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Date: Wed, 18 Mar 2015 09:42:40 -0700
From: josh@joshtriplett.org
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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        Stephen Hemminger <stephen@networkplumber.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        David Howells <dhowells@redhat.com>
Subject: Re: [RFC PATCH v14] sys_membarrier(): system/process-wide memory
 barrier (x86)
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On Wed, Mar 18, 2015 at 12:23:02PM -0400, Mathieu Desnoyers wrote:
> memory barriers in reader:    1701557485 reads, 3129842 writes
> signal-based scheme:          9825306874 reads,    5386 writes
> sys_membarrier:               7992076602 reads,     220 writes
> 
> The dynamic sys_membarrier availability check adds some overhead to
> the read-side compared to the signal-based scheme, but besides that,
> with the expedited scheme, we can see that we are close to the read-side
> performance of the signal-based scheme. However, this non-expedited
> sys_membarrier implementation has a much slower grace period than signal
> and memory barrier schemes.

Doesn't the query flag allow you to find out in advance rather than
dynamically within the reader?  What's the reader performance if you
hardcode availability of membarrier?

- Josh Triplett