On Wed, 2011-12-28 at 14:32 +0100, Arjan van de Ven wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 12/27/2011 10:20 AM, Mike Galbraith wrote: > > > > Quoting removal commit af5ab277ded04bd9bc6b048c5a2f0e7d70ef0867 > > Historically, Linux has tried to make the regular timer tick on > > the various CPUs not happen at the same time, to avoid contention > > on xtime_lock. > > > > Nowadays, with the tickless kernel, this contention no longer > > happens since time keeping and updating are done differently. In > > addition, this skew is actually hurting power consumption in a > > measurable way on many-core systems. End quote > > > > Contention remains a problem if NO_HZ is either not configured, or > > is nohz=off disabled due to workload constraints. The RT kernel > > running nohz=off was measured to be using > 1.4% CPU just ticking > > 64 CPUs, with tick perturbation reaching ~80us. For loads where > > measured (>100us) NO_HZ latencies are intolerable, a must have. > > I think we need to just say no to this, and kill the nohz=off option > entirely. > > Seriously, are people still running with ticks for any legitimate > reasons? (and not just because they goofed their config file) Yup. Realtime loads sometimes need it. Even without contention problems, entering/leaving nohz is a latency source. If every little bit counts, you may have the choice of letting the electric meter spin or not getting the job done at all. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
