From 217ffee38167229568a474051549123713ba3a9e Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Sun, 5 Apr 2026 19:56:00 +0300 Subject: [PATCH v12 11/13] Add alignment option to ShmemRequestStruct() The buffer blocks (in the next commit) are IO-aligned. This might come handy in other places too, so make it an explicit feature of ShmemRequestStruct(). Reviewed-by: Ashutosh Bapat Reviewed-by: Matthias van de Meent Reviewed-by: Daniel Gustafsson Discussion: https://www.postgresql.org/message-id/CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com --- src/backend/storage/ipc/shmem.c | 33 +++++++++++++++++++++++---------- src/include/storage/shmem.h | 6 ++++++ 2 files changed, 29 insertions(+), 10 deletions(-) diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c index 67fad32be82..0f103f17f29 100644 --- a/src/backend/storage/ipc/shmem.c +++ b/src/backend/storage/ipc/shmem.c @@ -136,6 +136,7 @@ #include "fmgr.h" #include "funcapi.h" #include "miscadmin.h" +#include "port/pg_bitutils.h" #include "port/pg_numa.h" #include "storage/lwlock.h" #include "storage/pg_shmem.h" @@ -236,7 +237,7 @@ typedef struct ShmemAllocatorData #define ShmemIndexLock (&ShmemAllocator->index_lock) -static void *ShmemAllocRaw(Size size, Size *allocated_size); +static void *ShmemAllocRaw(Size size, Size alignment, Size *allocated_size); /* shared memory global variables */ @@ -340,6 +341,7 @@ ShmemRequestInternal(ShmemStructOpts *options, ShmemRequestKind kind) { ShmemRequest *request; + /* Check the options */ if (options->name == NULL) elog(ERROR, "shared memory request is missing 'name' option"); @@ -358,6 +360,11 @@ ShmemRequestInternal(ShmemStructOpts *options, ShmemRequestKind kind) options->size, options->name); } + if (options->alignment != 0 && pg_nextpower2_size_t(options->alignment) != options->alignment) + elog(ERROR, "invalid alignment %zu for shared memory request for \"%s\"", + options->alignment, options->name); + + /* Check that we're in the right state */ if (shmem_request_state != SRS_REQUESTING) elog(ERROR, "ShmemRequestStruct can only be called from a shmem_request callback"); @@ -399,7 +406,8 @@ ShmemGetRequestedSize(void) { size = add_size(size, request->options->size); /* calculate alignment padding like ShmemAllocRaw() does */ - size = CACHELINEALIGN(size); + size = TYPEALIGN(Max(request->options->alignment, PG_CACHE_LINE_SIZE), + size); } return size; @@ -524,7 +532,9 @@ InitShmemIndexEntry(ShmemRequest *request) * We inserted the entry to the shared memory index. Allocate requested * amount of shared memory for it, and initialize the index entry. */ - structPtr = ShmemAllocRaw(request->options->size, &allocated_size); + structPtr = ShmemAllocRaw(request->options->size, + request->options->alignment, + &allocated_size); if (structPtr == NULL) { /* out of memory; remove the failed ShmemIndex entry */ @@ -753,7 +763,7 @@ ShmemAlloc(Size size) void *newSpace; Size allocated_size; - newSpace = ShmemAllocRaw(size, &allocated_size); + newSpace = ShmemAllocRaw(size, 0, &allocated_size); if (!newSpace) ereport(ERROR, (errcode(ERRCODE_OUT_OF_MEMORY), @@ -772,7 +782,7 @@ ShmemAllocNoError(Size size) { Size allocated_size; - return ShmemAllocRaw(size, &allocated_size); + return ShmemAllocRaw(size, 0, &allocated_size); } /* @@ -782,8 +792,9 @@ ShmemAllocNoError(Size size) * be equal to the number requested plus any padding we choose to add. */ static void * -ShmemAllocRaw(Size size, Size *allocated_size) +ShmemAllocRaw(Size size, Size alignment, Size *allocated_size) { + Size rawStart; Size newStart; Size newFree; void *newSpace; @@ -799,14 +810,15 @@ ShmemAllocRaw(Size size, Size *allocated_size) * structures out to a power-of-two size - but without this, even that * won't be sufficient. */ - size = CACHELINEALIGN(size); - *allocated_size = size; + if (alignment < PG_CACHE_LINE_SIZE) + alignment = PG_CACHE_LINE_SIZE; Assert(ShmemSegHdr != NULL); SpinLockAcquire(&ShmemAllocator->shmem_lock); - newStart = ShmemAllocator->free_offset; + rawStart = ShmemAllocator->free_offset; + newStart = TYPEALIGN(alignment, rawStart); newFree = newStart + size; if (newFree <= ShmemSegHdr->totalsize) @@ -820,8 +832,9 @@ ShmemAllocRaw(Size size, Size *allocated_size) SpinLockRelease(&ShmemAllocator->shmem_lock); /* note this assert is okay with newSpace == NULL */ - Assert(newSpace == (void *) CACHELINEALIGN(newSpace)); + Assert(newSpace == (void *) TYPEALIGN(alignment, newSpace)); + *allocated_size = newFree - rawStart; return newSpace; } diff --git a/src/include/storage/shmem.h b/src/include/storage/shmem.h index 8e0fc29dcac..060a4a8e5d2 100644 --- a/src/include/storage/shmem.h +++ b/src/include/storage/shmem.h @@ -51,6 +51,12 @@ typedef struct ShmemStructOpts */ ssize_t size; + /* + * Alignment of the starting address. If not set, defaults to cacheline + * boundary. Must be a power of two. + */ + size_t alignment; + /* * When the shmem area is initialized or attached to, pointer to it is * stored in *ptr. It usually points to a global variable, used to access -- 2.47.3