[Gluster-devel] Improvement of eager locking

[Pranith Kumar Karampuri]

On 01/16/2015 05:40 PM, Xavier Hernandez wrote:
> On 01/16/2015 04:58 AM, Pranith Kumar Karampuri wrote:
>>
>> On 01/15/2015 10:53 PM, Xavier Hernandez wrote:
>>> Hi,
>>>
>>> currently eager locking is implemented by checking the open-fd-count
>>> special xattr for each write. If there's more than one open on the
>>> same file, eager locking is disabled to avoid starvation.
>>>
>>> This works quite well for file writes, but makes eager locking
>>> unusable for other request types that do not involve an open fd (in
>>> fact, this method is only for writes on regular files, not reads or
>>> directories). This may cause a performance problem for other
>>> operations, like metadata.
>>>
>>> To be able to use eager locking for other purposes, what do you think
>>> about this proposal:
>>>
>>> Instead of implementing open-fd-count on posix xlator, do something
>>> similar but in locks xlator. The difference will be that locks xlator
>>> can use the pending locking information to determine if there are
>>> other processes waiting for a resource. If so, set a flag in the cbk
>>> xdata to let high level xlators know that they should not use eager
>>> locking (this can be done only upon request by xdata).
>>>
>>> I think this way provides a more precise way to avoid starvation and
>>> maximize performance at the same time, and it can be used for any
>>> request even if it doesn't depend on an fd.
>>>
>>> Another advantage is that if one file has been opened multiple times
>>> but all of them from the same glusterfs client, that client could use
>>> a single inodelk to manage all the accesses, not needing to release
>>> the lock. Current implementation in posix xlator cannot differentiate
>>> from opens from the same client or different clients.
>>>
>>> What do you think ?
>> I like the idea. So basically we can propagate list_empty information of
>> 'blocking_locks' list. And for sending locks, we need to use lk-owner
>> based on gfid so that locks from same client i.e. lkowner+transport are
>> granted irrespective of conflicting locks. The respective xls need to
>> make sure to order the fops so that they don't step on each other in a
>> single process. This can be used for entry-locks also.
>
> I don't understand what are the benefits of checking for 
> lkowner+transport to grant a lock bypassing conflicts. It seems 
> dangerous and I don't see exactly how this can help the upper xlator. 
> If this xlator already needs to take care of fop ordering for each 
> inode, it can share the same lock. It seems there's no need to do 
> additional locking calls. I may be missing some detail though.
Afr at the time of 3.2 or 3.3 used to take full file lock for doing 
self-heal. But this scheme was useless for VM healing. So we had to 
migrate the locking in a backward compatible way, so this strategy was 
employed where healing will take 128k chunk lock at a time heal that 
chunk and move to next chunk, but at no point we needed another 
self-heal to start. So the locking scheme came to be: Take a full file 
lock, find good/bad copies, take a lock on chunk-1, unlock full lock, 
heal chunk-1 then take a lock on chunk-2 unlock the lock on chunk-1 etc. 
To have this we needed the locks to be granted even when there were 
conflicting locks, so we chose (lk-owner+ transport) being same as a way 
to grant conflicting locks. We found that this can lead to another 
problem where truncate fop etc can hang, so we are moving to a different 
mechanism now. You can find the complete lock evolution document here 
https://github.com/gluster/glusterfs/blob/master/doc/code/xlators/cluster/afr/afr-locks-evolution.md

>
> Thinking a litle more about the way to detect multiple accesses to the 
> same inode using the list of pending locks, there's a case where some 
> more logic must be added to avoid unnecessary delays.
>
> Suppose you receive a request for an inode from one client. If there 
> isn't anyone else waiting, a flag is set into the answer indicating 
> that there's no conflict. After that the caller begins an eager lock 
> timer because there isn't anyone else waiting. During that timeout, 
> another client tries to access the same inode. It will block until the 
> eager lock timer expires (at this time it will release the inode lock) 
> or another request from the first client arrives (in this case the 
> request is served and the result will indicate that it should release 
> the lock since there are other clients waiting). When the lock is 
> released, it will be granted to the other client. It's possible that 
> this client completes the request before the first one tries to 
> acquire the lock again (because it had more requests pending), causing 
> that the second client initiates another eager lock timer because 
> there were no other client waiting at the moment of executing the 
> request. This is an unnecessary delay.
>
> To avoid this problem, we could add a flag in the inodelk/entrylk 
> calls to indicate that the lock is released to let other clients to 
> proceed, but we will want the lock again as soon as possible. It would 
> be as a combined unlock and lock on the same inode sent in a single 
> request. This way we avoid one round trip and locks xlator has more up 
> to date information to decide if there are concurrent accesses or not.
The problem I see with the combined unlock+lock is the order of arrival 
of these unlock requests + disconnects + possibility of deadlocks. i.e. 
In best case scenario, the new-lock requests that come as part of the 
unlock request go to pending-locks. But in the worst case it can end up 
with granted lock on some bricks and pending-lock on some bricks because 
of disconnects on just those bricks.
To prevent such cases, we need to wind the unlocks in an order. Which 
will add to the latency. One other way to get the desired behaviour of 
granted-lock from mount-2 to find that it shouldn't start eager-lock 
because of pending locks from mount-1, mount-1 should send the lock with 
different lk-owner before unlock of the previous one so that we are 
guaranteed to notice the presence of pending-locks at the time of 
granting lock from mount-2.

Pranith
>
> Xavi