/* ----------------------------------------------------------------------------- * * (c) The GHC Team 1998-2008 * * Generational garbage collector * * Documentation on the architecture of the Garbage Collector can be * found in the online commentary: * * https://gitlab.haskell.org/ghc/ghc/wikis/commentary/rts/storage/gc * * ---------------------------------------------------------------------------*/ #pragma once #include "WSDeque.h" #include "GetTime.h" // for Ticks #include "BeginPrivate.h" /* ----------------------------------------------------------------------------- General scheme ToDo: move this to the wiki when the implementation is done. We're only going to try to parallelise the copying GC for now. The Plan is as follows. Each thread has a gc_thread structure (see below) which holds its thread-local data. We'll keep a pointer to this in a thread-local variable, or possibly in a register. In the gc_thread structure is a gen_workspace for each generation. The primary purpose of the gen_workspace is to hold evacuated objects; when an object is evacuated, it is copied to the "todo" block in the thread's workspace for the appropriate generation. When the todo block is full, it is pushed to the global gen->todos list, which is protected by a lock. (in fact we intervene a one-place buffer here to reduce contention). A thread repeatedly grabs a block of work from one of the gen->todos lists, scavenges it, and keeps the scavenged block on its own ws->scavd_list (this is to avoid unnecessary contention returning the completed buffers back to the generation: we can just collect them all later). When there is no global work to do, we start scavenging the todo blocks in the workspaces. This is where the scan_bd field comes in: we can scan the contents of the todo block, when we have scavenged the contents of the todo block (up to todo_bd->free), we don't want to move this block immediately to the scavd_list, because it is probably only partially full. So we remember that we have scanned up to this point by saving the block in ws->scan_bd, with the current scan pointer in ws->scan. Later, when more objects have been copied to this block, we can come back and scan the rest. When we visit this workspace again in the future, scan_bd may still be the same as todo_bd, or it might be different: if enough objects were copied into this block that it filled up, then we will have allocated a new todo block, but *not* pushed the old one to the generation, because it is partially scanned. The reason to leave scanning the todo blocks until last is that we want to deal with full blocks as far as possible. ------------------------------------------------------------------------- */ /* ----------------------------------------------------------------------------- Generation Workspace A generation workspace exists for each generation for each GC thread. The GC thread takes a block from the todos list of the generation into the scanbd and then scans it. Objects referred to by those in the scan block are copied into the todo or scavd blocks of the relevant generation. ------------------------------------------------------------------------- */ typedef struct gen_workspace_ { generation * gen; // the gen for this workspace struct gc_thread_ * my_gct; // the gc_thread that contains this workspace // where objects to be scavenged go bdescr * todo_bd; StgPtr todo_free; // free ptr for todo_bd StgPtr todo_lim; // lim for todo_bd struct NonmovingSegment *todo_seg; // only available for oldest gen workspace WSDeque * todo_q; bdescr * todo_overflow; uint32_t n_todo_overflow; // where large objects to be scavenged go bdescr * todo_large_objects; // Objects that have already been scavenged. bdescr * scavd_list; StgWord n_scavd_blocks; // count of blocks in this list StgWord n_scavd_words; // Partially-full, scavenged, blocks bdescr * part_list; StgWord n_part_blocks; // count of above StgWord n_part_words; } gen_workspace ATTRIBUTE_ALIGNED(64); // align so that computing gct->gens[n] is a shift, not a multiply // fails if the size is <64, which is why we need the pad above /* ---------------------------------------------------------------------------- GC thread object Every GC thread has one of these. It contains all the generation specific workspaces and other GC thread local information. At some later point it maybe useful to move this other into the TLS store of the GC threads ------------------------------------------------------------------------- */ /* values for the wakeup field */ #define GC_THREAD_INACTIVE 0 #define GC_THREAD_STANDING_BY 1 #define GC_THREAD_RUNNING 2 #define GC_THREAD_WAITING_TO_CONTINUE 3 typedef struct gc_thread_ { Capability *cap; #if defined(THREADED_RTS) OSThreadId id; // The OS thread that this struct belongs to SpinLock gc_spin; SpinLock mut_spin; volatile StgWord wakeup; // NB not StgWord8; only StgWord is guaranteed atomic #endif uint32_t thread_index; // a zero based index identifying the thread bdescr * free_blocks; // a buffer of free blocks for this thread // during GC without accessing the block // allocators spin lock. // These two lists are chained through the STATIC_LINK() fields of static // objects. Pointers are tagged with the current static_flag, so before // following a pointer, untag it with UNTAG_STATIC_LIST_PTR(). StgClosure* static_objects; // live static objects StgClosure* scavenged_static_objects; // static objects scavenged so far W_ gc_count; // number of GCs this thread has done // block that is currently being scanned bdescr * scan_bd; // Remembered sets on this CPU. Each GC thread has its own // private per-generation remembered sets, so it can add an item // to the remembered set without taking a lock. The mut_lists // array on a gc_thread is the same as the one on the // corresponding Capability; we stash it here too for easy access // during GC; see recordMutableGen_GC(). bdescr ** mut_lists; // -------------------- // evacuate flags uint32_t evac_gen_no; // Youngest generation that objects // should be evacuated to in // evacuate(). (Logically an // argument to evacuate, but it's // static a lot of the time so we // optimise it into a per-thread // variable). bool failed_to_evac; // failure to evacuate an object typically // Causes it to be recorded in the mutable // object list bool eager_promotion; // forces promotion to the evac gen // instead of the to-space // corresponding to the object W_ thunk_selector_depth; // used to avoid unbounded recursion in // evacuate() for THUNK_SELECTOR // ------------------- // stats W_ copied; W_ scanned; W_ any_work; W_ no_work; W_ scav_find_work; Time gc_start_cpu; // thread CPU time Time gc_end_cpu; // thread CPU time Time gc_sync_start_elapsed; // start of GC sync Time gc_start_elapsed; // process elapsed time Time gc_end_elapsed; // process elapsed time W_ gc_start_faults; // ------------------- // workspaces // array of workspaces, indexed by gen->abs_no. This is placed // directly at the end of the gc_thread structure so that we can get from // the gc_thread pointer to a workspace using only pointer // arithmetic, no memory access. This happens in the inner loop // of the GC, see Evac.c:alloc_for_copy(). gen_workspace gens[]; } gc_thread; extern uint32_t n_gc_threads; extern gc_thread **gc_threads; #if defined(THREADED_RTS) && defined(CC_LLVM_BACKEND) extern ThreadLocalKey gctKey; #endif #include "EndPrivate.h"