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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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