Re: [LSF/MM TOPIC] Parallelize file operation (like creation, unlink) under large shared directory

[wangdi <di.wang@whamcloud.com>]

On 01/22/2012 05:55 AM, Boaz Harrosh wrote:
> On 01/21/2012 10:54 PM, wangdi wrote:
>> Single directory performance is a critical in some use cases. For
>> example the multiple application threads might create hundreds of
>> thousands of files in a single directory simultaneously within a short
>> window of time.
>>
> read NFSD here ;-)
>
>> Currently, both filename lookup and file system modifying operations
>> (such as create and unlink) are protected with a single lock for the
>> entire directory. It might be useful to remove this lock, so multiple
>> application threads can access the directory simultaneously.
>>
> I agree about create, unlink, and so on. But don't we have some lockless
> look up in place since a few Kernels ago?

  The lockless lookup is only for lookups in the dcache and not in the backing filesystem, which is serialized by i_mutex in the VFS(real_lookup).


> But yes the topic is very interesting, though I'd suspect its hard to
> implement.

We actually already implemented this for ext4, and we saw a lot performance improvement(at least 30% improvements for open/create in a single directory)for lustre stack,
but we want to make this improvement accessible through the VFS. Probably XFS and Btrfs could also benefit from this.

Thanks
WangDi


>> Thanks
>> WangDi
>>
> Thanks
> Boaz