[src/trunk]: src/usr.bin/make make(1): rewrap and reword the large comment fo...

[rillig <rillig%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/48b26d03296b
branches:  trunk
changeset: 941687:48b26d03296b
user:      rillig <rillig%NetBSD.org@localhost>
date:      Tue Oct 27 06:55:18 2020 +0000

description:
make(1): rewrap and reword the large comment for directory caching

Hashing the directories is not a problem at all, it's the caching that
makes things complicated.

diffstat:

 usr.bin/make/dir.c |  138 +++++++++++++++++++++++++---------------------------
 1 files changed, 67 insertions(+), 71 deletions(-)

diffs (184 lines):

diff -r 59c37aae047a -r 48b26d03296b usr.bin/make/dir.c
--- a/usr.bin/make/dir.c        Tue Oct 27 06:33:38 2020 +0000
+++ b/usr.bin/make/dir.c        Tue Oct 27 06:55:18 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: dir.c,v 1.190 2020/10/26 23:28:52 rillig Exp $ */
+/*     $NetBSD: dir.c,v 1.191 2020/10/27 06:55:18 rillig Exp $ */
 
 /*
  * Copyright (c) 1988, 1989, 1990 The Regents of the University of California.
@@ -69,11 +69,9 @@
  * SUCH DAMAGE.
  */
 
-/*-
- * dir.c --
- *     Directory searching using wildcards and/or normal names...
- *     Used both for source wildcarding in the Makefile and for finding
- *     implicit sources.
+/* Directory searching using wildcards and/or normal names.
+ * Used both for source wildcarding in the makefile and for finding
+ * implicit sources.
  *
  * The interface for this module is:
  *     Dir_Init        Initialize the module.
@@ -121,7 +119,8 @@
  *     Dir_ClearPath   Resets a search path to the empty list.
  *
  * For debugging:
- *     Dir_PrintDirectories    Print stats about the directory cache.
+ *     Dir_PrintDirectories
+ *                     Print stats about the directory cache.
  */
 
 #include <sys/types.h>
@@ -135,80 +134,77 @@
 #include "job.h"
 
 /*     "@(#)dir.c      8.2 (Berkeley) 1/2/94"  */
-MAKE_RCSID("$NetBSD: dir.c,v 1.190 2020/10/26 23:28:52 rillig Exp $");
+MAKE_RCSID("$NetBSD: dir.c,v 1.191 2020/10/27 06:55:18 rillig Exp $");
 
 #define DIR_DEBUG0(text) DEBUG0(DIR, text)
 #define DIR_DEBUG1(fmt, arg1) DEBUG1(DIR, fmt, arg1)
 #define DIR_DEBUG2(fmt, arg1, arg2) DEBUG2(DIR, fmt, arg1, arg2)
 
-/*
- *     A search path consists of a list of CachedDir structures. A CachedDir
- *     has in it the name of the directory and a hash table of all the files
- *     in the directory. This is used to cut down on the number of system
- *     calls necessary to find implicit dependents and their like. Since
- *     these searches are made before any actions are taken, we need not
- *     worry about the directory changing due to creation commands. If this
- *     hampers the style of some makefiles, they must be changed.
+/* A search path is a list of CachedDir structures. A CachedDir has in it the
+ * name of the directory and the names of all the files in the directory.
+ * This is used to cut down on the number of system calls necessary to find
+ * implicit dependents and their like. Since these searches are made before
+ * any actions are taken, we need not worry about the directory changing due
+ * to creation commands. If this hampers the style of some makefiles, they
+ * must be changed.
+ *
+ * All previously-read directories are kept in openDirs, which is checked
+ * first before a directory is opened.
  *
- *     A list of all previously-read directories is kept in the
- *     openDirectories Lst. This list is checked first before a directory
- *     is opened.
+ * The need for the caching of whole directories is brought about by the
+ * multi-level transformation code in suff.c, which tends to search for far
+ * more files than regular make does. In the initial implementation, the
+ * amount of time spent performing "stat" calls was truly astronomical.
+ * The problem with caching at the start is, of course, that pmake doesn't
+ * then detect changes to these directories during the course of the make.
+ * Three possibilities suggest themselves:
  *
- *     The need for the caching of whole directories is brought about by
- *     the multi-level transformation code in suff.c, which tends to search
- *     for far more files than regular make does. In the initial
- *     implementation, the amount of time spent performing "stat" calls was
- *     truly astronomical. The problem with hashing at the start is,
- *     of course, that pmake doesn't then detect changes to these directories
- *     during the course of the make. Three possibilities suggest themselves:
+ * 1)  just use stat to test for a file's existence. As mentioned above,
+ *     this is very inefficient due to the number of checks engendered by
+ *     the multi-level transformation code.
+ *
+ * 2)  use readdir() and company to search the directories, keeping them
+ *     open between checks. I have tried this and while it didn't slow down
+ *     the process too much, it could severely affect the amount of
+ *     parallelism available as each directory open would take another file
+ *     descriptor out of play for handling I/O for another job. Given that
+ *     it is only recently that UNIX OS's have taken to allowing more than
+ *     20 or 32 file descriptors for a process, this doesn't seem acceptable
+ *     to me.
  *
- *         1) just use stat to test for a file's existence. As mentioned
- *            above, this is very inefficient due to the number of checks
- *            engendered by the multi-level transformation code.
- *         2) use readdir() and company to search the directories, keeping
- *            them open between checks. I have tried this and while it
- *            didn't slow down the process too much, it could severely
- *            affect the amount of parallelism available as each directory
- *            open would take another file descriptor out of play for
- *            handling I/O for another job. Given that it is only recently
- *            that UNIX OS's have taken to allowing more than 20 or 32
- *            file descriptors for a process, this doesn't seem acceptable
- *            to me.
- *         3) record the mtime of the directory in the CachedDir structure and
- *            verify the directory hasn't changed since the contents were
- *            hashed. This will catch the creation or deletion of files,
- *            but not the updating of files. However, since it is the
- *            creation and deletion that is the problem, this could be
- *            a good thing to do. Unfortunately, if the directory (say ".")
- *            were fairly large and changed fairly frequently, the constant
- *            rehashing could seriously degrade performance. It might be
- *            good in such cases to keep track of the number of rehashes
- *            and if the number goes over a (small) limit, resort to using
- *            stat in its place.
+ * 3)  record the mtime of the directory in the CachedDir structure and
+ *     verify the directory hasn't changed since the contents were cached.
+ *     This will catch the creation or deletion of files, but not the
+ *     updating of files. However, since it is the creation and deletion
+ *     that is the problem, this could be a good thing to do. Unfortunately,
+ *     if the directory (say ".") were fairly large and changed fairly
+ *     frequently, the constant reloading could seriously degrade
+ *     performance. It might be good in such cases to keep track of the
+ *     number of reloadings and if the number goes over a (small) limit,
+ *     resort to using stat in its place.
  *
- *     An additional thing to consider is that pmake is used primarily
- *     to create C programs and until recently pcc-based compilers refused
- *     to allow you to specify where the resulting object file should be
- *     placed. This forced all objects to be created in the current
- *     directory. This isn't meant as a full excuse, just an explanation of
- *     some of the reasons for the caching used here.
+ * An additional thing to consider is that pmake is used primarily to create
+ * C programs and until recently pcc-based compilers refused to allow you to
+ * specify where the resulting object file should be placed. This forced all
+ * objects to be created in the current directory. This isn't meant as a full
+ * excuse, just an explanation of some of the reasons for the caching used
+ * here.
  *
- *     One more note: the location of a target's file is only performed
- *     on the downward traversal of the graph and then only for terminal
- *     nodes in the graph. This could be construed as wrong in some cases,
- *     but prevents inadvertent modification of files when the "installed"
- *     directory for a file is provided in the search path.
+ * One more note: the location of a target's file is only performed on the
+ * downward traversal of the graph and then only for terminal nodes in the
+ * graph. This could be construed as wrong in some cases, but prevents
+ * inadvertent modification of files when the "installed" directory for a
+ * file is provided in the search path.
  *
- *     Another data structure maintained by this module is an mtime
- *     cache used when the searching of cached directories fails to find
- *     a file. In the past, Dir_FindFile would simply perform an access()
- *     call in such a case to determine if the file could be found using
- *     just the name given. When this hit, however, all that was gained
- *     was the knowledge that the file existed. Given that an access() is
- *     essentially a stat() without the copyout() call, and that the same
- *     filesystem overhead would have to be incurred in Dir_MTime, it made
- *     sense to replace the access() with a stat() and record the mtime
- *     in a cache for when Dir_MTime was actually called.
+ * Another data structure maintained by this module is an mtime cache used
+ * when the searching of cached directories fails to find a file. In the past,
+ * Dir_FindFile would simply perform an access() call in such a case to
+ * determine if the file could be found using just the name given. When this
+ * hit, however, all that was gained was the knowledge that the file existed.
+ * Given that an access() is essentially a stat() without the copyout() call,
+ * and that the same filesystem overhead would have to be incurred in
+ * Dir_MTime, it made sense to replace the access() with a stat() and record
+ * the mtime in a cache for when Dir_MTime was actually called.
  */
 
 typedef List CachedDirList;
@@ -280,7 +276,7 @@
 static OpenDirs openDirs;      /* the list of all open directories */
 
 /*
- * Variables for gathering statistics on the efficiency of the hashing
+ * Variables for gathering statistics on the efficiency of the cashing
  * mechanism.
  */
 static int hits;               /* Found in directory cache */