From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Nehalem Date: Wed, 25 Mar 2009 08:32:45 -0400 Message-ID: <49CA246D.7080105@tmr.com> References: Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-smp-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Robert Hyatt Cc: linux-smp@vger.kernel.org Robert Hyatt wrote: > > I ran into an issue that may or may not be on the radar. Here goes: > > 1. The old hyperthreading fix works well for an old PIV with > hyperthreading, so that with two sockets, and 4 logical processors, > the compute-bound processes get balanced across the sockets, which > fixed the original hyper-threading bug everyone talked about. > > 2. I now have a dual-socket Nehalem box, 4 cores per socket. Someone > wanted to test hyper-threading, which I had disabled, and I found an > issue. > > It appears that the current process scheduling works fine for > balancing compute-bound processes across the two sockets to optimize > cache usage. But with hyper-threading, things go wrong. If I run 4 > compute-bound processes on this box, they will run two per socket just > fine. But on any one chip, it is probable that the two processes will > land on the same core, which is not good. > > My first thought was this needs a hiararchical approach. one big run > queue per socket, then N run queues per socket, one per physical core. > > Now the load can be balanced across the two sockets / chips using the > "high-level" pair of queues, and then balanced across the physical > cores on each socket using the low-level queues, to avoid running two > processes on one physical core, and none on another. > > Is a fix already in the works for this, or is this a new issue? I am > running 2.6.28.8 on this box. I am also not so happy with turbo-boost > either as it is giving some erratic timing data which I don't like for > my benchmark and tweak software development. But that's another > issue. not kernel-related. This might be an issue for me as well, I've just ordered parts to build several servers based on the i7 architecture, so I will have four cores + HT although they will all be in a single socket. I don't have any idea how well this will work, I suppose the HT can be turned off if needed, and it will run as well as the Q6600 system these will replace. -- bill davidsen CTO TMR Associates, Inc "You are disgraced professional losers. And by the way, give us our money back." - Representative Earl Pomeroy, Democrat of North Dakota on the A.I.G. executives who were paid bonuses after a federal bailout.