[pypy-commit] stmgc default: be smarter about finding a source page in the signal handler

[Raemi]

Author: Remi Meier <remi.meier at gmail.com>
Branch: 
Changeset: r2012:6cd15231609a
Date: 2017-02-10 11:18 +0100
http://bitbucket.org/pypy/stmgc/changeset/6cd15231609a/

Log:	be smarter about finding a source page in the signal handler

	It's unclear if this approach improves performance, but the previous
	strategy of simply finding the most recent revision sounded too
	simple. If only for the additional comment, the new approach makes
	more sense intuitively.

diff --git a/c8/stm/signal_handler.c b/c8/stm/signal_handler.c
--- a/c8/stm/signal_handler.c
+++ b/c8/stm/signal_handler.c
@@ -5,6 +5,7 @@
 
 
 
+
 static void setup_signal_handler(void)
 {
     struct sigaction act;
@@ -76,19 +77,37 @@
 
     assert(get_page_status_in(my_segnum, pagenum) == PAGE_NO_ACCESS);
 
-    /* find who has the most recent revision of our page */
+    /* find a suitable page to copy from in other segments:
+     * suitable means:
+     *  - if there is a revision around >= target_rev, get the oldest >= target_rev.
+     *  - otherwise find most recent revision
+     * Note: simply finding the most recent revision would be a conservative strategy, but
+     *       requires going back in time more often (see below)
+     */
     int copy_from_segnum = -1;
-    uint64_t most_recent_rev = 0;
+    uint64_t copy_from_rev = 0;
+    uint64_t target_rev = STM_PSEGMENT->last_commit_log_entry->rev_num;
     for (i = 1; i < NB_SEGMENTS; i++) {
         if (i == my_segnum)
             continue;
 
         struct stm_commit_log_entry_s *log_entry;
         log_entry = get_priv_segment(i)->last_commit_log_entry;
-        if (get_page_status_in(i, pagenum) != PAGE_NO_ACCESS
-            && (copy_from_segnum == -1 || log_entry->rev_num > most_recent_rev)) {
+
+        /* - if not found anything, initialise copy_from_rev
+         * - else if target_rev is higher than everything we found, find newest among them
+         * - else: find revision that is as close to target_rev as possible         */
+        bool accessible = get_page_status_in(i, pagenum) != PAGE_NO_ACCESS;
+        bool uninit = copy_from_segnum == -1;
+        bool find_most_recent = copy_from_rev < target_rev && log_entry->rev_num > copy_from_rev;
+        bool find_closest = copy_from_rev >= target_rev && (
+            log_entry->rev_num - target_rev < copy_from_rev - target_rev);
+
+        if (accessible && (uninit || find_most_recent || find_closest)) {
             copy_from_segnum = i;
-            most_recent_rev = log_entry->rev_num;
+            copy_from_rev = log_entry->rev_num;
+            if (copy_from_rev == target_rev)
+                break;
         }
     }
     OPT_ASSERT(copy_from_segnum != my_segnum);
@@ -118,21 +137,20 @@
     /* if there were modifications in the page, revert them. */
     copy_bk_objs_in_page_from(copy_from_segnum, pagenum, false);
 
-    /* we need to go from 'src_version' to 'target_version'.  This
+    dprintf(("handle_segfault_in_page: rev %lu to rev %lu\n",
+             copy_from_rev, target_rev));
+    /* we need to go from 'copy_from_rev' to 'target_rev'.  This
        might need a walk into the past. */
-    struct stm_commit_log_entry_s *src_version, *target_version;
-    src_version = get_priv_segment(copy_from_segnum)->last_commit_log_entry;
-    target_version = STM_PSEGMENT->last_commit_log_entry;
+    if (copy_from_rev > target_rev) {
+        /* adapt revision of page to our revision:
+           if our rev is higher than the page we copy from, everything
+           is fine as we never read/modified the page anyway */
+        struct stm_commit_log_entry_s *src_version, *target_version;
+        src_version = get_priv_segment(copy_from_segnum)->last_commit_log_entry;
+        target_version = STM_PSEGMENT->last_commit_log_entry;
 
-
-    dprintf(("handle_segfault_in_page: rev %lu to rev %lu\n",
-             src_version->rev_num, target_version->rev_num));
-    /* adapt revision of page to our revision:
-       if our rev is higher than the page we copy from, everything
-       is fine as we never read/modified the page anyway
-     */
-    if (src_version->rev_num > target_version->rev_num)
         go_to_the_past(pagenum, src_version, target_version);
+    }
 
     release_modification_lock_set(to_lock, my_segnum);
     release_all_privatization_locks();