Author: njn
Date: 2007-10-17 00:18:06 +0100 (Wed, 17 Oct 2007)
New Revision: 7009
Log:
Add a comment.
Modified:
trunk/coregrind/m_mallocfree.c
Modified: trunk/coregrind/m_mallocfree.c
===================================================================
--- trunk/coregrind/m_mallocfree.c 2007-10-16 21:07:43 UTC (rev 7008)
+++ trunk/coregrind/m_mallocfree.c 2007-10-16 23:18:06 UTC (rev 7009)
@@ -1104,6 +1104,24 @@
req_bszB = pszB_to_bszB(a, req_pszB);
// Scan through all the big-enough freelists for a block.
+ //
+ // Nb: this scanning might be expensive in some cases. Eg. if you
+ // allocate lots of small objects without freeing them, but no
+ // medium-sized objects, it will repeatedly scanning through the whole
+ // list, and each time not find any free blocks until the last element.
+ //
+ // If this becomes a noticeable problem... the loop answers the question
+ // "where is the first nonempty list above me?" And most of the time,
+ // you ask the same question and get the same answer. So it would be
+ // good to somehow cache the results of previous searches.
+ // One possibility is an array (with N_MALLOC_LISTS elements) of
+ // shortcuts. shortcut[i] would give the index number of the nearest
+ // larger list above list i which is non-empty. Then this loop isn't
+ // necessary. However, we'd have to modify some section [ .. i-1] of the
+ // shortcut array every time a list [i] changes from empty to nonempty or
+ // back. This would require care to avoid pathological worst-case
+ // behaviour.
+ //
for (lno = pszB_to_listNo(req_pszB); lno < N_MALLOC_LISTS; lno++) {
b = a->freelist[lno];
if (NULL == b) continue; // If this list is empty, try the next one.
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