Re: Nehalem

[Bill Davidsen <davidsen@tmr.com>]

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 <davidsen@tmr.com>
  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.