Re: find_busiest_group using lots of CPU

Jens Axboe
2009-10-06T11:20:44Z

On Tue, Oct 06 2009, Jens Axboe wrote:
> On Mon, Oct 05 2009, Peter Zijlstra wrote:
> > On Wed, 2009-09-30 at 10:18 +0200, Jens Axboe wrote:
> > > Hi,
> > >
> > > I stuffed a few more SSDs into my text box. Running a simple workload
> > > that just does streaming reads from 10 processes (throughput is around
> > > 2.2GB/sec), find_busiest_group() is using > 10% of the CPU time. This is
> > > a 64 thread box.
> > >
> > > The top two profile entries are:
> > >
> > > 10.86% fio [kernel] [k] find_busiest_group
> > > |
> > > |--99.91%-- thread_return
> > > | io_schedule
> > > | sys_io_getevents
> > > | system_call_fastpath
> > > | 0x7f4b50b61604
> > > | |
> > > | --100.00%-- td_io_getevents
> > > | io_u_queued_complete
> > > | thread_main
> > > | run_threads
> > > | main
> > > | __libc_start_main
> > > --0.09%-- [...]
> > >
> > > 5.78% fio [kernel] [k] cpumask_next_and
> > > |
> > > |--67.21%-- thread_return
> > > | io_schedule
> > > | sys_io_getevents
> > > | system_call_fastpath
> > > | 0x7f4b50b61604
> > > | |
> > > | --100.00%-- td_io_getevents
> > > | io_u_queued_complete
> > > | thread_main
> > > | run_threads
> > > | main
> > > | __libc_start_main
> > > |
> > > --32.79%-- find_busiest_group
> > > thread_return
> > > io_schedule
> > > sys_io_getevents
> > > system_call_fastpath
> > > 0x7f4b50b61604
> > > |
> > > --100.00%-- td_io_getevents
> > > io_u_queued_complete
> > > thread_main
> > > run_threads
> > > main
> > > __libc_start_main
> > >
> > > This is with SCHED_DEBUG=y and SCHEDSTATS=y enabled, I just tried with
> > > both disabled but that yields the same result (well actually worse, 22%
> > > spent in there. dunno if that's normal "fluctuation"). GROUP_SCHED is
> > > not set. This seems way excessive!
> >
> > io_schedule() straight into find_busiest_group() leads me to think this
> > could be SD_BALANCE_NEWIDLE, does something like:
> >
> > for i in /proc/sys/kernel/sched_domain/cpu*/domain*/flags;
> > do
> > val=`cat $i`; echo $((val & ~0x02)) > $i;
> > done
> >
> > [ assuming SCHED_DEBUG=y ]
> >
> > Cure things?
>
> I can try, as mentioned it doesn't look any better with SCHED_DEBUG=n

It does, it's gone from the profiles.

--
Jens Axboe