linux-kernel - Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Fri, 31 Jan 2014 16:08:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On Tue, Jan 28, 2014 at 01:19:10PM -0500, Waiman Long wrote:
> For single-thread performance (no contention), a 256K lock/unlock
> loop was run on a 2.4Ghz Westmere x86-64 CPU.  The following table
> shows the average time (in ns) for a single lock/unlock sequence
> (including the looping and timing overhead):
> 
>   Lock Type			Time (ns)
>   ---------			---------
>   Ticket spinlock		  14.1
>   Queue spinlock (Normal)	   8.8*

What CONFIG_NR_CPUS ?

Because for CONFIG_NR_CPUS < 128 (or 256 if you got !PARAVIRT), the fast
path code should be:

ticket:

  mov $0x100,eax
  lock xadd %ax,(%rbx)
  cmp %al,%ah
  jne ...

although my GCC is being silly and writes:

  mov $0x100,eax
  lock xadd %ax,(%rbx)
  movzbl %ah,%edx
  cmp %al,%dl
  jne ...

Which seems rather like a waste of a perfectly good cycle.

With a bigger NR_CPUS you do indeed need more ops:

  mov $0x10000,%edx
  lock xadd %edx,(%rbx)
  mov %edx,%ecx
  shr $0x10,%ecx
  cmp %dx,%cx
  jne ...


Whereas for the straight cmpxchg() you'd get something relatively simple
like:

  mov %edx,%eax
  lock cmpxchg %ecx,(%rbx)
  cmp %edx,%eax
  jne ...



Anyway, as soon as you get some (light) contention you're going to tank
because you have to pull in extra cachelines, which is sad.


I suppose we could from the ticket code more and optimize the
uncontended path, but that'll make the contended path more expensive
again, although probably not as bad as hitting a new cacheline.
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