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    SubjectRe: [patch] sched: implement cpu_clock(cpu) high-speed time source, take #2
    On Thu, Jul 19 2007, Jeremy Fitzhardinge wrote:
    > Jens Axboe wrote:
    > > On Thu, Jul 19 2007, Jeremy Fitzhardinge wrote:
    > >
    > >>> Index: linux/block/blktrace.c
    > >>> ===================================================================
    > >>> --- linux.orig/block/blktrace.c
    > >>> +++ linux/block/blktrace.c
    > >>> @@ -41,7 +41,7 @@ static void trace_note(struct blk_trace
    > >>> const int cpu = smp_processor_id();
    > >>>
    > >>> t->magic = BLK_IO_TRACE_MAGIC | BLK_IO_TRACE_VERSION;
    > >>> - t->time = sched_clock() - per_cpu(blk_trace_cpu_offset, cpu);
    > >>> + t->time = cpu_clock(cpu) - per_cpu(blk_trace_cpu_offset, cpu);
    > >>> t->device = bt->dev;
    > >>> t->action = action;
    > >>> t->pid = pid;
    > >>> @@ -159,7 +159,7 @@ void __blk_add_trace(struct blk_trace *b
    > >>>
    > >>> t->magic = BLK_IO_TRACE_MAGIC | BLK_IO_TRACE_VERSION;
    > >>> t->sequence = ++(*sequence);
    > >>> - t->time = sched_clock() - per_cpu(blk_trace_cpu_offset, cpu);
    > >>> + t->time = cpu_clock(cpu) - per_cpu(blk_trace_cpu_offset, cpu);
    > >>>
    > >>>
    > >> What's this measuring here? Time spend in IO? Wouldn't it be better
    > >> off with a measurement of real monotonic time?
    > >>
    > >
    > > It's not time spent in IO, it wants a nanosecond timestamp.
    > >
    >
    > What kind of nanoseconds? Real? Virtual?
    >
    > sched_clock() (and now cpu_clock()) measure the number of nanoseconds
    > available for running CPU instructions (regardless of whether it chooses
    > to use them or not), and so in a virtual environment doesn't include
    > time stolen by the hypervisor for other VCPUs.

    Real time, it's a timestamp that the user tools can use to see the time
    delta between A and B.

    So I guess blktrace currently doesn't work so well inside a gues...

    --
    Jens Axboe

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