Re: [RFC PATCH 00/14] nfsd/sunrpc: add support for a workqueue-based nfsd

[Tejun Heo <tj@kernel.org>]

Hello, Neil, Jeff.

On Tue, Dec 02, 2014 at 08:29:46PM -0500, Jeff Layton wrote:
> That's a good point. I had originally thought that max_active on an
> unbound workqueue would be the number of concurrent jobs that could run
> across all the CPUs, but now that I look I'm not sure that's really
> the case.

@max_active is a per-pool number.  By default, unbound wqs use
per-node pools, so @max_active would be per-node.  Currently,
@max_active is mostly meant as a protection against run-away
workqueues creating crazy number of workers, which has been enough for
the existing wq users.  *Maybe* it makes sense to make it actually
mean maximum concurrency which would prolly involve aggregated per-cpu
distribution mechanism so that we don't end up inc'ing and dec'ing the
same counter from all CPUs on each work item execution.

However, I do agree with Neil that making it user configurable is
almost always painful.  It's usually a question without a good answer
and the same value may behave differently depending on a lot of
implementation details and a better approach, probably, is to use
@max_active as the last resort protection mechanism while providing
automatic throttling of in-flight work items which is meaningful for
the specific use cases.

> I've heard random grumblings from various people in the past that
> workqueues have significant latency, but this is the first time I've
> really hit it in practice. If we can get this fixed, then that may be a
> significant perf win for all workqueue users. For instance, rpciod in
> the NFS client is all workqueue-based. Getting that latency down could
> really help things.
> 
> I'm currently trying to roll up a kernel module for benchmarking the
> workqueue dispatching code in the hopes that we can use that to help
> nail it down.

Definitely, there were some reportings but nothing really got tracked
down properly.  It'd be awesome to actually find out where the latency
is coming from.

Thanks!

-- 
tejun