On Fri, Mar 25, 2016 at 9:48 AM, Tom Lane <[email protected]> wrote:
> Robert Haas <[email protected]> writes:
>> It's stupid that we keep spending time and energy figuring out which
>> shared memory data structures require alignment and which ones don't.
>> Let's just align them *all* and be done with it. The memory cost
>> shouldn't be more than a few kB.
>
> I think such a proposal should come with a measurement, not just
> speculation about what it costs.
About 6kB with default settings. See below.
LOG: shared memory size: 148471808
LOG: size 48 -> maxalign 48, cachelinealign 128, delta 80
LOG: size 25988 -> maxalign 25992, cachelinealign 26112, delta 120
LOG: size 2904 -> maxalign 2904, cachelinealign 2944, delta 40
LOG: size 264 -> maxalign 264, cachelinealign 384, delta 120
LOG: size 4209296 -> maxalign 4209296, cachelinealign 4209408, delta 112
LOG: size 529216 -> maxalign 529216, cachelinealign 529280, delta 64
LOG: size 133600 -> maxalign 133600, cachelinealign 133632, delta 32
LOG: size 32 -> maxalign 32, cachelinealign 128, delta 96
LOG: size 267072 -> maxalign 267072, cachelinealign 267136, delta 64
LOG: size 66880 -> maxalign 66880, cachelinealign 66944, delta 64
LOG: size 133600 -> maxalign 133600, cachelinealign 133632, delta 32
LOG: size 948 -> maxalign 952, cachelinealign 1024, delta 72
LOG: size 2904 -> maxalign 2904, cachelinealign 2944, delta 40
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 20640 -> maxalign 20640, cachelinealign 20736, delta 96
LOG: size 28 -> maxalign 32, cachelinealign 128, delta 96
LOG: size 2904 -> maxalign 2904, cachelinealign 2944, delta 40
LOG: size 2904 -> maxalign 2904, cachelinealign 2944, delta 40
LOG: size 4100 -> maxalign 4104, cachelinealign 4224, delta 120
LOG: size 2904 -> maxalign 2904, cachelinealign 2944, delta 40
LOG: size 2904 -> maxalign 2904, cachelinealign 2944, delta 40
LOG: size 88 -> maxalign 88, cachelinealign 128, delta 40
LOG: size 2904 -> maxalign 2904, cachelinealign 2944, delta 40
LOG: size 24 -> maxalign 24, cachelinealign 128, delta 104
LOG: size 16 -> maxalign 16, cachelinealign 128, delta 112
LOG: size 133600 -> maxalign 133600, cachelinealign 133632, delta 32
LOG: size 16 -> maxalign 16, cachelinealign 128, delta 112
LOG: size 96 -> maxalign 96, cachelinealign 128, delta 32
LOG: size 95584 -> maxalign 95584, cachelinealign 95616, delta 32
LOG: size 1392 -> maxalign 1392, cachelinealign 1408, delta 16
LOG: size 1 -> maxalign 8, cachelinealign 128, delta 120
LOG: size 488 -> maxalign 488, cachelinealign 512, delta 24
LOG: size 16 -> maxalign 16, cachelinealign 128, delta 112
LOG: size 3016 -> maxalign 3016, cachelinealign 3072, delta 56
LOG: size 69144 -> maxalign 69144, cachelinealign 69248, delta 104
LOG: size 940 -> maxalign 944, cachelinealign 1024, delta 80
LOG: size 4760 -> maxalign 4760, cachelinealign 4864, delta 104
LOG: size 327720 -> maxalign 327720, cachelinealign 327808, delta 88
LOG: size 224 -> maxalign 224, cachelinealign 256, delta 32
LOG: size 56 -> maxalign 56, cachelinealign 128, delta 72
LOG: size 1192 -> maxalign 1192, cachelinealign 1280, delta 88
LOG: size 1356 -> maxalign 1360, cachelinealign 1408, delta 48
LOG: size 656 -> maxalign 656, cachelinealign 768, delta 112
LOG: size 1832 -> maxalign 1832, cachelinealign 1920, delta 88
LOG: size 66880 -> maxalign 66880, cachelinealign 66944, delta 64
LOG: total additional space consumed due to cache line alignment = 6096
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 36a04fc..72e774d 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -156,6 +156,7 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
* Create the shmem segment
*/
seghdr = PGSharedMemoryCreate(size, makePrivate, port, &shim);
+ elog(LOG, "shared memory size: %zu", size);
InitShmemAccess(seghdr);
@@ -269,4 +270,9 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
*/
if (shmem_startup_hook)
shmem_startup_hook();
+
+ {
+ extern Size cachealignwaste;
+ elog(LOG, "total additional space consumed due to cache line alignment = %zu", cachealignwaste);
+ }
}
diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 81506ea..ea7412d 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -85,6 +85,7 @@ slock_t *ShmemLock; /* spinlock for shared memory and LWLock
* allocation */
static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */
+Size cachealignwaste = 0;
/*
@@ -173,7 +174,12 @@ ShmemAlloc(Size size)
/*
* ensure all space is adequately aligned.
*/
- size = MAXALIGN(size);
+ if (MAXALIGN(size) != CACHELINEALIGN(size))
+ elog(LOG, "size %zu -> maxalign %zu, cachelinealign %zu, delta %zu",
+ size, MAXALIGN(size), CACHELINEALIGN(size),
+ CACHELINEALIGN(size) - MAXALIGN(size));
+ cachealignwaste += CACHELINEALIGN(size) - MAXALIGN(size);
+ size = CACHELINEALIGN(size);
Assert(ShmemSegHdr != NULL);
--
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