DragonFly On-Line Manual Pages

DSCHED_BFQ(4)	      DragonFly Kernel Interfaces Manual	 DSCHED_BFQ(4)

NAME
     dsched_bfq -- Budget Fair Queueing disk scheduling policy

SYNOPSIS
     To compile this driver into the kernel, place the following lines in your
     kernel configuration file:

	   options DSCHED_BFQ

     Alternatively, to load the driver as a module at boot time, place the
     following line in loader.conf(5):

	   dsched_bfq_load="YES"

DESCRIPTION
     The dsched_bfq disk scheduler is invented by Paolo Valente.  The current
     version of dsched_bfq in DragonFly is implemented according to his tech-
     nique report.  Also, some additional features are added into the current
     version; they are inspired by the Linux version, but are totally written
     from scratch.

     Like the CFQ (Complete Fair Queue) disk scheduler under Linux, dsched_bfq
     is a fair queueing scheduler that aims to improve the interactivity and
     lower the latency of the system.  Maximizing throughput, however, is not
     the major design goal of it.  So it is better to switch to dsched_bfq if
     the computer is for desktop usage, in which interactivity eclipses
     throughput in general.

   Switching To BFQ
     You may use sysctl(8) to switch to dsched_bfq (where ``ad0'' is the name
     of your hard drive):

	   sysctl dsched.policy.ad0=bfq

     A loader tunable of the same name is available for setting it perma-
     nently.

   Tuning BFQ
     Currently dsched_bfq has two tunable parameters: the global AS switch and
     the maximum budget.  They can be tuned by the sysctl(8) command, for
     example:

	   #enable the global AS feature:
	   sysctl dsched.bfq.ad0.as_switch=1

	   #set the max budget to 512KB:
	   sysctl dsched.bfq.ad0.max_budget=524288

	   #enable automatic max budget adapting:
	   sysctl dsched.bfq.ad0.auto_max_budget=0

     The AS feature
	     It is reported that turning AS on may affect the interactivity
	     and increase max latency greatly.	It is probably due to the
	     over-serialized implementation of BFQ.  However, the blogbench
	     shows that turning AS on will also increase the throughput
	     greatly.

	     Suggestion: turn on the AS feature, for it effects little on
	     average latency.

     The max budget
	     One thread could be assigned a budget no more than the max bud-
	     get.  Generally, a higher budget means higher throughput because
	     of less operations on WF2Q+ augtree, while a lower budget force
	     the scheduler cost more on those operations.

	     However, the real world experiments show that a too high budget
	     affects interactivity heavily.  A too low budget will also cause
	     higher latency, and if the budget is less than 64KB (65536),
	     which is smaller than the size of some bios, the scheduler will
	     retrograde to a round-robin scheduler, which is not a good form
	     for a disk scheduler.

	     Suggestions: Do not use automatic max budget, it is usually too
	     high.  A budget of 1/10 of the automatic max budget may be
	     proper.  In general, 512K(default), 256K, 192K can be good.  It
	     is better to determine the best max budget by binary selecting by
	     the result of some benchmarks.

SEE ALSO
     dsched(4), kldload(8), sysctl(8)

     http://leaf.dragonflybsd.org/~brillsp/bfq_doc/bfq.html

HISTORY
     The dsched_bfq scheduler first appeared in DragonFly 2.11.

AUTHORS
     The dsched_bfq scheduling policy was written by Brills Peng.

BUGS
     When switching to another dsched policy from BFQ, the system may dead-
     lock.

     Currently, the overall performance is not very good and it is not tested
     on large number of machines.  It is not recommended to use this version
     in a productivity environment.

DragonFly 4.3			August 19, 2011 		 DragonFly 4.3