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

[Thavatchai Makphaibulchoke]

On 01/28/2014 02:09 PM, Andreas Dilger wrote:
> 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
> 
> 
> 
> 
> 

Thanks George and Andreas for the comments.  Andreas you mentions a good point, e_bdev pointer is not needed when having one mb_cache for each block device.  I'll integrate that into my patch, removing the e_bdev pointer, and run some comparison between one large hash table vs multiple hash tables, as suggested by George.

Thanks,
Mak.

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