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8276549: Improve documentation about ContainerPtr encoding

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Reviewed-by: kbarrett, iwalulya
This commit is contained in:
Thomas Schatzl 2022-05-23 08:50:41 +00:00
parent 89a1d055d9
commit a0042de411
1 changed files with 25 additions and 5 deletions
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30
src/hotspot/share/gc/g1/g1CardSet.hpp
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@ -210,7 +210,7 @@ public:
//
// Possible encodings:
//
// 0...00000 free (Empty, should never happen)
// 0...00000 free (Empty, should never happen on a top-level ContainerPtr)
// 1...11111 full All card indexes in the whole area this ContainerPtr covers are part of this container.
// X...XXX00 inline-ptr-cards A handful of card indexes covered by this ContainerPtr are encoded within the ContainerPtr.
// X...XXX01 array of cards The container is a contiguous array of card indexes.
@ -218,11 +218,31 @@ public:
// X...XXX11 howl This is a card set container containing an array of ContainerPtr, with each ContainerPtr
// limited to a sub-range of the original range. Currently only one level of this
// container is supported.
using ContainerPtr = void*;
//
// The container's pointer starts off with an inline container and is then subsequently
// coarsened as more cards are added.
//
// Coarsening happens in the order below:
// ContainerInlinePtr -> ContainerArrayOfCards -> ContainerHowl -> Full
// Corsening of containers inside the ContainerHowl happens in the order:
// ContainerInlinePtr -> ContainerArrayOfCards -> ContainerBitMap -> Full
// ContainerInlinePtr -> ContainerArrayOfCards -> ContainerHowl -> Full
//
// There is intentionally no bitmap based container that covers a full region; first,
// a whole region is covered very well (and more flexibly) using the howl container and
// even then the overhead of the ContainerPtr array with all-bitmaps vs. a single bitmap
// is negligible, and most importantly transferring such a Howl container to a
// "Full Region Bitmap" is fairly hard without missing entries that are added by
// concurrent threads.
//
// Howl containers are basically arrays of containers. An entry starts off with
// Free. Further corsening of containers inside the ContainerHowl happens in the order:
//
// Free -> ContainerInlinePtr -> ContainerArrayOfCards -> ContainerBitMap -> Full
//
// Throughout the code it is assumed (and checked) that the last two bits of the encoding
// for Howl (0b11) is assumed to be the same as the last two bits for "FullCardSet"; this
// has been done in various places to not be required to check for a "FullCardSet" first
// all the time in iteration code (only if there is a Howl card set container, that is
// fairly uncommon).
using ContainerPtr = void*;
static const uintptr_t ContainerInlinePtr = 0x0;
static const uintptr_t ContainerArrayOfCards = 0x1;
static const uintptr_t ContainerBitMap = 0x2;