLockRelease(lock);
 }
 
 /*
@@ -261,6 +311,8 @@ StartupSUBTRANS(TransactionId oldestActiveXID)
 	FullTransactionId nextXid;
 	int64		startPage;
 	int64		endPage;
+	LWLock	   *prevlock;
+	LWLock	   *lock;
 
 	/*
 	 * Since we don't expect pg_subtrans to be valid across crashes, we
@@ -268,23 +320,47 @@ StartupSUBTRANS(TransactionId oldestActiveXID)
 	 * Whenever we advance into a new page, ExtendSUBTRANS will likewise zero
 	 * the new page without regard to whatever was previously on disk.
 	 */
-	LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
-
 	startPage = TransactionIdToPage(oldestActiveXID);
 	nextXid = TransamVariables->nextXid;
 	endPage = TransactionIdToPage(XidFromFullTransactionId(nextXid));
 
+	prevlock = SimpleLruGetBankLock(SubTransCtl, startPage);
+	LWLockAcquire(prevlock, LW_EXCLUSIVE);
 	while (startPage != endPage)
 	{
+		lock = SimpleLruGetBankLock(SubTransCtl, startPage);
+
+		/*
+		 * Check if we need to acquire the lock on the new bank then release
+		 * the lock on the old bank and acquire on the new bank.
+		 */
+		if (prevlock != lock)
+		{
+			LWLockRelease(prevlock);
+			LWLockAcquire(lock, LW_EXCLUSIVE);
+			prevlock = lock;
+		}
+
 		(void) ZeroSUBTRANSPage(startPage);
 		startPage++;
 		/* must account for wraparound */
 		if (startPage > TransactionIdToPage(MaxTransactionId))
 			startPage = 0;
 	}
-	(void) ZeroSUBTRANSPage(startPage);
 
-	LWLockRelease(SubtransSLRULock);
+	lock = SimpleLruGetBankLock(SubTransCtl, startPage);
+
+	/*
+	 * Check if we need to acquire the lock on the new bank then release the
+	 * lock on the old bank and acquire on the new bank.
+	 */
+	if (prevlock != lock)
+	{
+		LWLockRelease(prevlock);
+		LWLockAcquire(lock, LW_EXCLUSIVE);
+	}
+	(void) ZeroSUBTRANSPage(startPage);
+	LWLockRelease(lock);
 }
 
 /*
@@ -318,6 +394,7 @@ void
 ExtendSUBTRANS(TransactionId newestXact)
 {
 	int64		pageno;
+	LWLock	   *lock;
 
 	/*
 	 * No work except at first XID of a page.  But beware: just after
@@ -329,12 +406,13 @@ ExtendSUBTRANS(TransactionId newestXact)
 
 	pageno = TransactionIdToPage(newestXact);
 
-	LWLockAcquire(SubtransSLRULock, LW_EXCLUSIVE);
+	lock = SimpleLruGetBankLock(SubTransCtl, pageno);
+	LWLockAcquire(lock, LW_EXCLUSIVE);
 
 	/* Zero the page */
 	ZeroSUBTRANSPage(pageno);
 
-	LWLockRelease(SubtransSLRULock);
+	LWLockRelease(lock);
 }