[lkml]   [2009]   [Jul]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Dynamic configure max_cstate
> When running a fio workload, I found sometimes cpu C state has
> big impact on the result. Mostly, fio is a disk I/O workload
> which doesn't spend much time with cpu, so cpu switch to C2/C3
> freqently and the latency is big.
> If I start kernel with idle=poll or processor.max_cstate=1,
> the result is quite good. Consider a scenario that machine is
> busy at daytime and free at night. Could we add a dynamic
> configuration interface for processor.max_cstate or something
> similiar with sysfs? So user applications could change the
> max_cstate dynamically? For example, we could add a new
> parameter to function cpuidle_governor->select to mark the
> highest c state.

max_cstate is a debug param. It isn't a run-time API and never will be.
User-space shouldn't need to know or care about C-states,
and if it appears it needs to, then we have a bug we need to fix.

The interface in Documentation/power/pm_qos_interface.txt
is supposed to handle this. Though if the underlying code
is not noticing IO interrupts, then it can't help.

Another thing to look at is processor.latency_factor
which you can change at run-time in

We multiply the advertised exit latency by this
before deciding to enter a C-state. The concept
is that ACPI reports a performance number, but what
we really want is a power break-even. Anyway,
we know the default mulitple is too low, and will be
raising it shortly.

Of course if the current code is not predicting any
IO interrupts on your IO-only workload, this, like
pm_qos, will not help.

-Len Brown, Intel Open Source Technology Center

 \ /
  Last update: 2009-07-28 21:49    [W:0.064 / U:0.476 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site