Date: Thu, 11 May 2006 12:19:20 -0500
From: Nathan Lynch <ntl@pobox.com>
To: Ashok Raj <ashok.raj@intel.com>
Cc: Andrew Morton <akpm@osdl.org>, Shaohua Li <shaohua.li@intel.com>,
       linux-kernel@vger.kernel.org, zwane@linuxpower.ca, vatsa@in.ibm.com
Subject: Re: [PATCH 0/10] bulk cpu removal support
Message-ID: <20060511171920.GB10833@localdomain>
References: <1147067137.2760.77.camel@sli10-desk.sh.intel.com> <20060510230606.076271b2.akpm@osdl.org> <20060511095308.A15483@unix-os.sc.intel.com>
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Ashok Raj wrote:
> On Wed, May 10, 2006 at 11:06:06PM -0700, Andrew Morton wrote:
> Hi Andrew,
> 
> > Shaohua Li <shaohua.li@intel.com> wrote:
> > >
> > > CPU hotremove will migrate tasks and redirect interrupts off dead cpu.
> > 
> > This seems an awful lot of code for something which happens so infrequently.
> > 
> > How big is the problem you're fixing here, and what are the
> > user-observeable effects of these changes?
> 
> This is useful when say a NUMA node is being removed. With new multi-core
> CPUs comming up, considering a 2 core with HT, we could have up to 4 logical
> per socket. On NUMA node with 4 sockets, a node removal will mean we 
> do 16 single cpu offlines. Each time the process and interrupts could
> end up on a CPU that might be removed just immediatly.

But offlining all the cpus in a node is already something that just
works.  If the user is all that concerned about not thrashing the
tasks running on that node, they would have a workload manager that
migrates the tasks off the node before shooting down cpus.  Similar
argument applies to interrupt affinity.

I really haven't seen a compelling argument for why this is needed,
just a bunch of handwaving so far, sorry.

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