linux-kernel - Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

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Message-ID: <54E77D0E.6060000@linux.intel.com>
Date:	Fri, 20 Feb 2015 10:29:34 -0800
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	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,
	josh@...htriplett.org, 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 tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods

On 2/20/2015 10:27 AM, Paul E. McKenney wrote:
> On Fri, Feb 20, 2015 at 09:32:39AM -0800, Arjan van de Ven wrote:
>>>>>> Does it really make a machine boot much faster? Why are people using
>>>>>> synchronous gp primitives if they care about speed? Should we not fix
>>>>>> that instead?
>>>>>
>>>>> The report I heard was that it provided 10-15% faster boot times.
>>>>
>>>> That's not insignificant; got more details? I think we should really
>>>> look at why people are using the sync primitives.
>>>
>>> I must defer to the people who took the exact measurements.
>>>
>>> But yes, once I have that info, I should add it to the commit log.
>>
>> so the two most obvious cases are
>>
>> Registering sysrq keys ... even when the old key code had no handler
>> (have a patch pending for this)
>>
>> registering idle handlers
>> (this is more tricky, it's very obvious abuse but the fix is less clear)
>>
>> there's a few others as well that I'm chasing down...
>> .. but the flip side, prior to running ring 3 code, why NOT do fast expedites?
>
> It would be good to have before-and-after measurements of actual
> boot time.  Are these numbers available?

I'll make you pretty graphs when I get home from collab summit, which
should be later today

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