Author: Armin Rigo <ar...@tunes.org>
Branch: c7-more-segments
Changeset: r1024:4ab1f6857a09
Date: 2014-03-15 18:54 +0100
http://bitbucket.org/pypy/stmgc/changeset/4ab1f6857a09/
Log: Complete
diff --git a/c7/stm/pages.h b/c7/stm/pages.h
--- a/c7/stm/pages.h
+++ b/c7/stm/pages.h
@@ -15,16 +15,31 @@
last segment that sees this "shared" copy, and so it is actually not
shared with anybody else --- i.e. it is private already.
+ (This means that 'num_segments_sharing_page' is basically just an
+ optimization. Without it, we might need 'NB_SEGMENTS + 1' copies of
+ the same data; with it, we can bound the number to 'NB_SEGMENTS'.
+ This is probably important if NB_SEGMENTS is very small.)
+
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,
+ 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.
+
+ (*) an additional subtlety here is that the shared page should not
+ contain uncommitted changes; if 'num_segments_sharing_page' is 1 this
+ can occur.
+
+ Note that this page manipulation logic is independent from actually
+ tracking which objects are uncommitted, which occurs at the level of
+ segment-relative offsets; and propagating changes during commit,
+ which is done by copying objects (not pages) to the same offset
+ relative to a different segment.
*/
static uint8_t num_segments_sharing_page[NB_PAGES];
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