Re: [PATCH] record last user if malloc request is exact 4k

[Pekka Enberg <penberg@cs.helsinki.fi>]

Hi,

On Mon, Jan 30, Pekka Enberg wrote:
> > For architectures that have 4K pages, adding debugging overhead to 4K
> > objects is pretty much the worst case. Any particular reason you want
> > this?

On Mon, 2006-01-30 at 21:25 +0100, Olaf Hering wrote:
> I'm just curious.

Oh, okay. One or more pages are allocated for each slab depending on
object size. Each slab is then divided into equal-sized buffers which
must fit at one object. A buffer also has optional padding so that
objects respect alignment rules given to a cache. In addition, when
CONFIG_DEBUG_SLAB is enabled, the buffer contains space for red-zone on
left and right of the object and last user information.

When all debugging is enable, the total overhead is padding plus 4 *
sizeof(void *) for red-zoning and one more sizeof(void *) for the last
caller address. If you allow debugging for 4K objects, you have huge
internal fragmentation for both 4 KB and 8 KB pages (almost one full
page). The current 4095 limit isn't perfect either but increasing will
only make things worse. I think it's designed for
<linux/kmalloc_sizes.h> so that the last general object size with
debugging is 2048.

			Pekka