Re: slab: introduce the flag SLAB_MINIMIZE_WASTE

[Vlastimil Babka <vbabka@suse.cz>]

On 04/16/2018 04:37 PM, Mikulas Patocka wrote:
>>> Can you or Mikulas briefly summarize how the dependency is avoided, and
>>> whether if (something like) SLAB_MINIMIZE_WASTE were implemented, the
>>> dm-bufio code would happily switch to it, or not?
>>
>> git log eeb67a0ba04df^..45354f1eb67224669a1 -- drivers/md/dm-bufio.c
>>
>> But the most signficant commit relative to SLAB_MINIMIZE_WASTE is: 
>> 359dbf19ab524652a2208a2a2cddccec2eede2ad ("dm bufio: use slab cache for 
>> dm_buffer structure allocations")
>>
>> So no, I don't see why dm-bufio would need to switch to
>> SLAB_MINIMIZE_WASTE if it were introduced in the future.
> 
> Currently, the slab cache rounds up the size of the slab to the next power 
> of two (if the size is large). And that wastes memory if that memory were 
> to be used for deduplication tables.
> 
> Generally, the performance of the deduplication solution depends on how 
> much data can you put to memory. If you round 640KB buffer to 1MB (this is 
> what the slab and slub subsystem currently do), you waste a lot of memory. 
> Deduplication indices with 640KB blocks are already used in the wild, so 
> it can't be easily changed.

Thank you both for the clarification.

Vlastimil

> 
>> Mike
> 
> Mikulas
>