linux-kernel - Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones

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Date:	Wed, 1 Jul 2015 14:20:01 -0700
From:	josh@...htriplett.org
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods
 encouraging normal ones

On Wed, Jul 01, 2015 at 01:09:36PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 01, 2015 at 07:02:42PM +0200, Peter Zijlstra wrote:
> > USB sure, but a backing dev is involved in nfs clients, loopback and all
> > sorts of block/filesystem like setups.
> > 
> > unmount an NFS mount and voila expedited rcu, unmount a loopback, tada.
> > 
> > All you need is a regular server workload triggering any of that on a
> > semi regular basis and even !rt people might start to notice something
> > is up.
> 
> I don't believe that latency-sensitive systems are going to be messing
> with remapping their storage at runtime, let alone on a regular basis.
> If they are not latency sensitive, and assuming that the rate of
> storage remapping is at all sane, I bet that they won't notice the
> synchronize_rcu_expedited() overhead.  The overhead of the actual
> remapping will very likely leave the synchronize_rcu_expedited() overhead
> way down in the noise.
> 
> And if they are doing completely insane rates of storage remapping,
> I suspect that the batching in the synchronize_rcu_expedited()
> implementation will reduce the expedited-grace-period overhead still
> further as a fraction of the total.

Consider the case of container-based systems, calling mount as part of
container setup and umount as part of container teardown.

And those workloads are often sensitive to latency, not throughput.

- Josh Triplett
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