Re: HT (Hyper Threading) aware process scheduling doesn't work as it should

[Chris Friesen <chris.friesen@genband.com>]

On 10/30/2011 03:26 PM, Henrique de Moraes Holschuh wrote:
> On Sun, 30 Oct 2011, Artem S. Tashkinov wrote:
>> I've found out that even on Linux 3.0.8 the process scheduler doesn't correctly distributes
>> the load amongst virtual CPUs. E.g. on a 4-core system (8 total virtual CPUs) the process
>> scheduler often run some instances of four different tasks on the same physical CPU.
>
> Please check how your sched_mc_power_savings and sched_smt_power_savings
> tunables.   Here's the doc from lesswats.org:
>
> 'sched_mc_power_savings' tunable under /sys/devices/system/cpu/ controls
> the Multi-core related tunable. By default, this is set to '0' (for
> optimal performance). By setting this to '1', under light load
> scenarios, the process load is distributed such that all the cores in a
> processor package are busy before distributing the process load to other
> processor packages.
>
> [...]
>
> 'sched_smt_power_savings' tunable under /sys/devices/system/cpu/
> controls the multi-threading related tunable. By default, this is set to
> '0' (for optimal performance). By setting this to '1', under light load
> scenarios, the process load is distributed such that all the threads in
> a core and all the cores in a processor package are busy before
> distributing the process load to threads and cores, in other processor
> packages.


I'm currently running Fedora 14 (2.6.35.14-97.fc14.x86_64 kernel) on an 
i5 560M cpu.  It's supposed to have 2 cores, with hyperthreading.

I created a tiny cpu burner program that just busy-loops.  Running two 
instances on my system they were always scheduled on separate physical 
cpus regardless of the values in sched_mc_power_savings or 
sched_smt_power_savings.

Running four instances they were always spread across all four "cpus".

With the newer 8-core chips (plus HT) in multi-socket boards with 
package-level turbo-boost and NUMA memory access this is going to get 
really interesting...

Chris


-- 
Chris Friesen
Software Developer
GENBAND
chris.friesen@genband.com
www.genband.com