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From: Nick Piggin on 20 May 2010 03:10
Just wondering whether there is a good reason to have a full 16 bits of
sequence in ipc ids? 32K indexes is pretty easy to overflow, if only in
stress tests for now. I was doing some big aim7 stress testing, which
required this patch, but it's not exactly a realistic workload :)

But the sequence seems like it just helps slightly with buggy apps, and
if the app is buggy then it can by definition mess up its own ids
anyway? So I don't see that such amount of seq is required.

Index: linux-2.6/ipc/util.h
===================================================================
--- linux-2.6.orig/ipc/util.h
+++ linux-2.6/ipc/util.h
@@ -14,7 +14,16 @@
#include <linux/err.h>

/* IPCMNI_MAX should be <= MAX_INT, absolute limit for ipc arrays */
-#define IPCMNI_MAX_SHIFT 15
+/*
+ * IPC ids consist of an index into the idr, which allocates from the bottom
+ * up, and a sequence number which is continually incremented. Valid indexes
+ * are from 0..IPCMNI_MAX (or further constrained by sysctls or other limits).
+ * The sequence number prevents ids from being reused quickly. The sequence
+ * number resides in the top part of the 'int' after IPCMNI_MAX.
+ *
+ * Increasing IPCMNI_MAX reduces the sequence wrap interval.
+ */
+#define IPCMNI_MAX_SHIFT 20
#define IPCMNI_MAX (1 << IPCMNI_MAX_SHIFT)

#define SEQ_SHIFT IPCMNI_MAX_SHIFT
--
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