linux-kernel - Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING

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Date:	Wed, 15 Jun 2016 15:31:51 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <linux-alpha@...r.kernel.org>,
	<linux-ia64@...r.kernel.org>, <linux-s390@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS
 for better disambiguation

On 06/15/2016 01:43 PM, Peter Zijlstra wrote:
> On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote:
>> even the reduced maximum of about 16k (32-bit) or 1G (64-bit) should
>> be more than enough for the foreseeable future.
> So what happens if I manage to create 16k+ threads on my 32bit kernel
> and get them all to do mmap() or so at the same time.
>
> That doesn't seem too far fetched.
>
> Then again, with double that (the current limit) that doesn't seem
> impossible either.

To hit the limit, we need to have all the threads calling down_write() 
at exactly the same instance in time which, I think, is pretty hard to 
do. Also, I don't believe you will ever see a 16k-cpu massive SMP system 
running on 32-bit kernel. I can imagine such a system running on 64-bit 
kernel, but certainly not 32-bit.

Cheers,
Longman