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IBM Systems & Technolgy Groupz/OS Basics: Virtual StorageManagement (VSM) OverviewElpida Tzortzatos – email: elpida@us.ibm.com1 2009 IBM Corporation

Agendaz/OS Memory/Storage Types z/OS Memory Managers 31-Bit VSM Basics Recent Enhancements 64-BitVSM/RSMBasics Recent Enhancements Appendices CSA Common Storage Tracker2

z/OS Memory/Storage Types3

z/OS Memory Types Memory management viewsPageDatasetsReal storageframes (physicalCPU memory)Virtual pages inmultiple address spaces(created through DAT)Auxiliary slots on(DASD) volumes(paging datasets)4

z/OS Memory Controls Real Virtual Mixture of z/OS internal thresholds and system-wideparametersCustomer-specified policies via JCL, SystemManagement Facility (SMF) parameters and exitsGenerally either prevent work exceeding the limitsfrom starting or causes work to terminate if itexceeds a limit during executionProcess-level granularityAux z/OS internal thresholds coupled with paging datasetconfiguration: number of datasets, size of each5

Storage Types (all with respect to z/OS’s point of view) Virtual memory/storage DATDAT-on addressingExplicitly allocated either by the system or applications24, 31, or 64 bit residency mode (rmode(rmode))3131-bit virtual is allocated/freed in multiple of 8 byte chunks6464-bit virtual is allocated in 1MB multiples on a 1M boundaryVirtual storage attributes are specified when virtual storage is allocated Real memory/storage Based on addressing mode of application and whether or not the “real view”view” will be usedExample: the ordinary 3131-bit application requires 3131-bit virtual pages but the frame backingeach page may have a 6464-bit real addressExpanded storage DATDAT-off addressing, Real Space ALET, I/O, some privileged opop-codes specify real addresses4 KB page frames (often: just “frames”frames”), 1MB page frames (often just “large pages”pages”)24, 31, or 64 bit residency mode (rmode(rmode))Each memory allocation request to z/OS specifies allowable virtual and real rmodes Fixed (pinned), DREF, pageable,pageable, 1MB pagesWhen physical resources (real memory/paging space) are assigned to virtual storage depends on thevirtual storage attributesNo longer used by z/OSMidMid-1980s to early 2000s, electronic storage with characteristics betweenbetween real and auxiliaryin terms of response time and monetary costFunctioned mostly as a fast synchronous paging deviceAuxiliary (storage, on Disk/DASD) Used for paging and swapping4 KB slots in one or more paging datasetsInvisible to applications6

Virtual/Real MapTheoretical Max 16 EB (2**64)64 bitaddressable4 TBCurrent maximumreal storagesupported by z/OS“The Bar” 2 GB“The Line” 16 MB31 bit addressable24 bit addressableLow memory address: 07

z/OS Memory Managers8

z/OS Memory Managers: VSMVirtual Storage ManagerAddress Space-centric view of the system andprocessesObjectives 1.2.Control the allocation/deallocation of 31/64-bit virtual storageaddressesObey policies imposed on it by customer specified limits1.3. Each installation can use virtual storage parameters (inSys1.Parmlib,JCL, SMF) to specify how certain virtual storageareas are to be allocated to programsEfficiency – minimum overhead per requestAssociate a storage protection key with each virtualstorage block requestedMaintain storage use information by generating SMFrecords.9

z/OS Memory Managers: RSMReal Storage ManagerFrame-centric view of the system and processesObjectives 1.2.3.4.Keep the system upObey policies imposed on it by the customer, SRMEfficiency – minimum overhead per requestManage 64-bit storage requestsResource pools Expanded frames (historical only)Real storage frames Individual framesFrame pairs: 2 contiguous in real on particular boundaryQuad frames: 4 contiguous in real on particular boundaryMajor policy knobs that control its behavior Optional per-process minimum/maximum number of frames Processes below minimum stolen from last, above maximum firstSystem-wide thresholds controlling paging Begin stealing when less than X frames unallocated, steal until Y frames available10

Dynamic Address Translation V ir t u a l A d d r e s s S p a c e seSzASCEA S C E .6 0 -6 11 1 - R e g io n 1 s t ta b le1 0 - R e g io n 2 n d t a b le0 1 - R e g io n 3 r d t a b le0 0 - S e g m e n t t a b leP a g e Ta b leerisD a ta P a g eSegm entTa b leR e g io n 3 r dTa b leR e g io n 2 n dTa b leE ffe c tiv e V ir tu a l A d d r e s sR e g io n 1 s tTa b le11RFX111111812RSXRTXSXPXBXT r a n s la tio n c a n s ta r t a t R 1 T , R 2 T , R 3 T o r S G TT h e s ta r tin g p o in t o f th e tr a n s la tio n is d e s ig n a te d inth e A d d r e s s S p a c e C o n tr o l E le m e n t ( A S C E .6 0 - 6 1 )C o p y r ig h t IB M C o r p o r a tio n 2 0 0 0 , 2 0 0 111

Large (1MB) Page DATArchitectureASCEASCE.60-6111 - Region 1st table10 - Region 2nd table01 - Region 3rd table00 - Segment tableLarge Data PageSegmentTableRegion 2ndTableRegion 1stTable.Region 3rdTableEffective Virtual Address11RFX11RSX11RTX11SX20BXTranslation can start at R1T, R2T, R3T or SGTThe starting point of the translation is designated inthe Address Space Control Element (ASCE.60-61)12

z/OS Memory Managers: ASMAuxiliary Storage ManagerPaging-centric view of the system; virtually no process awarenessObjectives 1.Efficiency – minimum overhead per request Highly optimized Slot allocation efficiency falls dramatically once utilization ofof a dataset rises above 30%ASM tries to equalize the number of slots allocated across the datasets,datasets, not their % allocatedRelies on other managers to only give it sensible requestsAn auxiliary storage slot is allocated to a page when the page isis paged outAn aux slot is freed when the page is freed or reused when the pagepage is paged outagainPeriodically RSM calls ASM to free slots for pages that are changedchanged in real storageResource pools 1-n Paging datasets formatted into 4 KB slots Called paging datasets, but used for both paged out and physicallyphysically swapped framesMajor policy knobs that control its behavior Number of paging datasets and size of each Datasets may be different sizes13

31-Bit VSM14

Virtual memory/storage DAT-on addressingExplicitly allocated either by the system or applications24, 31, or 64 bit residency mode (rmode)31-bit virtual is allocated/freed in multiple of 8 byte chunks64-bit virtual is allocated in 1MB multiples on a 1M boundaryVirtual storage attributes are specified when virtual storage isallocated Fixed (pinned), DREF, pageable, 1MB pagesWhen physical resources (real memory/paging space) are assignedto virtual storage depends on the virtual storage attributes15

31-Bit Address Space Memory Map2 Gig 80000000Extended Private16 Meg 1000000Extended CSAExtended LPAExtended SQAExtended NucleusNucleusSQALPACSAPrivate600020000System RegionPSAGlobal storage includes Nucleus,Extended Nucleus, SQA, ESQA,LPA, ELPA, CSA, and ECSAGlobal storage managed by VSMincludes SQA, ESQA, CSA, andECSAUpper boundary of ECSA storageand lower boundary of CSAstorage are always on aMegabyte boundarySizes of SQA, ESQA, CSA, andECSA storage are specified at IPLtime via the IEASYSxx parmlibmember16

Local Storage AreaLSQA / SWA / high pvtUser regionAuthorized storage (LSQA, SWA, and highprivate) is assigned from the high end of the localstorage box. In other words, this storage growsfrom the top of the box down.Unauthorized storage (user region) isassigned from the low end of the localstorage box. It grows from the bottom of thebox up.17

z/OS Memory Managers: VSMV S M S to ra g e M a n a g e m e n t R u le sM V S m a n a g e s s to ra g e th ro u g h th e u s e o f s u b p o o lsd e s ig n e d to a c c o m m o d a te a v a rie ty o f s to ra g e n e e d sS to ra g e is a llo c a te d o r a s s ig n e d to a s u b p o o l in o n ep a g e (4 K ) m u ltip le sS to ra g e b e lo n g in g to d iffe re n t s u b p o o ls c a n n o t o c c u p yth e s a m e p a g eS to ra g e w ith d iffe re n t s to ra g e k e y s c a n n o t o c c u p y th esam e pageS to ra g e b e lo n g in g to d iffe re n t T C B s c a n n o t o c c u p y th esam e page18

z/OS Memory Managers: VSMV S M S to ra g e M a n a g e m e n t R u le sW h e n th e re is n o t e n o u g h s to ra g e a b o v e th e lin e tofu lfill a n a b o v e th e lin e s to ra g e re q u e s t, V S M w illa tte m p t to h o n o r th e re q u e s t fro m b e lo w th e lin ein s te a dL S Q A / S W A / h ig h p riv a te p a g e s m a y n o t in te rm ixw ith u s e r re g io n p a g e sU n le s s o th e rw is e d ire c te d o n th e G E T M A INre q u e s t, V S M w ill g iv e o u t s to ra g e a t th e h ig h e n do f th e p a g e firs t19

z/OS Memory Managers: VSMP riv a te S u b p o o l A ttrib u te sS u b p o o l n u m b e rs 0 - 2 5 5S to r a g e p ro te c tio n K e y s 0 - 1 5U s e r R e g io n s u b p o o ls0 - 132, 250 - 252T C B -re la te dK e y e d s to ra g eU n a u th o riz e dG e n e ra l p u rp o s e s u b p o o ls* S e e M V S D ia g n o s is : R e fe r e n c e , C h a p te r 8 ,fo r a d d itio n a l s u b p o o l in fo r m a tio n .H ig h P riv a te s u b p o o ls229, 230, 249T C B -re la te dK e y e d s to ra g eA u th o riz e dS p e c ia l a u th o riz e da p p lic a tio n s to ra g eneedsLSQA2 5 5 (m a in ly )F ix e d , k e y 0 s to ra g eA d d re s s s p a c e -re la te d ,n o t T C B -re la te d20

Virtual Storage Allocation GETMAIN, STORAGE OBTAIN, or CPOOL macro requiredfor virtual storage allocation.GETMAIN will get storage from subpool 0STORAGE OBTAIN obtains storage in the primaryaddress space (by default) or in the address spacedefined through the ALET parameterNOTE: Compared to GETMAIN, STORAGE OBTAINprovides an easier-to-use interface and has fewerrestrictions. For programs running in AR mode or crossmemory mode use the STORAGE OBTAIN macro toobtain storage.Cellpool (carve storage as wanted after doing an initialGETMAIN - specify size of storage needed at each time)21

GETMAIN and STORAGE OBTAINUsing LOC Option LOC parameter on GETMAIN macro and/or STORAGE OBTAINindicates location of requested virtual storage and real storage whenthe page is fixed.Allows storage to be acquired anywhere in the 2 gigabyte addressrange.Caller can be in 24, 31, or 64 bit AMODEAll values and addresses are treated as 31-bit values and addresses.Specifying LOC(xx,64) indicates that central storage (real) can belocated anywhere in 64-bit storage.The LOC parameter is only valid for the following GETMAIN options RU - Register, Unconditional (get storage, if not available, ABEND)RC - Register, Conditional (get storage, return code, won't ABEND)VRU - Variable, Unconditional (between 200 and 500 bytes long, will tryto give 500 at worst, give 200, if not 200 ABEND)VRC - Variable, Conditional (if not minimum, get return code)22

Returning Virtual Storage STORAGE RELEASE releases storage in theprimary address space (by default) or in theaddress space defined through the ALETparameterFREEMAIN options Register, UnconditionalRegister, ConditionalTo free storage above the 16 MB line the RU/RCoptions of the FREEMAIN macro must be used.23

Obtaining Storage via CPOOL The CPOOL macro provides faster storage managementthan GETMAIN macroThe following services are available: Create a Cell Pool (Build) Obtain a Cell from an existing pool ?CPOOL (GET) UNCOND . . . ;Return a Cell to a Cell pool ?CPOOL (GET) COND . . . ;Extend a Cell pool, if necessary ?CPOOL (BUILD) . . . ;?CPOOL (FREE) . . . ;Free an entire Cell Pool ?CPOOL (DELETE) . . . ;24

IBM Systems & Technology GroupVSM GETMAIN Changes in z/OS 1.10 Problem:– A large number of datasets are opened in theDB2 address space. This causes VSAM to do alot of GETMAINs/STORAGE OBTAINs** insubpool 252. This creates a long chain of DQEs for subpool 252 inthe DB2 address space. Long DQE chains causeperformance problems** Subsequent references to GETMAIN also apply to STORAGEOBTAIN25 2009 IBM Corporation