Re: [PATCH v4 0/3] ext4: increase mbcache scalability

[Andreas Dilger]

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@xxxxxxxxxxxxxx> wrote:
> 
> T Makphaibulchoke <tmac@xxxxxx> 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@xxxxxxxxxxxxxxx -- Speaking for myself only
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