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Re: [Cluster-devel] GFS2: Use new workqueue scheme



Hi,

On Thu, 2010-09-09 at 15:48 +0200, Tejun Heo wrote:
> Hello,
> 
> On 09/09/2010 03:45 PM, Steven Whitehouse wrote:
> > On Thu, 2010-09-09 at 15:18 +0200, Tejun Heo wrote:
> >> Hello, Steven.
> >>
> >> Thanks for working on this.
> >>
> > I think it will be a big win for GFS2, particularly as the number of cpu
> > cores increases
> 
> Awesome. :-)
> 
> >>> -	glock_workqueue = create_workqueue("glock_workqueue");
> >>> +	glock_workqueue = alloc_workqueue("glock_workqueue", WQ_RESCUER |
> >>> +					  WQ_HIGHPRI | WQ_CPU_INTENSIVE |
> >>> +					  WQ_FREEZEABLE, 0);
> >>
> >> Does this really need WQ_HIGHRPI and WQ_CPU_INTENSIVE?
> >>
> > This would be a tasklet were it not for the fact that it needs to be
> > able to submit block I/O from time to time. It does need to be as fast
> > as possible since it directly affects the latency of operations using
> > large numbers of inodes.
> > 
> > I read your latest set of docs before assigning the flags, so I hope
> > I've understood it correctly.
> > 
> > The glock workqueue is involved in sending requests to the DLM and
> > processing the results of those requests, waking up waiting processes as
> > quickly as possible.
> 
> I see but then wouldn't WQ_CPU_INTENSIVE be unnecessary?  It's high
> priority but doesn't sound like they're gonna hog huge amount of CPU
> cycles.  Also, please note that the high priority is global across all
> workqueues and thus _must_ be used judiciously.  Well, if you were
> gonna use tasklets for it, it probably is a good candidate tho.
> 
Ah, I see. Maybe I misunderstood. I read the bit about using both
WQ_HIGHPRO and WQ_CPU_INTENSIVE which says:

"Work items queued on a highpri CPU-intensive wq start execution as soon
as resources are available and don't affect execution of other work
items."

and assumed that was what I needed, but maybe I don't need to
WQ_CPU_INTENSIVE as you suggest.

> >>>  	gfs_recovery_wq = alloc_workqueue("gfs_recovery",
> >>> -					  WQ_NON_REENTRANT | WQ_RESCUER, 0);
> >>> +					  WQ_NON_REENTRANT | WQ_RESCUER |
> >>> +					  WQ_UNBOUND | WQ_FREEZEABLE, 0);
> >>
> >> And do these need to be WQ_UNBOUND?  Unless the flags are specifically
> >> needed, I think it would be better to stick with the default.  I'm
> >> currently working on the documentation.  It's still not complete but
> >> please take a look for more information the behaviors of each flag.
> >
> > I wouldn't say that it was 100% a requirement, but they are long running
> > (potentially a few seconds, or even as far as a minute or two in extreme
> > cases). The recovery workqueue seems to meet this criteria:
> 
> Long running doesn't matter.  Normal workqueues can handle them
> perfectly fine.  The only cases you would want to use unbound
> workqueues are long running CPU hogs and (very) high fluctuation in
> the number of concurrent work items.
> 
It sounds like maybe the delete workqueue needs that, but that the
recovery one certainly doesn't in that case.

> >> 	* Long running CPU intensive workloads which can be better
> >> 	  managed by the system scheduler.
> > 
> > and the delete_workqueue seems to meet this criteria:
> > 
> >> 	* Wide fluctuation in the concurrency level requirement is
> >> 	  expected and using bound wq may end up creating large number
> >> 	  of mostly unused workers across different CPUs as the issuer
> >> 	  hops through different CPUs.
> > 
> > It may be that I didn't understand the docs correctly, but I think I've
> > found the right flags. The delete_workqueue is usually unused during
> > normal fs operation, but occasionally it might have a lot to do. It was
> > made a separate workqueue because it needs to be able to manipulate
> > glocks and thus must never block the glock workqueue.
> 
> Heh, these being one of the first conversions, I just wanna make sure.
> Long running CPU-intensive tasks would be things like works running
> RAID checksums, crypto stuff, IOW, stuff which are actually gonna
> perform a long calculation.  If a work is just gonna be blocking on
> locks for long period of time, there's no need to use the unbound
> ones.  So, unless I'm misunderstanding, I don't really think
> WQ_UNBOUND is necessary for the latter two.
> 
> Thanks.
> 
Yes, I'll try it without and see if that is ok. I am also trying to be a
bit cautious about the flags in case I accidentally introduce some
dependency which was not there before.

I'll follow up with an updated patch shortly,

Steve.



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