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

From: Tejun Heo
Date: Tue Dec 02 2014 - 14:18:23 EST

Hello, Jeff.

On Tue, Dec 02, 2014 at 01:24:09PM -0500, Jeff Layton wrote:
> 2) get some insight about the latency from those with a better
> understanding of the CMWQ code. Any thoughts as to why we might be
> seeing such high latency here? Any ideas of what we can do about it?

The latency is prolly from concurrency management. Work items which
participate in concurrency management (the ones on per-cpu workqueues
w/o WQ_CPU_INTENSIVE set) tend to get penalized on latency side quite
a bit as the "run" durations for all such work items end up being
serialized on the cpu. Setting WQ_CPU_INTENSIVE on the workqueue
disables concurrency management and so does making the workqueue
unbound. If strict cpu locality is likely to be beneficial and each
work item isn't likely to consume huge amount of cpu cycles,
WQ_CPU_INTENSIVE would fit better; otherwise, WQ_UNBOUND to let the
scheduler do its thing.


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