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@xxxxxxx>
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.
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