CPU numbering & hyperthreading

CPU numbering & hyperthreading

[Andrew Athan]

On an Intel Xeon dual CPU machine running 2.6.16 and up...

I have two highly CPU/memory/network intensive processes with 3-5 
threads each.  I am using sched_setaffinity calls to make sure these two 
processes never compete for the same physical CPU.  Am I right to assume 
that CPU #0 and #1 vs CPU #2 and #3 are separate physical CPUs on a 
2-CPU w/ hyperthreading box?

I've spent some time looking, but I did not find documentation on 
exactly how CPUs are numbered in a hyperthreaded box.

For a process with N threads where N is generally <=5, where each thread 
shares access to the same large (300Mb) data structure across several 
threads, and which pumps the data from memory to a TCP socket, making 
many futex, select, write(), send() network calls (but no disk I/O), I 
assume it is best to keep said process on the same physical CPU but 
allow use both logical processors on that CPU (vs. keeping it to a 
single logical CPU)?

Thanks,
A.

Re: CPU numbering & hyperthreading

[Keith Mannthey]

On 7/17/06, Andrew Athan <aathan_linux_kernel_1542@cloakmail.com> wrote:
>
> On an Intel Xeon dual CPU machine running 2.6.16 and up...
>
> I have two highly CPU/memory/network intensive processes with 3-5
> threads each.  I am using sched_setaffinity calls to make sure these two
> processes never compete for the same physical CPU.  Am I right to assume
> that CPU #0 and #1 vs CPU #2 and #3 are separate physical CPUs on a
> 2-CPU w/ hyperthreading box?
>
> I've spent some time looking, but I did not find documentation on
> exactly how CPUs are numbered in a hyperthreaded box.

It is up to the bios how the cpus are layed out.  Any easy way to
check for the info you want is to look at /proc/cpuinfo (you can also
get the info out of /sys/devices/system/cpu/cpu[x]/topology).  Cpus
with the same physical id are in the same package.  The info (core_id
core_siblings  physical_package_id  thread_siblings
and like kind from proc) describes the topology.

Thanks,
  Keith

Re: CPU numbering & hyperthreading

[Chuck Ebbert]

In-Reply-To: <44BC4200.90308@cloakmail.com>

On Mon, 17 Jul 2006 22:05:52 -0400, Andrew Athan wrote:
>
> I have two highly CPU/memory/network intensive processes with 3-5 
> threads each.  I am using sched_setaffinity calls to make sure these two 
> processes never compete for the same physical CPU.  Am I right to assume 
> that CPU #0 and #1 vs CPU #2 and #3 are separate physical CPUs on a 
> 2-CPU w/ hyperthreading box?
> 
> I've spent some time looking, but I did not find documentation on 
> exactly how CPUs are numbered in a hyperthreaded box.

There's no fixed numbering.

If you want to know the die/core/thread layout, check out
Documentation/cputopology.txt.

-- 
Chuck
And did we tell you the name of the game, boy, we call it Riding the Gravy Train.