[dm-devel] [PATCH] slab: introduce the flag SLAB_MINIMIZE_WASTE

[Mikulas Patocka]

On Wed, 21 Mar 2018, Christopher Lameter wrote:

> One other thought: If you want to improve the behavior for large scale
> objects allocated through kmalloc/kmemcache then we would certainly be
> glad to entertain those ideas.
> 
> F.e. you could optimize the allcations > 2x PAGE_SIZE so that they do not
> allocate powers of two pages. It would be relatively easy to make
> kmalloc_large round the allocation to the next page size and then allocate
> N consecutive pages via alloc_pages_exact() and free the remainder unused
> pages or some such thing.

It may be possible, but we'd need to improve the horrible complexity of 
alloc_pages_exact().

This is a trade-of between performance and waste. A power-of-two 
allocation can be done quicky, but it wastes a lot of space. 
alloc_pages_exact() wastes less space, but it is slow.

The question is - how many of these large-kmalloc allocations are 
short-lived and how many are long-lived? I don't know, I haven't measured 
it.

Mikulas