Adaptive Disk Spin-down Policies for Mobile Computers

Adaptive Disk Spin-down Policies for Mobile ComputersFred DouglisAT&T Bell LaboratoriesP.KrishnanBrown UniversityBrian BershadUniversity of WashingtonAbstractMobile computers typically spin down their hard disk af-ter afixed period of inactivity.If this threshold is toolong,the disk wastes energy;if it is too short,the delaydue to spinning the disk up again frustrates the -age patterns change over time,so a singlefixed thresholdmay not be appropriate at all times.Also,different usersmay have varying prioritieswith respect to trading off en-ergy conservation against performance.We describe amethod for varying the spin-down threshold dynamicallyby adapting to the user’s access patterns and priorities.Adaptive spin-down can in some circumstances reduce byup to50%the number of disk spin-ups that are deemedby the user to be inconvenient,while only moderately in-creasing energy consumption.1IntroductionIn today’s mobile computers,the hard disk is typicallyspun down after afixed period of inactivity in order toconserve energy.When the disk is next accessed,it isspun up again,which can cause a delay of a few seconds.This spin-up delay may be acceptable to the user,whoknows that the delay is in exchange for extending bat-tery life,or it may be bothersome.We have developeda method for distinguishing between undesirable and ac-ceptable spin-up delays and varying the idle-time thresh-old for spinning down the disk based on the user’s toler-There are situations when caching can eliminate just enough I/Os that the disk spins down due to inactivity,but not enough to amortize the cost of the spin-down.In this case,the overhead from spinning up the disk can increase over-all energy consumption.Douglis,Krishnan,Second USENIX Symposium on MobileThe subscript denotes an additive adjustment and the subscript denotes a multiplicative adjustment.as after“close calls.”This approach can be modi-fied,for example,to decrease the spin-down thresh-old only when several spin-ups in a row are accept-able.Variable penalties Thus far,the description of adaptive spin-down has assumed that either a spin-up is ac-ceptable,or it is not.There is in fact a continuum of degrees of acceptance:spinning up just after the disk has spun down is worse than spinning up just before the point at which the spin-up delay would be deemed acceptable.In fact,the worst type of spin-up is one that occurs before the disk has completely spun down,since the delay will be greater(the disk must typically be fully spun down before the order to spin up can be issued).An improvement on the method above,therefore,is to increment the spin-down threshold by a greater amount when the delay due to a spin-up is especially egregious than when it is barely above the user’s threshold.4ExperimentsThe effectiveness of adaptive policies depends on a num-ber of factors:workload,hardware characteristics(i.e., disk parameters,DRAM size,and SRAM size),user per-ception(i.e.,the acceptability ratio),and threshold ad-justment ranges and modifiers.We performed several ex-periments to quantify the effect of these factors.To keep the study manageable,wefixed DRAM at1Mbyte and SRAM at32Kbytes except where noted.To study workload,we simulated both adaptive and fixed-threshold spin-down policies on three traces used in a previous study of mobile storage management:a 3.5-hour Macintosh PowerBook trace,a1.5-hour Win-dows3.1trace,and a4.4-day HP-UX trace[4].Ta-ble1,reproduced from that study,summarizes informa-tion about the traces.In addition,the MAC trace has a distinctive quality that has a significant impact on the effectiveness of the SRAM write buffer:it has a very high locality of write accesses,with36%of writes go-ing to just one1-Kbyte block and24%of writes going to another.Note as well that the HP trace contains disk-level rather thanfile-level accesses,below the level of the buffer cache,so we do not simulate a DRAM buffer cache when considering the HP trace.With respect to hardware characteristics,we con-MAC DOS HPApplications Finder,Excel,NewtonToolkitFramemaker,Powerpoint,Wordemail,editingNumber of distinctKbytes accessed220001630032000 Block size(Kbytes)10.51Mean write size (blocks)1.2 3.4 6.2Max Max MaxInter-arrival time(s)0.0780.570.52810.811.1112.3 sider two magnetic disks:a Western Digital Caviar Ul-tralite CU140,and a Quantum Go Drive.The CU140is available with lightweight mobile systems such as theHewlett-Packard OmniBook300PC,and typically spinsup the disk in about1s.However,the disk may take amaximum of5s to respond after spinning down[17].Oursimulator considers average behavior,so we model theCU140by charging2.5s to spin down the CU140and1sto spin it up again.The Go Drive is a bigger disk andtakes6s to spin down and2.5s to spin up[15].We varied the acceptability ratio among three val-ues:0.02,0.05,and0.2.Putting aside for a moment theissue of a request arriving at the disk while it is spinningdown,a ratio of0.05means that a CU140would have tobe idle for20s for a spin-up delay of1s to be acceptable,while for the Go Drive idle time would normally have tobe50s for the2.5s delay to be acceptable.Based on sub-jective personal experience,these idle-time requirementsseem like a fair trade-off between delay and energy sav-ings,and we use the ratio of0.05as the canonical exam-ple of the adaptive approach.A ratio of0.02is more re-strictive,requiring long idle times to reduce the numberof bumps,while a ratio of0.2is forgiving:for the CU140,the system need be idle only5s for a spin-up to be accept-able,though again,accessing the disk while it is spinningDouglis,Krishnan,Second USENIX Symposium on Mobile2.000.55.000.751.000.751.000.92.000.9(a)Adjustment values.The left two columns list the additive values studied in this paper,while the right two list the multiplicative values.Starting Maximum value value22510is spinning.In the experiments reported here,SRAM blocks are written to disk either when there is no room for new data,or when the disk is already spinning and fewer than5%of SRAM is available for new writes;blocks are written out until the next user I/O occurs or10%of SRAM is free.We consider a more aggressive write pol-icy in Section5.5.5ResultsTo evaluate different sets of parameters,we plot for each trace the count of bumps against energy consumption. We consider general comparisons of adaptive andfixed-threshold policies in Section5.1,variations in the adjust-ment values in Section5.2,variations in the acceptability ratio in Section5.3,disk parameters in Section5.4,and DRAM and SRAM sizes in Section5.5.5.1Adaptive versus Fixed ThresholdsW INDOWSFigure1shows the effect of spin-down policies on en-ergy consumption and bumps,using the Windows trace on the CU140disk with.For this trace/hardware configuration,afixed threshold of2s consumes the least energy;this follows from previous studies[5,14].How-ever,the short threshold results in over50bumps over a 1.5hour period.Increasing thefixed threshold or moving to an adaptive policy can decrease the number of bumps in exchange for higher energy consumption.At the ex-treme case,a spin-down threshold of30s results in no bumps,but also an increase of48%in energy pared to the2s threshold,afixed threshold of 10s decreases bumps by two-thirds and increases energy by just15%.Figure1demonstrates that adaptive policies span a roughly linear range of points between thefixed-threshold policies of5s and30s thresholds,depending on the parameters of the adaptive policy.Generally they are in the“desirable region”(below or to the left of compa-rablefixed-threshold points).For instance,consider an adaptive policy that varies its threshold between5and30s,increasing the threshold by s upon a bump and reducing it by s upon an acceptable spin-up.(This point ismarked by the solid arrow in Figure1.)This configura-tion increases energy by just8%compared to the energy-optimal2sfixed threshold,and decreases the number of bumps by65%.As another example(shown as the dashed arrow), consider an adaptive policy that varies the threshold be-tween5s and30s,multiplies the threshold by on a bump,and multiplies it by on an acceptable pared to the5sfixed threshold policy,this adaptive policy consumes just0.5%more energy while reducing bumps by a third.Each of the other multiplica-tive policies within the5–30s range consumes slightlyDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileThis contrasts with the result reported in[4],which did not buffer writes to a spun-down disk as effectively as possible,and showed only about a20%improvement due to SRAM buffering.Douglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on MobileDouglis,Krishnan,Second USENIX Symposium on Mobile。