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SubjectRe: [patch] sched: implement cpu_clock(cpu) high-speed time source, take #2
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.

J
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