Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size

From: Minchan Kim
Date: Thu May 10 2012 - 20:25:45 EST

On 05/11/2012 09:03 AM, Dan Magenheimer wrote:

>> From: Minchan Kim [mailto:minchan@xxxxxxxxxx]
>> Subject: Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size
>> On 05/08/2012 11:00 PM, Dan Magenheimer wrote:
>>>> From: Minchan Kim [mailto:minchan@xxxxxxxxxx]
>>>>> zcache can potentially create a lot of pools, so the latter will save
>>>>> some memory.
>>>> Dumb question.
>>>> Why should we create pool per user?
>>>> What's the problem if there is only one pool in system?
>>> zcache doesn't use zsmalloc for cleancache pages today, but
>>> that's Seth's plan for the future. Then if there is a
>>> separate pool for each cleancache pool, when a filesystem
>>> is umount'ed, it isn't necessary to walk through and delete
>>> all pages one-by-one, which could take quite awhile.
>>> ramster needs one pool for each client (i.e. machine in the
>>> cluster) for frontswap pages for the same reason, and
>>> later, for cleancache pages, one per mounted filesystem
>>> per client
>> Fair enough.
>> But some subsystems can't want a own pool for not waste unnecessary memory.
>> Then, how about this interfaces like slab?
>> 1. zs_handle zs_malloc(size_t size, gfp_t flags) - share a pool by many subsystem(like kmalloc)
>> 2. zs_handle zs_malloc_pool(struct zs_pool *pool, size_t size) - use own pool(like kmem_cache_alloc)
>> Any thoughts?
> I don't have any objections to adding this kind of
> capability to zsmalloc. But since we are just speculating
> that this capability would be used by some future
> kernel subsystem, isn't it normal kernel protocol for
> this new capability NOT to be added until that future
> kernel subsystem creates a need for it.

Now zram makes pool per block device and a embedded system may use zram
for several block device, ex) swap device, several compressed tmpfs
In such case, share pool is better than private pool because embedded system
don't mount/umount frequently on such directories since booting.

> As I said in reply to the other thread, there is missing
> functionality in zsmalloc that is making it difficult for
> it to be used by zcache. It would be good if Seth
> and Nitin (and any other kernel developers) would work

So, if you guys post TODO list, it helps fix the direction.

> on those issues before adding capabilities for non-existent
> future users of zsmalloc.

I think it's not urgent than zs_handle mess.

> Again, that's just my opinion.

> Dan
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Kind regards,
Minchan Kim
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