Align large shared memory allocations - Mailing list pgsql-patches

Attached is a patch that aligns large shared memory allocations beyond
MAXIMUM_ALIGNOF. The reason for this is that Intel's cpus have a fast
path for bulk memory copies that only works with aligned addresses. It's
possible that other cpus have similar restrictions.
With 7.3.4, it achives a 5% performance gain with pgbench. It has no
effect with 7.3.3, because the buffers are already aligned by chance. I
haven't properly tested 7.4cvs yet.

One problem is the "32" - it's arbitrary, it probably belongs into an
arch dependant header file. But where?

--
    Manfred
diff -u pgsql.orig/src/backend/storage/ipc/shmem.c pgsql/src/backend/storage/ipc/shmem.c
--- pgsql.orig/src/backend/storage/ipc/shmem.c    2003-09-20 20:17:08.000000000 +0200
+++ pgsql/src/backend/storage/ipc/shmem.c    2003-09-20 20:34:21.000000000 +0200
@@ -131,6 +131,7 @@
 void *
 ShmemAlloc(Size size)
 {
+    uint32        newStart;
     uint32        newFree;
     void       *newSpace;

@@ -146,10 +147,21 @@

     SpinLockAcquire(ShmemLock);

-    newFree = shmemseghdr->freeoffset + size;
+    newStart = shmemseghdr->freeoffset;
+    if (size >= BLCKSZ)
+    {
+        /* Align BLCKSZ sized buffers even further:
+         * - the costs are small
+         * - some cpus (most notably Intel Pentium III)
+         *   prefer well-aligned addresses for memory copies
+         */
+        newStart = TYPEALIGN(32, newStart);
+    }
+
+    newFree = newStart + size;
     if (newFree <= shmemseghdr->totalsize)
     {
-        newSpace = (void *) MAKE_PTR(shmemseghdr->freeoffset);
+        newSpace = (void *) MAKE_PTR(newStart);
         shmemseghdr->freeoffset = newFree;
     }
     else