Information for setting up SMT related parameters on linux 2.6.16 on POWER5

Will Schmidt will_schmidt at
Tue May 9 06:03:38 EST 2006

On Mon, 2006-05-08 at 12:09 -0600, Meswani, Mitesh wrote:
> Hello 


> I am looking to use the SMT related parameters like Snooze delay, HMT
> thread priorities, SMT ON/Off and wanted to know how to invoke and set
> them. I am running Open Suse 10 with 2.6.16.rc4-3-ppc64 kernel on
> eServer p590 2-way POWER5 partition. 

> I noticed the parameters in /sys/devices/system/cpu/cpu#   with the
> following :
> mmcr0  online       pmc2  pmc5  smt_snooze_delay
> mmcr1  physical_id  pmc3  pmc6  topology
> crash_notes  mmcra  pmc1         pmc4  purr

Some of this is unique to Logical Partitions and cpu's on POWER5 pSeries
hardware..    I think you've already found the useful entry "online",
and the rest would be just trivial information.

On the kernel boot commandline, you can add parms such as
"smt-snooze-delay=xxxx" to set the snooze_delay value, and
"smt-enabled=off" to turn off the secondary threads. 

As you've already found, you can echo values into the sys entries to
cause the cpus to go online or offline.  0=offline, 1=online.   You can
also adjust the value for snooze-delay.  This one defaults to '0'.  This
controls the amount of time the processor thread spins before declaring
that it's got no useful work to do and cedes itself. 

"mmcr*" and "pmc*" are performance counter registers and values.  These
are used by oprofile.

"purr" is a Processor Utilization Resource Register, indicating the
number of ticks that the processor has been in use.  

I believe "crash_notes" has to do with lkcd or crashdump.   

No idea on the "topology" entry.  possibly related to NUMA. 

> setting the value in online to 0 seems to turn off the logical
> processor, but I am not sure what the others are for and the meaning
> of their hex values? 
> It seems that there is include/asm-ppc64/processor.h  with macros like
> HMT_very_low()  ,  wonder if these can be set on command line
>  since I am running unmodified app binaries. 

the HMT_* macros are telling firmware that "this processor thread should
run at this priority".  Typically used when we're waiting on a spinlock.
I.e. When we are waiting on a spinlock, we hit the HMT_low macro to drop
our threads priority, allowing the other thread to use those extra
cycles finish it's stuff quicker, and maybe even release the lock we're
waiting for.          HMT_* is all within the kernel though, no exposure
to userspace apps.  

> Thanks, 
> Mitesh

Hope that is helpful..  

> Mitesh R. Meswani 
> Ph.D. Candidate 
> Research Associate, PLS2 Group
> Room 106 F, Department of Computer Science
> The University of Texas at El Paso, 
> El Paso, Texas 79968
> Tel: 915 747 8012 (O)
> Email: mmeswani at
> ______________________________________________________________________
> From: at on behalf of
> Geoff Levand
> Sent: Fri 5/5/2006 6:00 PM
> To: Paul Mackerras
> Cc: Arnd Bergmann; Levand,Geoffrey; linux-kernel at;
> linuxppc-dev at; Arnd Bergmann; cbe-oss-dev at
> Subject: Re: [PATCH 04/13] cell: remove broken __setup_cpu_be function

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