diff options
Diffstat (limited to 'rts/sm/GC.c')
| -rw-r--r-- | rts/sm/GC.c | 79 |
1 files changed, 74 insertions, 5 deletions
diff --git a/rts/sm/GC.c b/rts/sm/GC.c index 2911aad7a0..1a71bd7bf0 100644 --- a/rts/sm/GC.c +++ b/rts/sm/GC.c @@ -120,6 +120,8 @@ bool unload_mark_needed; */ static W_ g0_pcnt_kept = 30; // percentage of g0 live at last minor GC +static int consec_idle_gcs = 0; + /* Mut-list stats */ #if defined(DEBUG) // For lack of a better option we protect mutlist_scav_stats with oldest_gen->sync @@ -261,6 +263,7 @@ addMutListScavStats(const MutListScavStats *src, void GarbageCollect (uint32_t collect_gen, const bool do_heap_census, + const bool is_overflow_gc, const bool deadlock_detect, uint32_t gc_type USED_IF_THREADS, Capability *cap, @@ -981,11 +984,26 @@ GarbageCollect (uint32_t collect_gen, } #endif - /* If the amount of data remains constant, next major GC we'll - * require (F+1)*live + prealloc. We leave (F+2)*live + prealloc - * in order to reduce repeated deallocation and reallocation. #14702 - */ - need = need_prealloc + (RtsFlags.GcFlags.oldGenFactor + 2) * need_live; + // Reset the counter if the major GC was caused by a heap overflow + consec_idle_gcs = is_overflow_gc ? 0 : consec_idle_gcs + 1; + + // See Note [Scaling retained memory] + double scaled_factor = + RtsFlags.GcFlags.returnDecayFactor > 0 + ? RtsFlags.GcFlags.oldGenFactor / pow(2, (float) consec_idle_gcs / RtsFlags.GcFlags.returnDecayFactor) + : RtsFlags.GcFlags.oldGenFactor; + + debugTrace(DEBUG_gc, "factors: %f %d %f", RtsFlags.GcFlags.oldGenFactor, consec_idle_gcs, scaled_factor ); + + // Unavoidable need depends on GC strategy + // * Copying need 2 * live + // * Compacting need 1.x * live (we choose 1.2) + // * Nonmoving needs ~ 1.x * live + double unavoidable_need_factor = (oldest_gen->compact || RtsFlags.GcFlags.useNonmoving) + ? 1.2 : 2; + W_ scaled_needed = (scaled_factor + unavoidable_need_factor) * need_live; + debugTrace(DEBUG_gc, "factors_2: %f %d", unavoidable_need_factor, scaled_needed); + need = need_prealloc + scaled_needed; /* Also, if user set heap size, do not drop below it. */ @@ -1003,6 +1021,7 @@ GarbageCollect (uint32_t collect_gen, need = BLOCKS_TO_MBLOCKS(need); got = mblocks_allocated; + debugTrace(DEBUG_gc,"Returning: %d %d", got, need); uint32_t returned = 0; if (got > need) { @@ -2208,3 +2227,53 @@ bool doIdleGCWork(Capability *cap STG_UNUSED, bool all) * work_stealing is "mostly immutable". We set it to false when we begin the * final sequential collections, for the benefit of notifyTodoBlock. * */ + +/* Note [Scaling retained memory] + * Tickets: #19381 #19359 #14702 + * + * After a spike in memory usage we have been conservative about returning + * allocated blocks to the OS in case we are still allocating a lot and would + * end up just reallocating them. The result of this was that up to 4 * live_bytes + * of blocks would be retained once they were allocated even if memory usage ended up + * a lot lower. + * + * For a heap of size ~1.5G, this would result in OS memory reporting 6G which is + * both misleading and worrying for users. + * In long-lived server applications this results in consistent high memory + * usage when the live data size is much more reasonable (for example ghcide) + * + * Therefore we have a new (2021) strategy which starts by retaining up to 4 * live_bytes + * of blocks before gradually returning uneeded memory back to the OS on subsequent + * major GCs which are NOT caused by a heap overflow. + * + * Each major GC which is NOT caused by heap overflow increases the consec_idle_gcs + * counter and the amount of memory which is retained is inversely proportional to this number. + * By default the excess memory retained is + * oldGenFactor (controlled by -F) / 2 ^ (consec_idle_gcs * returnDecayFactor) + * + * On a major GC caused by a heap overflow, the `consec_idle_gcs` variable is reset to 0 + * (as we could continue to allocate more, so retaining all the memory might make sense). + * + * Therefore setting bigger values for `-Fd` makes the rate at which memory is returned slower. + * Smaller values make it get returned faster. Setting `-Fd0` means no additional memory + * is retained. + * + * The default is `-Fd4` which results in the following scaling: + * + * > mapM print [(x, 1/ (2**(x / 4))) | x <- [1 :: Double ..20]] + * (1.0,0.8408964152537146) + * ... + * (4.0,0.5) + * ... + * (12.0,0.125) + * ... + * (20.0,3.125e-2) + * + * So after 12 consecutive GCs only 0.1 of the maximum memory used will be retained. + * + * Further to this decay factor, the amount of memory we attempt to retain is + * also influenced by the GC strategy for the oldest generation. If we are using + * a copying strategy then we will need at least 2 * live_bytes for copying to take + * place, so we always keep that much. If using compacting or nonmoving then we need a lower number, + * so we just retain at least `1.2 * live_bytes` for some protection. + */ |