[Sbcl-commits] 1.0.46.12: faster core startup on x86-64

[Nikodemus S. <nik...@ra...>]

1.0.46.12: faster core startup on x86-64

 Patch by Lutz Euler, lp#557357.

 Remove unnecessary page_table initialization loop: it is allocated
 using calloc, which zero-initializes it -- which as it happens is
 enough for us.

Index: NEWS
===================================================================
RCS file: /cvsroot/sbcl/sbcl/NEWS,v
retrieving revision 1.1882
diff -u -r1.1882 NEWS
--- NEWS	20 Feb 2011 11:48:50 -0000	1.1882
+++ NEWS	20 Feb 2011 16:31:32 -0000
@@ -6,6 +6,7 @@
   * enhancement: (FORMAT "foo" ...) and similar signal a compile-time
warning. (lp#327223)
   * optimization: SLOT-VALUE &co are faster in the presence of
SLOT-VALUE-USING-CLASS
     and its compatriots.
+  * optimization: core startup time is reduced by 30% on x86-64. (lp#557357)
   * bug fix: SB-DEBUG:BACKTRACE-AS-LIST guards against potentially leaking
     stack-allocated values out of their dynamic-extent. (lp#310175)
   * bug fix: attempts to use SB-SPROF for wallclock profiling on threaded
Index: version.lisp-expr
===================================================================
RCS file: /cvsroot/sbcl/sbcl/version.lisp-expr,v
retrieving revision 1.5196
diff -u -r1.5196 version.lisp-expr
--- version.lisp-expr	20 Feb 2011 11:48:50 -0000	1.5196
+++ version.lisp-expr	20 Feb 2011 16:31:32 -0000
@@ -20,4 +20,4 @@
 ;;; checkins which aren't released. (And occasionally for internal
 ;;; versions, especially for internal versions off the main CVS
 ;;; branch, it gets hairier, e.g. "0.pre7.14.flaky4.13".)
-"1.0.46.11"
+"1.0.46.12"
Index: src/runtime/gencgc.c
===================================================================
RCS file: /cvsroot/sbcl/sbcl/src/runtime/gencgc.c,v
retrieving revision 1.167
diff -u -r1.167 gencgc.c
--- src/runtime/gencgc.c	7 Aug 2010 13:46:26 -0000	1.167
+++ src/runtime/gencgc.c	20 Feb 2011 16:31:34 -0000
@@ -4487,6 +4487,10 @@
     page_table_pages = dynamic_space_size/PAGE_BYTES;
     gc_assert(dynamic_space_size == npage_bytes(page_table_pages));

+    /* The page_table must be allocated using "calloc" to initialize
+     * the page structures correctly. There used to be a separate
+     * initialization loop (now commented out; see below) but that was
+     * unnecessary and did hurt startup time. */
     page_table = calloc(page_table_pages, sizeof(struct page));
     gc_assert(page_table);

@@ -4502,14 +4506,38 @@

     heap_base = (void*)DYNAMIC_SPACE_START;

-    /* Initialize each page structure. */
-    for (i = 0; i < page_table_pages; i++) {
-        /* Initialize all pages as free. */
-        page_table[i].allocated = FREE_PAGE_FLAG;
-        page_table[i].bytes_used = 0;
-
-        /* Pages are not write-protected at startup. */
-        page_table[i].write_protected = 0;
+    /* The page structures are initialized implicitly when page_table
+     * is allocated with "calloc" above. Formerly we had the following
+     * explicit initialization here (comments converted to C99 style
+     * for readability as C's block comments don't nest):
+     *
+     * // Initialize each page structure.
+     * for (i = 0; i < page_table_pages; i++) {
+     *     // Initialize all pages as free.
+     *     page_table[i].allocated = FREE_PAGE_FLAG;
+     *     page_table[i].bytes_used = 0;
+     *
+     *     // Pages are not write-protected at startup.
+     *     page_table[i].write_protected = 0;
+     * }
+     *
+     * Without this loop the image starts up much faster when dynamic
+     * space is large -- which it is on 64-bit platforms already by
+     * default -- and when "calloc" for large arrays is implemented
+     * using copy-on-write of a page of zeroes -- which it is at least
+     * on Linux. In this case the pages that page_table_pages is stored
+     * in are mapped and cleared not before the corresponding part of
+     * dynamic space is used. For example, this saves clearing 16 MB of
+     * memory at startup if the page size is 4 KB and the size of
+     * dynamic space is 4 GB.
+     * FREE_PAGE_FLAG must be 0 for this to work correctly which is
+     * asserted below: */
+    {
+      /* Compile time assertion: If triggered, declares an array
+       * of dimension -1 forcing a syntax error. The intent of the
+       * assignment is to avoid an "unused variable" warning. */
+      char assert_free_page_flag_0[(FREE_PAGE_FLAG) ? -1 : 1];
+      assert_free_page_flag_0[0] = assert_free_page_flag_0[0];
     }

     bytes_allocated = 0;