Re: What is the posix_memalign() equivalent for the PostgreSQL?

The unexpected errors are problems to free the memory that was previously
allocated with posix_memalign. After a number of memory allocation and free
calls, the next free call crashes the system.

On the other hand, if I replace the posix_memalign and the free calls for
the following malloc/free aligned implementations, it works.

void *malloc_aligned(size_t alignment, size_t bytes) {
void *mallocPtr; //Initial pointer returned from malloc
void *newMallocPtr; //New pointer after adjustment
void *alignedPtr;
size_t alignMask; //Need this to get the aligned address
size_t totalBytes = 0;

/* Make sure alignment is power of 2 and it is not zero
* because zero is not power of 2 */
if ( !(!(alignment & (alignment-1)) && alignment) )
return NULL;

/* We need to allocate extra memory to make sure the allocated
* memory will be aligned and need sizeof(size_t) bytes more for
* storing the value of the bytes we padded.
*/
totalBytes = bytes + alignment + sizeof(size_t);

mallocPtr = palloc(totalBytes);

if (NULL == mallocPtr)
return NULL;

newMallocPtr = (void*)((char*)mallocPtr + sizeof(size_t));

alignMask = ~(alignment - 1);

/* Value of alignedPtr should be multiple of alignment */
alignedPtr = (void *)(((size_t)newMallocPtr + alignment) & alignMask);

/* Store the extra bytes info right before alignedPtr */
*((size_t*)alignedPtr - 1) = (size_t)alignedPtr - (size_t)mallocPtr;

return alignedPtr;
}

void free_aligned(void *raw_data) {
void *mallocPtr; //Initial malloc pointer
size_t extraBytes;

if (NULL == raw_data)
return;

/* Retrieve the extra padded byte info */
extraBytes = *((size_t*)raw_data - 1);

/* Get initial malloc ptr */
mallocPtr = (void*) ((size_t)raw_data - extraBytes);

pfree(mallocPtr);
}

Please note that I am using in these functions, the palloc and pfree
instead of malloc and free respectively. But the problem is that the
free_aligned function is not indeed freeing the allocated memory. Thus, I
would like to know if the PostgreSQL provides a memory function that
allocates aligned memory. If not, according to your experience, is there a
significance difference between the performance of the O_DIRECT or not?

Thank you,
Anderson

2016-09-02 7:24 GMT-03:00 Craig Ringer <craig(at)2ndquadrant(dot)com>:

> On 2 September 2016 at 01:12, Anderson Carniel <accarniel(at)gmail(dot)com>
> wrote:
> > Dear all,
> >
> > I am developing an extension for the PostgreSQL that write/read some
> > external files from the PostgreSQL. In order to write/read, I am using
> the
> > O_DIRECT flag and using the posix_memalign to allocate memory. I would
> like
> > to know if the postgresql internal library provides an equivalent
> function
> > for the posix_memalign since I am getting unexpected errors.
>
> "unexpected errors". Details please?
>
> If you're trying to allocate aligned memory, I believe PostgreSQL
> typically uses the TYPEALIGN macros (see c.h) but I'm painfully
> clueless in the area, so ... yeah. Don't trust me.
>
> I was a bit surprised not to see a MemoryContextAlloc or palloc
> variant that returns memory aligned to a given boundary.
>
> > All my
> > allocations are in the TopMemoryContext since I am working with several
> > buffers that must be alive while the PostgreSQL Server is activated.
>
> You can't posix_memalign into TopMemoryContext. Such memory is outside
> the memory context system, like memory directly malloc()'d.
>
> --
> Craig Ringer http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>