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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
