From 63d68bd657b7386c9c58b4439a100ea0ccbd633e Mon Sep 17 00:00:00 2001 From: Roberto Ierusalimschy Date: Wed, 29 Nov 2023 16:22:09 -0300 Subject: Comments detailing the ages for generational GC Plus other comments and small details. --- lgc.h | 44 +++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 41 insertions(+), 3 deletions(-) (limited to 'lgc.h') diff --git a/lgc.h b/lgc.h index 959465ec..f06427c2 100644 --- a/lgc.h +++ b/lgc.h @@ -123,6 +123,43 @@ #define changeage(o,f,t) \ check_exp(getage(o) == (f), (o)->marked ^= ((f)^(t))) +/* +** In generational mode, objects are created 'new'. After surviving one +** cycle, they become 'survival'. Both 'new' and 'survival' can point +** to any other object, as they are traversed at the end of the cycle. +** We call them both 'young' objects. +** If a survival object survives another cycle, it becomes 'old1'. +** 'old1' objects can still point to survival objects (but not to +** new objects), so they still must be traversed. After another cycle +** (that, being old, 'old1' objects will "survive" no matter what) +** finally the 'old1' object becomes really 'old', and then they +** are no more traversed. +** +** To keep its invariants, the generational mode uses the same barriers +** also used by the incremental mode. If a young object is caught in a +** foward barrier, it cannot become old immediately, because it can +** still point to other young objects. Instead, it becomes 'old0', +** which in the next cycle becomes 'old1'. So, 'old0' objects is +** old but can point to new and survival objects; 'old1' is old +** but cannot point to new objects; and 'old' cannot point to any +** young object. +** +** If any old object ('old0', 'old1', 'old') is caught in a back +** barrier, it becomes 'touched1' and goes into a gray list, to be +** visited at the end of the cycle. There it evolves to 'touched2', +** which can point to survivals but not to new objects. In yet another +** cycle then it becomes 'old' again. +** +** The generational mode must also control the colors of objects, +** because of the barriers. While the mutator is running, young objects +** are kept white. 'old', 'old1', and 'touched2' objects are kept black, +** as they cannot point to new objects; exceptions are threads and open +** upvalues, which age to 'old1' and 'old' but are kept gray. 'old0' +** objects may be gray or black, as in the incremental mode. 'touched1' +** objects are kept gray, as they must be visited again at the end of +** the cycle. +*/ + /* Default Values for GC parameters */ @@ -161,9 +198,10 @@ ** (value * original parameter / 100). ** ** For most parameters, which are typically larger than 100%, 2^n is -** 16 (2^4), allowing maximum values up to 1599. For the minor -** multiplier, which is typically smaller, 2^n is 64 (2^6) to allow more -** precision. +** 16 (2^4), allowing maximum values up to ~1500%, with a granularity +** of ~6%. For the minor multiplier, which is typically smaller, +** 2^n is 64 (2^6) to allow more precision. In that case, the maximum +** value is ~400%, with a granularity of ~1.5%. */ #define gcparamshift(p) \ (offsetof(global_State, p) == offsetof(global_State, genminormul) ? 6 : 4) -- cgit v1.2.3-55-g6feb