[Git][ghc/ghc][wip/andreask/allocationArea] Increase -A and -O rts defaults to 4MB.

[Andreas Klebinger]

Andreas Klebinger pushed to branch wip/andreask/allocationArea at Glasgow Haskell Compiler / GHC


Commits:
444c1e0e by Andreas Klebinger at 2020-07-13T13:47:58+02:00
Increase -A and -O rts defaults to 4MB.

Fixes #16499

- - - - -


2 changed files:

- docs/users_guide/runtime_control.rst
- rts/RtsFlags.c


Changes:

=====================================
docs/users_guide/runtime_control.rst
=====================================
@@ -401,7 +401,7 @@ performance.
 
 .. rts-flag:: -A ⟨size⟩
 
-    :default: 1MB
+    :default: 4MB
 
     .. index::
        single: allocation area, size
@@ -410,15 +410,22 @@ performance.
     collector. The allocation area (actually generation 0 step 0) is
     fixed and is never resized (unless you use :rts-flag:`-H [⟨size⟩]`, below).
 
-    Increasing the allocation area size may or may not give better
-    performance (a bigger allocation area means worse cache behaviour
-    but fewer garbage collections and less promotion).
+    Optimal settings depend on the actual machine, program, and other RTS options.
+    Increasing the allocation area size means worse cache behaviour
+    but fewer garbage collections and less promotion.
+
+    In general settings >= 4MB can reduce performance in some cases, in particular for single
+    threaded operation. However in a parallel setting increasing the allocation area
+    to ``16MB``, or even ``64MB`` can increase gc throughput significantly.
 
     With only 1 generation (e.g. ``-G1``, see :rts-flag:`-G ⟨generations⟩`) the
     ``-A`` option specifies the minimum allocation area, since the actual size
     of the allocation area will be resized according to the amount of data in
     the heap (see :rts-flag:`-F ⟨factor⟩`, below).
 
+    When heap profiling using a smaller allocation area can increase accuracy as more frequent
+    major garbage collections also results in more frequent heap snapshots
+
 .. rts-flag:: -AL ⟨size⟩
 
     :default: :rts-flag:`-A <-A ⟨size⟩>` value
@@ -450,7 +457,7 @@ performance.
 
 .. rts-flag:: -O ⟨size⟩
 
-    :default: 1m
+    :default: 4m
 
     .. index::
        single: old generation, size


=====================================
rts/RtsFlags.c
=====================================
@@ -152,10 +152,11 @@ void initRtsFlagsDefaults(void)
     RtsFlags.GcFlags.stkChunkSize       = (32 * 1024) / sizeof(W_);
     RtsFlags.GcFlags.stkChunkBufferSize = (1 * 1024) / sizeof(W_);
 
-    RtsFlags.GcFlags.minAllocAreaSize   = (1024 * 1024)       / BLOCK_SIZE;
+    /* -A default. See #16499 for a discussion about the tradeoffs */
+    RtsFlags.GcFlags.minAllocAreaSize   = (4 * 1024 * 1024)       / BLOCK_SIZE;
     RtsFlags.GcFlags.largeAllocLim      = 0; /* defaults to minAllocAreasize */
     RtsFlags.GcFlags.nurseryChunkSize   = 0;
-    RtsFlags.GcFlags.minOldGenSize      = (1024 * 1024)       / BLOCK_SIZE;
+    RtsFlags.GcFlags.minOldGenSize      = (4 * 1024 * 1024)       / BLOCK_SIZE; /* -O default */
     RtsFlags.GcFlags.maxHeapSize        = 0;    /* off by default */
     RtsFlags.GcFlags.heapLimitGrace     = (1024 * 1024);
     RtsFlags.GcFlags.heapSizeSuggestion = 0;    /* none */



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/444c1e0e9df29e3323bec9cf5d50fb88c57eaafa

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/444c1e0e9df29e3323bec9cf5d50fb88c57eaafa
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