Subject: RE: Mainline kernel OLTP performance update
From: Steven Rostedt <srostedt@redhat.com>
To: Chris Mason <chris.mason@oracle.com>
Cc: "Ma, Chinang" <chinang.ma@intel.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Matthew Wilcox <matthew@wil.cx>,
       "Wilcox, Matthew R" <matthew.r.wilcox@intel.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "Tripathi, Sharad C" <sharad.c.tripathi@intel.com>,
       "arjan@linux.intel.com" <arjan@linux.intel.com>,
       "Kleen, Andi" <andi.kleen@intel.com>,
       "Siddha, Suresh B" <suresh.b.siddha@intel.com>,
       "Chilukuri, Harita" <harita.chilukuri@intel.com>,
       "Styner, Douglas W" <douglas.w.styner@intel.com>,
       "Wang, Peter Xihong" <peter.xihong.wang@intel.com>,
       "Nueckel, Hubert" <hubert.nueckel@intel.com>,
       "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
       Andrew Vasquez <andrew.vasquez@qlogic.com>,
       Anirban Chakraborty <anirban.chakraborty@qlogic.com>,
       Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>,
       Gregory Haskins <ghaskins@novell.com>,
       Rusty Russell <rusty@rustcorp.com.au>
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Organization: Red Hat
Date: Mon, 19 Jan 2009 14:07:35 -0500
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On Mon, 2009-01-19 at 13:55 -0500, Chris Mason wrote:

> I wasn't actually looking at the cost of the checks, even though they do
> look higher (if they are using CONFIG_CPUMASK_OFFSTACK anyway).
> 
> The 2.6.24 code would trigger a rescheduling interrupt only when the
> prio of the inbound task was higher than the running task.
> 
> This workload has a large number of equal priority rt tasks that are not
> bound to a single CPU, and so I think it should trigger more
> preempts/reschedules with the today's check_preempt_equal_prio().

Ah yeah. This is one of the things that shows RT being more "responsive"
but less on performance. An RT task wants to run ASAP even if that means
there's a chance of more interrupts and higher cache misses.

The old way would be much faster in general through put, but I measured
RT tasks taking up to tens of milliseconds to get scheduled. This is
unacceptable for an RT task.

-- Steve


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