ShmemAlloc() alignment patch
In ShmemAlloc() we have:
newStart = BUFFERALIGN(newStart);
newSpace = (void *) (ShmemBase + newStart);
return newSpace;
Notice that though newStart is ALIGNOF_BUFFER, ShmemBase is not. Thus the
newSpace is not aligned as we disired.
Attached please find the patch.
Regards,
Qingqing
Index: src/backend/storage/ipc/shmem.c
===================================================================
RCS file: /projects/cvsroot/pgsql/src/backend/storage/ipc/shmem.c,v
retrieving revision 1.92
diff -c -r1.92 shmem.c
*** src/backend/storage/ipc/shmem.c 13 Jul 2006 16:49:16 -0000 1.92
--- src/backend/storage/ipc/shmem.c 14 Jul 2006 06:47:43 -0000
***************
*** 169,178 ****
SpinLockAcquire(ShmemLock);newStart = shmemseghdr->freeoffset;
/* extra alignment for large requests, since they are probably
buffers */
if (size >= BLCKSZ)
! newStart = BUFFERALIGN(newStart);
newFree = newStart + size;
if (newFree <= shmemseghdr->totalsize)
--- 169,179 ----
SpinLockAcquire(ShmemLock); newStart = shmemseghdr->freeoffset;
+ newSpace = (void *) MAKE_PTR(newStart);
/* extra alignment for large requests, since they are probably
buffers */
if (size >= BLCKSZ)
! newStart = MAKE_OFFSET(BUFFERALIGN(newSpace));
newFree = newStart + size;
if (newFree <= shmemseghdr->totalsize)
On Fri, Jul 14, 2006 at 02:50:31PM +0800, Qingqing Zhou wrote:
In ShmemAlloc() we have:
newStart = BUFFERALIGN(newStart);
newSpace = (void *) (ShmemBase + newStart);
return newSpace;Notice that though newStart is ALIGNOF_BUFFER, ShmemBase is not. Thus the
newSpace is not aligned as we disired.
How can ShmemBase not be aligned? Surely it's page-aligned?
Have a nice day,
--
Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
Show quoted text
From each according to his ability. To each according to his ability to litigate.
Martijn van Oosterhout <kleptog@svana.org> writes:
On Fri, Jul 14, 2006 at 02:50:31PM +0800, Qingqing Zhou wrote:
Notice that though newStart is ALIGNOF_BUFFER, ShmemBase is not. Thus the
newSpace is not aligned as we disired.
How can ShmemBase not be aligned? Surely it's page-aligned?
That's certainly what the code expects. I'm disinclined to apply this
patch unless you can identify a real system where ShmemBase might not
point to a page boundary.
(Note: in a standalone backend, the "shared memory segment" is just a
huge malloc chunk, and so depending on your platform it might not be
page-aligned. I don't feel a need to add cycles to ShmemAlloc to
optimize this case, though. We only care about performance in the
normal shared-memory case.)
regards, tom lane
Tom Lane wrote:
Martijn van Oosterhout <kleptog@svana.org> writes:
On Fri, Jul 14, 2006 at 02:50:31PM +0800, Qingqing Zhou wrote:
Notice that though newStart is ALIGNOF_BUFFER, ShmemBase is not. Thus the
newSpace is not aligned as we disired.How can ShmemBase not be aligned? Surely it's page-aligned?
That's certainly what the code expects. I'm disinclined to apply this
patch unless you can identify a real system where ShmemBase might not
point to a page boundary.(Note: in a standalone backend, the "shared memory segment" is just a
huge malloc chunk, and so depending on your platform it might not be
page-aligned. I don't feel a need to add cycles to ShmemAlloc to
optimize this case, though. We only care about performance in the
normal shared-memory case.)
Should we add an assert?
--
Bruce Momjian bruce@momjian.us
EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Bruce Momjian <bruce@momjian.us> writes:
How can ShmemBase not be aligned? Surely it's page-aligned?
Should we add an assert?
No, because even if it's not page-aligned, there's no correctness issue
here. (Besides, how would you know what the page size is on any given
platform?)
regards, tom lane