Author: Armin Rigo <[email protected]>
Branch: c7-more-segments
Changeset: r1023:fa8bfb4eb410
Date: 2014-03-15 18:42 +0100
http://bitbucket.org/pypy/stmgc/changeset/fa8bfb4eb410/
Log: Write a long comment about the new model I'm aiming for.
diff --git a/c7/stm/pages.h b/c7/stm/pages.h
--- a/c7/stm/pages.h
+++ b/c7/stm/pages.h
@@ -1,21 +1,32 @@
-enum /* flag_page_private */ {
- /* The page is not in use. Assume that each segment sees its own copy. */
- FREE_PAGE=0,
+/* For every page, 'num_segments_sharing_page' stores a number that
+ counts the number of segments that share the page. If 0, the page is
+ not used so far.
- /* The page is shared by all segments. Each segment sees the same
- physical page (the one that is within the segment 0 mmap address). */
- SHARED_PAGE,
+ When the page is first initialized, 'num_segments_sharing_page' is
+ set to NB_SEGMENTS. When later a segment wants a private copy, it
+ looks first in its own 'private_page_mapping' tree, which maps shared
+ pages to private copies. If not found, then it proceeds like this:
- /* For only one range of pages at a time, around the call to
- remap_file_pages() that un-shares the pages (SHARED -> PRIVATE). */
- REMAPPING_PAGE,
+ If 'num_segments_sharing_page' is greater than 1, then it is
+ decremented and a private copy of the page is made.
- /* Page is private for each segment. */
- PRIVATE_PAGE,
-};
+ If 'num_segments_sharing_page' is equal to 1, then we know we are the
+ last segment that sees this "shared" copy, and so it is actually not
+ shared with anybody else --- i.e. it is private already.
-static uint8_t flag_page_private[NB_PAGES];
+ The shared copy of a page is stored in the mmap at the file offset
+ corresponding to the segment 0 offset (with all other segments
+ remapping to the segment 0 offset). Private copies are made in the
+ offset from segment 1 (and if full, more segments afterwards),
+ picking file offsets that are simply the next free ones. This is
+ probably good for long-term memory usage: a major collection looks
+ for pages that are no-longer-used private copies of some shared page,
+ and discard them, remapping the address to the shared page. The
+ pages thus freed are recorded into a free list, and can be reused as
+ the private copies of the following (unrelated) pages.
+*/
+static uint8_t num_segments_sharing_page[NB_PAGES];
static void _pages_privatize(uintptr_t pagenum, uintptr_t count);
static void pages_initialize_shared(uintptr_t pagenum, uintptr_t count);
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