Message-ID: <47BD44FF.7070104@linux.vnet.ibm.com>
Date: Thu, 21 Feb 2008 15:01:43 +0530
From: Balbir Singh <balbir@linux.vnet.ibm.com>
Reply-To: balbir@linux.vnet.ibm.com
Organization: IBM
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To: Ingo Molnar <mingo@elte.hu>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
       Dhaval Giani <dhaval@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
Subject: Re: Make yield_task_fair more efficient
References: <20080221053321.GA26918@balbir.in.ibm.com> <20080221060427.GA9159@elte.hu> <47BD1F75.5030506@linux.vnet.ibm.com> <20080221070733.GA13694@elte.hu> <47BD2A99.3010608@linux.vnet.ibm.com> <1203583439.6243.119.camel@lappy> <47BD3B56.3090404@linux.vnet.ibm.com> <20080221090430.GA20055@elte.hu>
In-Reply-To: <20080221090430.GA20055@elte.hu>
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Ingo Molnar wrote:
> * Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> 
>> If you insist that sched_yield() is bad, I might agree, but how does 
>> my patch make things worse. [...]
> 
> it puts new instructions into the hotpath.
> 
>> [...] In my benchmarks, it has helped the sched_yield case, why is 
>> that bad? [...]
> 
> I had the same cache for the rightmost task in earlier CFS (it's a 
> really obvious thing) but removed it. It wasnt a bad idea, but it hurt 
> the fastpath hence i removed it. Algorithms and implementations are a 
> constant balancing act.

This is more convincing, was the code ever in git? How did you measure the
overhead? What are your plans for reports with regressions where
kernel.compat_sched_yield is set to 1?

I have an alternate approach in mind (that I need to find time for),
threaded-rbtrees. Walking the tree is really efficient, specially finding
successor of a node.


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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