[Gluster-devel] [RFC] inode table locking contention reduction experiment

[Changwei Ge]

Hi Amar,

On 2019/10/31 6:30 下午, Amar Tumballi wrote:
> 
> 
> On Wed, Oct 30, 2019 at 4:32 PM Xavi Hernandez <jahernan at redhat.com 
> <mailto:jahernan at redhat.com>> wrote:
> 
>     Hi Changwei,
> 
>     On Tue, Oct 29, 2019 at 7:56 AM Changwei Ge <chge at linux.alibaba.com
>     <mailto:chge at linux.alibaba.com>> wrote:
> 
>         Hi,
> 
>         I am recently working on reducing inode_[un]ref() locking
>         contention by
>         getting rid of inode table lock. Just use inode lock to protect
>         inode
>         REF. I have already discussed a couple rounds with several
>         Glusterfs
>         developers via emails and Gerrit and basically get understood on
>         major
>         logic around.
> 
>         Currently, inode REF can be ZERO and be reused by increasing it
>         to ONE.
>         This is IMO why we have to burden so much work for inode table when
>         REF/UNREF. It makes inode [un]ref() and inode table and
>         dentries(alias)
>         searching hard to run concurrently.
> 
>         So my question is in what cases, how can we find a inode whose
>         REF is ZERO?
> 
>         As Glusterfs store its inode memory address into kernel/fuse,
>         can we
>         conclude that only fuse_ino_to_inode() can bring back a REF=0 inode?
> 
> 
> Xavi's answer below provides some insights. and same time, assuming that 
> only fuse_ino_to_inode() can bring back inode from ref=0 state (for 
> now), is a good start.
> 
> 
>     Yes, when an inode gets refs = 0, it means that gluster code is not
>     using it anywhere, so it cannot be referenced again unless kernel
>     sends new requests on the same inode. Once refs=0 and nlookup=0, the
>     inode can be destroyed.
> 
>     Inode code is quite complex right now and I haven't had time to
>     investigate this further, but I think we could simplify inode
>     management significantly (specially unref) if we add a reference
>     when nlookup becomes > 0, and remove a reference when
>     nlookup becomes 0 again. Maybe with this approach we could avoid
>     inode table lock in many cases. However we need to make sure we
>     correctly handle invalidation logic to keep inode table size under
>     control.
> 
> 
> My suggestion is, don't wait for a complete solution for posting the 
> patch. Let us get a chance to have a look at WorkInProgress patches, so 
> we can have discussions on code itself. It would help to reach better 
> solutions sooner.

Agree.

I have almost implemented my draft design for this experiment.
The immature code has been pushed to my personal Glusterfs repo[1].

Now it's a single large patch, I will split it to patches when I decide 
to push it to Gerrit for review convenience. If you prefer to push it to 
Gerrit for a early review and discussion, I can do that :-). But I am 
still doing some debug stuff.

My work includes:

1. Move inode refing and unrefing logic unrelated logic out from 
`__inode_[un]ref()` hence to reduce their arguments.
2. Add a specific ‘ref_lock’ to inode to keep ref/unref atomicity.
3. As `inode_table::active_size` is only used for debug purpose, convert 
it to atomic variable.
4. Factor out pruning inode.
5. In order to run inode search and grep run concurrently, firstly  use 
RDLOCK  and then convert it WRLOCK if necessary.
6. Inode table lock is not necessary for inode ref/unref unless we have 
to move it between table lists.

etc...

Any comments, ideas, suggestions are kindly welcomed.

Thanks,
Changwei

[1]:
https://github.com/changweige/glusterfs/commit/d7226d2458281212af19ec8c2ca3d8c8caae1330

> 
>     Regards,
> 
>     Xavi
> 
> 
> 
>         Thanks,
>         Changwei
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