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@xxxxxxxxxx
<mailto:jahernan@xxxxxxxxxx>> wrote:
Hi Changwei,
On Tue, Oct 29, 2019 at 7:56 AM Changwei Ge <chge@xxxxxxxxxxxxxxxxx
<mailto:chge@xxxxxxxxxxxxxxxxx>> 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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