Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS VOLUME 3 REV 2.3, Manual

Page: 570 / 1898

2:322 Volume 2, Part 1: Processor Abstraction Layer
As seen above, for a HCDD, the PAL_GET_PSTATE procedure allows the caller to get
feedback on the dynamic performance of the processor over a software-controlled time
period. The caller can use this information to get better system utilization over a
subsequent time period by changing the P-state in correlation with the current
workload demand. The caller can also use PAL_GET_PSTATE to see the most recent
P-state set for this logical processor (
type =0) and the instantaneous current P-state
that the domain parameters are set to ( type =3). Platform power-caps do not affect
either of these return values.
HIDD: If the logical processor belongs to a hardware-independent dependency domain,
a weighted-average performance index can be returned by PAL_GET_PSTATE (
type =1
or 2). Since software could calculate the performance index based on P-states it set,
the weighted-average performance index is only of value when factoring in the effect of
platform power-caps.
Note that P-state transitions typically do not happen instantaneously. An
implementation-specific amount of time is required for a given transition to complete.
The computation of the weighted-average
performance_index may not take into
account the fact that transitions of power/performance are gradual, but may be done as
though they were instantaneous at the point when the transition starts. The
expectation is that any errors in computing the
performance_index due to
non-instantaneous transitions to higher and lower P-states will tend to cancel out, and
to the extent that they do not, will be insignificant.
11.6.1.4 Variable P-state Performance
Some processors support variable P-state performance which allows the frequency to
vary within a given P-state in order to achieve the maximum performance for that
P-state's power budget. The PAL_PROC_GET_FEATURES procedure indicates whether
the processor supports variable P-state performance (see
“PAL_PROC_GET_FEATURES
– Get Processor Dependent Features (17)” on page 2:446 for details).
Since the frequency within a P-state can vary, the performance index calculation is
slightly different when a processor supports variable P-state performance. Frequencies
for a given P-state are represented by an index value
F
x,y
. The value x is the P-state
number and y represents a frequency point in the range from 0 to N. A value of 0
represents the minimum frequency index value for the given P-state. For example:
F
0,0
to
F
0,N
– Frequency index values for the P0 state
F
1,0
to
F
1,N
– Frequency index values for the P1 state
…etc.
F
0,0
is the minimum frequency index for the P0 state and its value is 100.
F
0,1
represents a higher frequency point for P0 and will have a value greater than 100. For
example, if
F
0,1
frequency is 5% greater than
F
0,0
it would have a value of 105.
The
performance_index
equation for P0 is calculated as follows:
((
F
0,0
* time spent in
F
0,0
) + (
F
0,1
* time spent in
F
0,1
)+ .. (
F
0,N
* time spent in
F
0,N
)) /
(Total Time spent in
P
0
)