BTreeVM.mesa

Interface for the VM to support Taft's IFS BTree

Created by M. D. Schroeder

Last Edited by: Schroeder, March 24, 1983 11:36 am

Last Edited by: Taft, March 20, 1983 1:11 pm

DIRECTORY

BTreeVM: CEDAR DEFINITIONS = BEGIN

FilePagesPerPage: TYPE = [1..16];

CacheSize: TYPE = [8..255]; -- in BTree pages --

Handle: TYPE = REF VMObject;

VMObject: TYPE;

Open: PROC [ file: File.Capability, filePagesPerPage: FilePagesPerPage,
cacheSize: CacheSize, base: File.PageNumber ← 0 ] RETURNS [ Handle ];

The Handle returned should be passed to BTree.Open as the "storage" argument. If you want the BTree on the disk to be maintained in a consistent state, then the BTree should be opened with "maintainRecomputableState" set TRUE. If you don't want this, then you probably should be using another package. "base" specifies which file page is the origin for BTree page zero.

The BTreeVM package has a finialization procedure that will return any virtual memory allocated to the cache whenever all references to the "handle" from outside the package disappear. The BTreeVM package also takes the precautions necessary to maintain the BTree state across a rollback to a previously saved checkpoint.

ReferencePage: BTree.ReferencePage;

Used by the BTree package to read pages of the BTree file. BTreeVM will automatically extend the backing file as the BTree grows. It never shrinks the backing file, and make no effort to keep the byte count and created-date correct.

ReleasePage: BTree.ReleasePage;

Used by the BTree package to write pages of the BTree file.

StartLongUpdate: PROC [ h: Handle ];

EndLongUpdate: PROC [ h: Handle ];

The BTree package designates a special statePage. If the BTree is opened with "maintainRecomputableState" set to TRUE, then this page contains an "update in progress" flag which is set at the beginning of any multipage update, cleared at the end, and checked during Open. The "update in progress" flag is a reliable indication of the consistency of the BTree, as BTreeVM writes the statePage to the disk at the beginning of an update and then again at the end. Ocassionally the client will have a large number of updates to do all at once and not want to incur the extra disk writes necessary to have a consistent BTree on disk after each update. Calling BTreeVM.StartLongUpdate causes the statePage to be written to disk at the beginning of the next update, and then not written again until the conclusion of the last update started before BTreeVM.EndLongUpdate. In between, the only disk writes performed are those necessary to obtain free pages in the cache. If execution is interrupted during a "long update", then the BTree will most likely appear inconsistent the next time a BTree is open on the file. Single page updates do not bother with extra writes of the statePage.

GetStats: PROC [ h: Handle ] RETURNS [hits, misses, reads, writes, cumChainLength: LONG CARDINAL, cumReadWriteTime: System.Pulses];

Returns performance statistics, measured from the time the handle was created. Hits and misses are the number of ReferencePage operations for pages that were and were not already in the cache. Reads and writes are the number of actual reads and writes to the backing file; and cumReadWriteTime is the total amount of time spent waiting for reads and writes. cumChainLength is the total number of hash chain entries searched during all lookups; the average search length is cumChainLength/(hits+misses).

CreatePackageRef: PRIVATE PROC [ h: Handle ];

DestroyPackageRef: PRIVATE PROC [ h: Handle ];

EnumeratePackageRefs: PRIVATE PROC [ p: PROC [ h: Handle ] ];

END..