[Darshan-commits] [Darshan] branch, dev-modular, updated. darshan-2.3.1-78-g905e0ac

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- Log -----------------------------------------------------------------
commit 905e0ac4cccc3cee2bf5324b572799ce64a9c8ec
Author: Phil Carns <carns at mcs.anl.gov>
Date:   Wed Apr 1 09:54:54 2015 -0500

    extended comments on posix_file_runtime_ref

commit 2f7e0da108c8fcb07b5246c79ead55e6e647ba25
Author: Phil Carns <carns at mcs.anl.gov>
Date:   Wed Apr 1 09:52:06 2015 -0500

    extended comments on darshan_file_runtime struct

-----------------------------------------------------------------------

Summary of changes:
 darshan-runtime/lib/darshan-posix.c |   27 +++++++++++++++++++++++++++
 1 files changed, 27 insertions(+), 0 deletions(-)


Diff of changes:
diff --git a/darshan-runtime/lib/darshan-posix.c b/darshan-runtime/lib/darshan-posix.c
index 41b2838..a06cbd0 100644
--- a/darshan-runtime/lib/darshan-posix.c
+++ b/darshan-runtime/lib/darshan-posix.c
@@ -97,6 +97,22 @@ enum posix_io_type
  * assists with the instrumenting of specific statistics in the file record.
  * 'hlink' is a hash table link structure used to add/remove this record
  * from the hash table of POSIX file records for this process. 
+ *
+ * RATIONALE: the POSIX module needs to track some stateful, volatile 
+ * information about each open file (like the current file offset, most recent 
+ * access time, etc.) to aid in instrumentation, but this information can't be
+ * stored in the darshan_posix_file struct because we don't want it to appear in
+ * the final darshan log file.  We therefore associate a posix_file_runtime
+ * struct with each darshan_posix_file struct in order to track this information.
+  *
+ * NOTE: There is a one-to-one mapping of posix_file_runtime structs to
+ * darshan_posix_file structs.
+ *
+ * NOTE: The posix_file_runtime struct contains a pointer to a darshan_posix_file
+ * struct (see the *file_record member) rather than simply embedding an entire
+ * darshan_posix_file struct.  This is done so that all of the darshan_posix_file
+ * structs can be kept contiguous in memory as a single array to simplify
+ * reduction, compression, and storage.
  */
 struct posix_file_runtime
 {
@@ -122,6 +138,17 @@ struct posix_file_runtime
  * available. This structure includes another hash table link, since separate
  * hashes are maintained for posix_file_runtime structures and posix_file_runtime_ref
  * structures.
+ *
+ * RATIONALE: In theory the fd information could be included in the
+ * posix_file_runtime struct rather than in a separate structure here.  The
+ * reason we don't do that is because the same file could be opened multiple
+ * times by a given process with different file descriptors and thus
+ * simulataneously referenced using different file descriptors.  This practice is
+ * not common, but we must support it.
+ *
+ * NOTE: there are potentially multiple posix_file_runtime_ref structures
+ * referring to a single posix_file_runtime structure.  Most of the time there is
+ * only one, however.
  */
 struct posix_file_runtime_ref
 {


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