[Cluster-devel] [PATCH 0/2] GFS2: inplace_reserve performance improvements

[Steven Whitehouse]

Hi,


On 28/09/18 14:43, Tim Smith wrote:
> On Friday, 28 September 2018 14:18:59 BST Steven Whitehouse wrote:
>> Hi,
>>
>> On 28/09/18 13:50, Mark Syms wrote:
>>> Hi Bon,
>>>
>>> The patches look quite good and would seem to help in the intra-node
>>> congestion case, which our first patch was trying to do. We haven't tried
>>> them yet but I'll pull a build together and try to run it over the
>>> weekend.
>>>
>>> We don't however, see that they would help in the situation we saw for the
>>> second patch where rgrp glocks would get bounced around between hosts at
>>> high speed and cause lots of state flushing to occur in the process as
>>> the stats don't take any account of anything other than network latency
>>> whereas there is more involved with a rgrp glock when state needs to be
>>> flushed.
>>>
>>> Any thoughts on this?
>>>
>>> Thanks,
>>>
>>> 	Mark.
>> There are a few points here... the stats measure the latency of the DLM
>> requests. Since in order to release a lock, some work has to be done,
>> and the lock is not released until that work is complete, the stats do
>> include that in their timings.
> I think what's happening for us is that the work that needs to be done to
> release an rgrp lock is happening pretty fast and is about the same in all
> cases, so the stats are not providing a meaningful distinction. We see the
> same lock (or small number of locks) bouncing back and forth between nodes
> with neither node seeming to consider them congested enough to avoid, even
> though the FS is <50% full and there must be plenty of other non-full rgrps.
>

It could well be that is the case. The system was designed to deal with 
inter-node contention on resource group locks. If there is no inter-node 
contention then the times should be similar and the system should have 
little effect. If the contention is all intra-node then we'd prefer a 
solution which increases the parallelism there - it covers more use 
cases than just allocation. Also it will help to keep related blocks 
closer too each other, particularly as the filesystem ages.

If might also be that there is a bug too - so worth looking closely at 
the numbers just to make sure that it is working as intended,

Steve.