linux-kernel - Re: [RFC][PATCH] fs: optimize inotify/fsnotify code for unwatched files

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Date:	Wed, 24 Jun 2015 09:50:50 -0700
From:	Dave Hansen <dave@...1.net>
To:	paulmck@...ux.vnet.ibm.com
CC:	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <ak@...ux.intel.com>, dave.hansen@...ux.intel.com,
	akpm@...ux-foundation.org, jack@...e.cz, viro@...iv.linux.org.uk,
	eparis@...hat.com, john@...nmccutchan.com, rlove@...ve.org,
	tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] fs: optimize inotify/fsnotify code for unwatched
 files

On 06/22/2015 05:26 PM, Paul E. McKenney wrote:
> OK, here is an experimental patch that provides a fast-readers variant
> of RCU, forward-ported from v3.3.  Because we didn't have call_srcu()
> and srcu_barrier() back then, it is not a drop-in replacement for SRCU,
> so you need to adapt the code to the API, which means putting an "fr"
> in front of the "srcu" in the API members.
> 
> Understood on the overhead of the memory-barrier instruction showing
> up consistently.  My point was instead that getting rid of this
> memory-barrier instruction does not come for free, as it greatly
> increases the latency of synchronize_frsrcu().  In a real workload,
> it is entirely possible that the savings from eliminating the memory
> barrier are overwhelmed by the increased grace-period latency.
> 
> Anyway, the patch is below.  Very lightly tested.

This does give a very similar performance boost as the other
optimization I posted.  I measured this patch to boost the writes/second
by 11.0% while my previous optimization did 10.8%.

I don't think this workload will see any of the overhead of the
synchronize_frsrcu(), though, but this helps confirm the source of the
overhead.
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