Preallocate some DSM space at startup.

Create an optional region in the main shared memory segment that can be used to acquire and release "fast" DSM segments, and can benefit from huge pages allocated at cluster startup time, if configured. Fall back to the existing mechanisms when that space is full. The size is controlled by a new GUC min_dynamic_shared_memory, defaulting to 0. Main region DSM segments initially contain whatever garbage the memory held last time they were used, rather than zeroes. That change revealed that DSA areas failed to initialize themselves correctly in memory that wasn't zeroed first, so fix that problem. Discussion: https://postgr.es/m/CA%2BhUKGLAE2QBv-WgGp%2BD9P_J-%3Dyne3zof9nfMaqq1h3EGHFXYQ%40mail.gmail.com
2025-11-06 07:49:08 +03:00 · 2020-07-31 17:27:09 +12:00
parent 7b1110d2fd
commit 84b1c63ad4
9 changed files with 216 additions and 26 deletions
--- a/src/backend/storage/ipc/dsm.c
+++ b/src/backend/storage/ipc/dsm.c
@@ -35,10 +35,12 @@

 #include "lib/ilist.h"
 #include "miscadmin.h"
+#include "port/pg_bitutils.h"
 #include "storage/dsm.h"
 #include "storage/ipc.h"
 #include "storage/lwlock.h"
 #include "storage/pg_shmem.h"
+#include "utils/freepage.h"
 #include "utils/guc.h"
 #include "utils/memutils.h"
 #include "utils/resowner_private.h"
@@ -76,6 +78,8 @@ typedef struct dsm_control_item
 {
 	dsm_handle	handle;
 	uint32		refcnt;			/* 2+ = active, 1 = moribund, 0 = gone */
+	size_t		first_page;
+	size_t		npages;
 	void	   *impl_private_pm_handle; /* only needed on Windows */
 	bool		pinned;
 } dsm_control_item;
@@ -95,10 +99,15 @@ static dsm_segment *dsm_create_descriptor(void);
 static bool dsm_control_segment_sane(dsm_control_header *control,
 									 Size mapped_size);
 static uint64 dsm_control_bytes_needed(uint32 nitems);
+static inline dsm_handle make_main_region_dsm_handle(int slot);
+static inline bool is_main_region_dsm_handle(dsm_handle handle);

 /* Has this backend initialized the dynamic shared memory system yet? */
 static bool dsm_init_done = false;

+/* Preallocated DSM space in the main shared memory region. */
+static void *dsm_main_space_begin = NULL;
+
 /*
 * List of dynamic shared memory segments used by this backend.
 *
@@ -171,7 +180,7 @@ dsm_postmaster_startup(PGShmemHeader *shim)
 	{
 		Assert(dsm_control_address == NULL);
 		Assert(dsm_control_mapped_size == 0);
-		dsm_control_handle = random();
+		dsm_control_handle = random() << 1; /* Even numbers only */
 		if (dsm_control_handle == DSM_HANDLE_INVALID)
 			continue;
 		if (dsm_impl_op(DSM_OP_CREATE, dsm_control_handle, segsize,
@@ -247,8 +256,12 @@ dsm_cleanup_using_control_segment(dsm_handle old_control_handle)
 		if (refcnt == 0)
 			continue;

-		/* Log debugging information. */
+		/* If it was using the main shmem area, there is nothing to do. */
 		handle = old_control->item[i].handle;
+		if (is_main_region_dsm_handle(handle))
+			continue;
+
+		/* Log debugging information. */
 		elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u (reference count %u)",
 			 handle, refcnt);

@@ -348,8 +361,11 @@ dsm_postmaster_shutdown(int code, Datum arg)
 		if (dsm_control->item[i].refcnt == 0)
 			continue;

-		/* Log debugging information. */
 		handle = dsm_control->item[i].handle;
+		if (is_main_region_dsm_handle(handle))
+			continue;
+
+		/* Log debugging information. */
 		elog(DEBUG2, "cleaning up orphaned dynamic shared memory with ID %u",
 			 handle);

@@ -418,6 +434,45 @@ dsm_set_control_handle(dsm_handle h)
 }
 #endif

+/*
+ * Reserve some space in the main shared memory segment for DSM segments.
+ */
+size_t
+dsm_estimate_size(void)
+{
+	return 1024 * 1024 * (size_t) min_dynamic_shared_memory;
+}
+
+/*
+ * Initialize space in the main shared memory segment for DSM segments.
+ */
+void
+dsm_shmem_init(void)
+{
+	size_t		size = dsm_estimate_size();
+	bool		found;
+
+	if (size == 0)
+		return;
+
+	dsm_main_space_begin = ShmemInitStruct("Preallocated DSM", size, &found);
+	if (!found)
+	{
+		FreePageManager *fpm = (FreePageManager *) dsm_main_space_begin;
+		size_t		first_page = 0;
+		size_t		pages;
+
+		/* Reserve space for the FreePageManager. */
+		while (first_page * FPM_PAGE_SIZE < sizeof(FreePageManager))
+			++first_page;
+
+		/* Initialize it and give it all the rest of the space. */
+		FreePageManagerInitialize(fpm, dsm_main_space_begin);
+		pages = (size / FPM_PAGE_SIZE) - first_page;
+		FreePageManagerPut(fpm, first_page, pages);
+	}
+}
+
 /*
 * Create a new dynamic shared memory segment.
 *
@@ -434,6 +489,10 @@ dsm_create(Size size, int flags)
 	dsm_segment *seg;
 	uint32		i;
 	uint32		nitems;
+	size_t		npages = 0;
+	size_t		first_page = 0;
+	FreePageManager *dsm_main_space_fpm = dsm_main_space_begin;
+	bool		using_main_dsm_region = false;

 	/* Unsafe in postmaster (and pointless in a stand-alone backend). */
 	Assert(IsUnderPostmaster);
@@ -444,20 +503,48 @@ dsm_create(Size size, int flags)
 	/* Create a new segment descriptor. */
 	seg = dsm_create_descriptor();

-	/* Loop until we find an unused segment identifier. */
-	for (;;)
+	/*
+	 * Lock the control segment while we try to allocate from the main shared
+	 * memory area, if configured.
+	 */
+	if (dsm_main_space_fpm)
 	{
-		Assert(seg->mapped_address == NULL && seg->mapped_size == 0);
-		seg->handle = random();
-		if (seg->handle == DSM_HANDLE_INVALID)	/* Reserve sentinel */
-			continue;
-		if (dsm_impl_op(DSM_OP_CREATE, seg->handle, size, &seg->impl_private,
-						&seg->mapped_address, &seg->mapped_size, ERROR))
-			break;
+		npages = size / FPM_PAGE_SIZE;
+		if (size % FPM_PAGE_SIZE > 0)
+			++npages;
+
+		LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+		if (FreePageManagerGet(dsm_main_space_fpm, npages, &first_page))
+		{
+			/* We can carve out a piece of the main shared memory segment. */
+			seg->mapped_address = (char *) dsm_main_space_begin +
+				first_page * FPM_PAGE_SIZE;
+			seg->mapped_size = npages * FPM_PAGE_SIZE;
+			using_main_dsm_region = true;
+			/* We'll choose a handle below. */
+		}
 	}

-	/* Lock the control segment so we can register the new segment. */
-	LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+	if (!using_main_dsm_region)
+	{
+		/*
+		 * We need to create a new memory segment.  Loop until we find an
+		 * unused segment identifier.
+		 */
+		if (dsm_main_space_fpm)
+			LWLockRelease(DynamicSharedMemoryControlLock);
+		for (;;)
+		{
+			Assert(seg->mapped_address == NULL && seg->mapped_size == 0);
+			seg->handle = random() << 1;	/* Even numbers only */
+			if (seg->handle == DSM_HANDLE_INVALID)	/* Reserve sentinel */
+				continue;
+			if (dsm_impl_op(DSM_OP_CREATE, seg->handle, size, &seg->impl_private,
+							&seg->mapped_address, &seg->mapped_size, ERROR))
+				break;
+		}
+		LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+	}

 	/* Search the control segment for an unused slot. */
 	nitems = dsm_control->nitems;
@@ -465,6 +552,14 @@ dsm_create(Size size, int flags)
 	{
 		if (dsm_control->item[i].refcnt == 0)
 		{
+			if (using_main_dsm_region)
+			{
+				seg->handle = make_main_region_dsm_handle(i);
+				dsm_control->item[i].first_page = first_page;
+				dsm_control->item[i].npages = npages;
+			}
+			else
+				Assert(!is_main_region_dsm_handle(seg->handle));
 			dsm_control->item[i].handle = seg->handle;
 			/* refcnt of 1 triggers destruction, so start at 2 */
 			dsm_control->item[i].refcnt = 2;
@@ -479,9 +574,12 @@ dsm_create(Size size, int flags)
 	/* Verify that we can support an additional mapping. */
 	if (nitems >= dsm_control->maxitems)
 	{
+		if (using_main_dsm_region)
+			FreePageManagerPut(dsm_main_space_fpm, first_page, npages);
 		LWLockRelease(DynamicSharedMemoryControlLock);
-		dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
-					&seg->mapped_address, &seg->mapped_size, WARNING);
+		if (!using_main_dsm_region)
+			dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
+						&seg->mapped_address, &seg->mapped_size, WARNING);
 		if (seg->resowner != NULL)
 			ResourceOwnerForgetDSM(seg->resowner, seg);
 		dlist_delete(&seg->node);
@@ -495,6 +593,12 @@ dsm_create(Size size, int flags)
 	}

 	/* Enter the handle into a new array slot. */
+	if (using_main_dsm_region)
+	{
+		seg->handle = make_main_region_dsm_handle(nitems);
+		dsm_control->item[i].first_page = first_page;
+		dsm_control->item[i].npages = npages;
+	}
 	dsm_control->item[nitems].handle = seg->handle;
 	/* refcnt of 1 triggers destruction, so start at 2 */
 	dsm_control->item[nitems].refcnt = 2;
@@ -580,6 +684,12 @@ dsm_attach(dsm_handle h)
 		/* Otherwise we've found a match. */
 		dsm_control->item[i].refcnt++;
 		seg->control_slot = i;
+		if (is_main_region_dsm_handle(seg->handle))
+		{
+			seg->mapped_address = (char *) dsm_main_space_begin +
+				dsm_control->item[i].first_page * FPM_PAGE_SIZE;
+			seg->mapped_size = dsm_control->item[i].npages * FPM_PAGE_SIZE;
+		}
 		break;
 	}
 	LWLockRelease(DynamicSharedMemoryControlLock);
@@ -597,8 +707,9 @@ dsm_attach(dsm_handle h)
 	}

 	/* Here's where we actually try to map the segment. */
-	dsm_impl_op(DSM_OP_ATTACH, seg->handle, 0, &seg->impl_private,
-				&seg->mapped_address, &seg->mapped_size, ERROR);
+	if (!is_main_region_dsm_handle(seg->handle))
+		dsm_impl_op(DSM_OP_ATTACH, seg->handle, 0, &seg->impl_private,
+					&seg->mapped_address, &seg->mapped_size, ERROR);

 	return seg;
 }
@@ -688,8 +799,9 @@ dsm_detach(dsm_segment *seg)
 	 */
 	if (seg->mapped_address != NULL)
 	{
-		dsm_impl_op(DSM_OP_DETACH, seg->handle, 0, &seg->impl_private,
-					&seg->mapped_address, &seg->mapped_size, WARNING);
+		if (!is_main_region_dsm_handle(seg->handle))
+			dsm_impl_op(DSM_OP_DETACH, seg->handle, 0, &seg->impl_private,
+						&seg->mapped_address, &seg->mapped_size, WARNING);
 		seg->impl_private = NULL;
 		seg->mapped_address = NULL;
 		seg->mapped_size = 0;
@@ -729,10 +841,15 @@ dsm_detach(dsm_segment *seg)
 			 * other reason, the postmaster may not have any better luck than
 			 * we did.  There's not much we can do about that, though.
 			 */
-			if (dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
+			if (is_main_region_dsm_handle(seg->handle) ||
+				dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
 							&seg->mapped_address, &seg->mapped_size, WARNING))
 			{
 				LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+				if (is_main_region_dsm_handle(seg->handle))
+					FreePageManagerPut((FreePageManager *) dsm_main_space_begin,
+									   dsm_control->item[control_slot].first_page,
+									   dsm_control->item[control_slot].npages);
 				Assert(dsm_control->item[control_slot].handle == seg->handle);
 				Assert(dsm_control->item[control_slot].refcnt == 1);
 				dsm_control->item[control_slot].refcnt = 0;
@@ -894,10 +1011,15 @@ dsm_unpin_segment(dsm_handle handle)
 		 * pass the mapped size, mapped address, and private data as NULL
 		 * here.
 		 */
-		if (dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
+		if (is_main_region_dsm_handle(handle) ||
+			dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
 						&junk_mapped_address, &junk_mapped_size, WARNING))
 		{
 			LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+			if (is_main_region_dsm_handle(handle))
+				FreePageManagerPut((FreePageManager *) dsm_main_space_begin,
+								   dsm_control->item[control_slot].first_page,
+								   dsm_control->item[control_slot].npages);
 			Assert(dsm_control->item[control_slot].handle == handle);
 			Assert(dsm_control->item[control_slot].refcnt == 1);
 			dsm_control->item[control_slot].refcnt = 0;
@@ -1094,3 +1216,28 @@ dsm_control_bytes_needed(uint32 nitems)
 	return offsetof(dsm_control_header, item)
 		+ sizeof(dsm_control_item) * (uint64) nitems;
 }
+
+static inline dsm_handle
+make_main_region_dsm_handle(int slot)
+{
+	dsm_handle	handle;
+
+	/*
+	 * We need to create a handle that doesn't collide with any existing extra
+	 * segment created by dsm_impl_op(), so we'll make it odd.  It also
+	 * mustn't collide with any other main area pseudo-segment, so we'll
+	 * include the slot number in some of the bits.  We also want to make an
+	 * effort to avoid newly created and recently destroyed handles from being
+	 * confused, so we'll make the rest of the bits random.
+	 */
+	handle = 1;
+	handle |= slot << 1;
+	handle |= random() << (pg_leftmost_one_pos32(dsm_control->maxitems) + 1);
+	return handle;
+}
+
+static inline bool
+is_main_region_dsm_handle(dsm_handle handle)
+{
+	return handle & 1;
+}