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SubjectRe: [PATCH v4 0/3] ext4: increase mbcache scalability
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I think the ext4 block groups are locked with the blockgroup_lock that has about the same number of locks as the number of cores, with a max of 128, IIRC.  See blockgroup_lock.h. 

While there is some chance of contention, it is also unlikely that all of the cores are locking this area at the same time.

Cheers, Andreas

> On Jan 24, 2014, at 14:38, Andi Kleen <andi@firstfloor.org> wrote:
>
> T Makphaibulchoke <tmac@hp.com> writes:
>
>> The patch consists of three parts.
>>
>> The first part changes the implementation of both the block and hash chains of
>> an mb_cache from list_head to hlist_bl_head and also introduces new members,
>> including a spinlock to mb_cache_entry, as required by the second part.
>
> spinlock per entry is usually overkill for larger hash tables.
>
> Can you use a second smaller lock table that just has locks and is
> indexed by a subset of the hash key. Most likely a very small
> table is good enough.
>
> Also I would be good to have some data on the additional memory consumption.
>
> -Andi
>
> --
> ak@linux.intel.com -- Speaking for myself only


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