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

From: Tejun Heo
Date: Wed Dec 03 2014 - 10:56:59 EST

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.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at