* Paul Mackerras <[email protected]> wrote:
> PowerPC's sched_clock() currently measures real time. On POWER5 and
> POWER6 machines we could change it to use a register called the "PURR"
> (for Processor Utilization of Resources Register), which only measures
> time spent while the partition is running. But the PURR has another
> function as well: it measures the distribution of dispatch cycles
> between the two hardware threads on each core when running in SMT
> mode. That is, the cpu dispatches instructions from one thread or the
> other (not both) on each CPU cycle, and each thread's PURR only gets
> incremented on cycles where the cpu dispatches instructions for that
> thread. So the sum of the two threads' PURRs adds up to real time.
>
> Do you think this makes the PURR more useful for CFS, or less? To me
> it looks like this would mean that CFS can make a more equitable
> distribution of CPU time if, for example, you had 3 runnable tasks on
> a 2-core x dual-threaded machine (4 virtual CPUs).
there's one complication: sched_clock() still needs to increase while
the CPU (or thread) is idle, so that we can have a correct measurement
of the CPU's utilization, for SMP load-balancing. CFS constructs another
clock from sched_clock() [the rq->fair_clock] which does stop while the
CPU is idle.
So perhaps a combination of the PURR and real-time might work as
sched_clock(): when a hardware thread is in cpu_idle(), it should
advance its sched clock with _half_ the rate of real-time [so that the
sum of advance of all threads if they are all idle is equal to real
time], and use the PURR if they are not idle. This would still correctly
keep a meaningful load-average if the physical CPU is under-utilized.
If you do such a change you'll immediately see whether the approach is
right: monitor the cpu_load[] values in /proc/sched_debug, they should
match the intuitive 'load average' of that CPU (if divided by 1024), and
check whether 'top' still works fine.
> BTW, what does "time spent running during sleep" mean? Does it mean
> "time that other tasks are running while this task is sleeping"?
yeah. It's "the amount of fair runtime i missed out on while others were
running".
> > still, CFS needs time measurement across idle periods as well, for
> > another purpose: to be able to do precise task statistics for /proc.
> > (for top, ps, etc.) So it's still true that sched_clock() should
> > include idle periods too.
>
> As with s390, 64-bit PowerPC also uses CONFIG_VIRT_CPU_ACCOUNTING.
> That affects how tsk->utime and tsk->stime are accumulated (we call
> account_user_time and account_system_time directly rather than calling
> update_process_times) as well as the system hardirq/softirq time, idle
> time, and stolen time.
tsk->utime and tsk->stime is only used for a single purpose: to
determine the 'split' factor of how to split up the precise total time
between user and system time.
> When you say "precise task statistics for /proc", where are they
> accumulated? I don't see any changes to the way that tsk->utime and
> ctime are computed.
we now use p->se.sum_exec_runtime that measures (in nanoseconds) the
precise amount of time spent executing (sum of system and user time) -
and ->stime and ->utime is used to determine the 'split'. [this allows
us to gather ->stime and ->utime via low-resolution sampling, while
keeping the 'total' precise. Accounting at every system entry point
would be quite expensive on most platforms.]
Ingo
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