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Message-ID: <20091020005921.GE10727@kernel.dk> Date: Tue, 20 Oct 2009 02:59:22 +0200 From: Jens Axboe <jens.axboe@...cle.com> To: Corrado Zoccolo <czoccolo@...il.com> Cc: Linux-Kernel <linux-kernel@...r.kernel.org>, Jeff Moyer <jmoyer@...hat.com> Subject: Re: [RFC PATCH 4/5] cfq-iosched: enable idling for last queue on priority class On Mon, Oct 19 2009, Corrado Zoccolo wrote: > cfq can disable idling for queues in various circumstances. > When workloads of different priorities are competing, if the higher > priority queue has idling disabled, lower priority queues may steal > its disk share. For example, in a scenario with an RT process > performing seeky reads vs a BE process performing sequential reads, > on an NCQ enabled hardware, with low_latency unset, > the RT process will dispatch only the few pending requests every full > slice of service for the BE process. > > The patch solves this issue by always performing idle on the last > queue at a given priority class > idle. If the same process, or one > that can pre-empt it (so at the same priority or higher), submits a > new request within the idle window, the lower priority queue won't > dispatch, saving the disk bandwidth for higher priority ones. > > Note: this doesn't touch the non_rotational + NCQ case (no hardware > to test if this is a benefit in that case). > > +static bool cfq_should_idle(struct cfq_data *cfqd, struct cfq_queue *cfqq) > +{ > + if (cfq_class_idle(cfqq)) > + return false; > + if (cfq_cfqq_idle_window(cfqq)) > + return true; > + { > + enum wl_prio_t prio = cfq_class_rt(cfqq) ? RT_WL : BE_WL; > + struct cfq_rb_root *service_tree = service_tree_for(prio, cfqd); > + return service_tree->count == 0 || > + (service_tree->count == 1 > + && cfq_rb_first(service_tree) == cfqq); > + } > +} This, again, looks more like a debug patch than a piece of tested production code. Fix it. And comment it! -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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