com.oracle.svm.core.genscavenge

Class CardTable

java.lang.Object

com.oracle.svm.core.genscavenge.CardTable

public final class CardTable
extends Object

A card remembered set table is a remembered set that summarizes pointer stores into a region. A card is "dirty" if a pointer has been stored recently into the memory summarized by the card, or "clean" otherwise. When looking for roots into the young space, the whole old space need not be searched for pointers, only the parts of the old space covered by dirty cards. (The card table works in concert with the FirstObjectTable to find objects that cross onto memory covered by a card.) At each pointer store the card corresponding to the destination of the store is dirtied. At each collection, the dirty cards are scanned and the corresponding memory examined for pointers to the young space. When the memory has been scanned, the corresponding card is cleaned. Implementation notes: - Since the card remembered set table is not a real object, the methods here are all static, and if necessary take a Pointer to the table as their first argument. - In theory, I only need "clean" and "dirty" values in the table, but since bit manipulations are expensive (particularly atomic bit manipulations), I trade space for time and make each entry a byte. - The "dirty" value is 0, since that makes dirtying a card a single "clearByte" instructions which is available in all the instruction set architectures I care about, whereas a "setByte" with a non-zero value takes more instruction space. - There are various proposals for other entry values since no one likes using a byte to hold one bit. For example, one could have a "pre-cleaning" thread that went over the memory and where it found only one interesting pointer, change the card from "dirty" to the offset of that pointer within the card. That feature is not in this version.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected static class	CardTable.ReferenceToYoungObjectReferenceVisitor Visit an object reference and return false if it is a reference to the young space.
protected static class	CardTable.ReferenceToYoungObjectVisitor
static class	CardTable.TestingBackDoor For testing and debugging.
static interface	CardTable.Visitor An interface for visitors to a card remembered set table.

Field Summary

Fields

Modifier and Type	Field and Description
static org.graalvm.word.LocationIdentity	CARD_REMEMBERED_SET_LOCATION A LocationIdentity to distinguish card locations from other locations.

Method Summary

Modifier and Type

Method and Description

protected static boolean

verify(org.graalvm.word.Pointer ctStart, org.graalvm.word.Pointer fotStart, org.graalvm.word.Pointer objectsStart, org.graalvm.word.Pointer objectsLimit) Check that: every clean card indicates an object with no pointers to young space. that every object with a pointer to young space has a corresponding marked card. I would like to check that every dirty card has a pointer to young space, but a card may be dirtied by the storing of a null, which doesn't point to young space.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

CARD_REMEMBERED_SET_LOCATION

public static final org.graalvm.word.LocationIdentity CARD_REMEMBERED_SET_LOCATION

A LocationIdentity to distinguish card locations from other locations.

Method Detail

verify

protected static boolean verify(org.graalvm.word.Pointer ctStart,
                                org.graalvm.word.Pointer fotStart,
                                org.graalvm.word.Pointer objectsStart,
                                org.graalvm.word.Pointer objectsLimit)

Check that:

• every clean card indicates an object with no pointers to young space.
• that every object with a pointer to young space has a corresponding marked card.

I would like to check that every dirty card has a pointer to young space, but a card may be dirtied by the storing of a null, which doesn't point to young space. For extra credit, make CardTable.getMemoryBytesPerEntry() 8 so there's at most one object per card to weed out ambiguous marked cards.