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SubjectRe: [RFC 0/2] guarantee natural alignment for kmalloc()
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On 3/21/19 3:23 AM, Matthew Wilcox wrote:
> On Wed, Mar 20, 2019 at 10:48:03PM +0100, Vlastimil Babka wrote:
>>
>> Well, looks like that's what happens. This is with SLAB, but the alignment
>> calculations should be common:
>>
>> slabinfo - version: 2.1
>> # name <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables <limit> <batchcount> <sharedfactor> : slabdata <active_slabs> <num_slabs> <sharedavail>
>> kmalloc-96 2611 4896 128 32 1 : tunables 120 60 8 : slabdata 153 153 0
>> kmalloc-128 4798 5536 128 32 1 : tunables 120 60 8 : slabdata 173 173 0
>
> Hmm. On my laptop, I see:
>
> kmalloc-96 28050 35364 96 42 1 : tunables 0 0 0 : slabdata 842 842 0
>
> That'd take me from 842 * 4k pages to 1105 4k pages -- an extra megabyte of
> memory.
>
> This is running Debian's 4.19 kernel:
>
> # CONFIG_SLAB is not set
> CONFIG_SLUB=y

Ah, you're right. SLAB creates kmalloc caches with:

#ifndef ARCH_KMALLOC_FLAGS
#define ARCH_KMALLOC_FLAGS SLAB_HWCACHE_ALIGN
#endif

create_kmalloc_caches(ARCH_KMALLOC_FLAGS);

While SLUB just:

create_kmalloc_caches(0);

even though it uses SLAB_HWCACHE_ALIGN for kmem_cache_node and
kmem_cache caches.


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