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Date: Wed, 21 Mar 2018 15:25:07 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
Subject: Re: [PATCH] slab: introduce the flag SLAB_MINIMIZE_WASTE
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To: Christopher Lameter <cl@linux.com>
Cc: Matthew Wilcox <willy@infradead.org>, Pekka Enberg <penberg@kernel.org>, David Rientjes <rientjes@google.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, dm-devel@redhat.com, Mike Snitzer <msnitzer@redhat.com>



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