[src/trunk]: src/share/man/man4 Document T-states. One less item in CAVEATS.

[jruoho <jruoho%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/61c4a335dd8d
branches:  trunk
changeset: 757107:61c4a335dd8d
user:      jruoho <jruoho%NetBSD.org@localhost>
date:      Sun Aug 15 05:09:30 2010 +0000

description:
Document T-states. One less item in CAVEATS.

diffstat:

 share/man/man4/acpicpu.4 |  78 +++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 68 insertions(+), 10 deletions(-)

diffs (128 lines):

diff -r 429403df1f82 -r 61c4a335dd8d share/man/man4/acpicpu.4
--- a/share/man/man4/acpicpu.4  Sun Aug 15 04:35:16 2010 +0000
+++ b/share/man/man4/acpicpu.4  Sun Aug 15 05:09:30 2010 +0000
@@ -1,4 +1,4 @@
-.\" $NetBSD: acpicpu.4,v 1.8 2010/08/11 17:28:59 jruoho Exp $
+.\" $NetBSD: acpicpu.4,v 1.9 2010/08/15 05:09:30 jruoho Exp $
 .\"
 .\" Coyright (c) 2010 Jukka Ruohonen <jruohonen%iki.fi@localhost>
 .\" All rights reserved.
@@ -24,7 +24,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd August 11, 2010
+.Dd August 15, 2010
 .Dt ACPICPU 4
 .Os
 .Sh NAME
@@ -44,9 +44,6 @@
 Typically the
 .Tn ACPI
 processor functionality is grouped into so-called C-, P-, and T-states.
-Presently,
-.Nm
-supports C- and P-states.
 .Ss C-states
 The processor power states, or C-states,
 are low-power modes that can be used when the
@@ -186,6 +183,68 @@
 implementation reacts to these events by imposing the dynamic maximum, but
 .Nm
 does not take any actions to manipulate the frequencies by itself.
+.Ss T-states
+Processor T-states, or
+.Dq throttling states ,
+can be used to actively modulate the
+time a processor is allowed to execute.
+Outside the
+.Tn ACPI
+nomenclature, throttling may refer to
+.Dq on-demand clock modulation
+.Pq Tn ODCM ,
+among others.
+.Pp
+The concept of
+.Dq duty cycle
+is relevant to T-states.
+It is generally defined to be a fraction of time that a system is in an
+.Dq active
+state.
+The T0-state has always a duty cycle of 100 \&%,
+and thus, comparable to the C0-state, the processor is fully active.
+Each additional higher-numbered T-state indicates lower duty cycles.
+At most eight T-states may be available, although also T-states use
+.Tn DVFS ;
+both the maximum and the minimum available T-state may change dynamically.
+.Pp
+The duty cycle does not refer to the actual clock signal,
+but to the time period in which the clock signal is allowed
+to drive the processor chip.
+For instance, if a T-state has a duty cycle of 75 \&%, the
+.Tn CPU
+runs at the same clock frequency and uses the same voltage,
+but 25 \&% of the time the
+.Tn CPU
+is forced to idle.
+Because of this, the use of T-states may
+severely reduce system performance.
+.Pp
+There are two typical situations for throttling:
+power management and thermal control.
+As a technique to save power,
+T-states are largely an artefact from the past.
+There was a short period in the x86 lineage when P-states
+were not yet available and throttling was considered
+as an option to modulate the processor power consumption.
+The approach was however quickly abandoned.
+In modern x86 systems P-states should be preferred in all circumstances.
+It is also more beneficial to move from the C0-state
+to deeper C-states than it is to actively force down the
+duty cycle of a processor.
+.Pp
+But T-states have retained their use as a last line
+of defence against critical thermal conditions.
+Many x86 processors include a catastrophic shutdown detector.
+When the processor core temperature reaches this factory defined trip-point,
+the processor execution is halted without any software control.
+Before this fatal condition, it is possible to use throttling
+for a short period of time in order to force the temperatures to lower levels.
+The thermal control modulation is typically started only when
+the system is in the highest-power P-state and
+a high temperature situation exists.
+After the temperatures have returned to non-critical levels,
+the modulation ceases.
 .Ss Statistics
 The
 .Nm
@@ -197,7 +256,7 @@
 .Fl e
 flag).
 Note that valid per-CPU data is available only for C-states.
-.Ss System information variables
+.Ss System control variables
 The
 .Nm
 driver uses the same
@@ -216,6 +275,7 @@
 may however remove the system control variables without further notice.
 .Sh SEE ALSO
 .Xr acpi 4 ,
+.Xr acpitz 4 ,
 .Xr cpu_idle 9
 .Sh HISTORY
 The
@@ -251,11 +311,9 @@
 is currently supported only on Intel
 .Tn CPUs .
 .It
-Neither processor thermal control (see
+Processor thermal control (see
 .Xr acpitz 4 )
-nor
-.Dq throttling
-are currently supported.
+is not yet supported.
 .It
 There is currently neither a well-defined, machine-independent
 .Tn API