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

[Andreas Dilger]

On Jan 28, 2014, at 5:26 AM, George Spelvin <linux@xxxxxxxxxxx> wrote:
>> The third part of the patch further increases the scalablity of an ext4
>> filesystem by having each ext4 fielsystem allocate and use its own private
>> mbcache structure, instead of sharing a single mcache structures across all
>> ext4 filesystems, and increases the size of its mbcache hash tables.
> 
> Are you sure this helps?  The idea behind having one large mbcache is
> that one large hash table will always be at least as well balanced as
> multiple separate tables, if the total size is the same.
> 
> If you have two size 2^n hash tables, the chance of collision is equal to
> one size  2^(n+1) table if they're equally busy, and if they're unequally
> busy. the latter is better.  The busier file system will take less time
> per search, and since it's searched more often than the less-busy one,
> net win.
> 
> How does it compare with just increasing the hash table size but leaving
> them combined?

Except that having one mbcache per block device would avoid the need
to store the e_bdev pointer in thousands/millions of entries.  Since
the blocks are never shared between different block devices, there
is no caching benefit even if the same block is on two block devices.

Cheers, Andreas

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