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Re: [PATCH v10 3/3] read-cache: speed up add_index_entry during checkout






On 4/15/2017 1:55 PM, René Scharfe wrote:
Am 14.04.2017 um 21:12 schrieb git@xxxxxxxxxxxxxxxxx:
From: Jeff Hostetler <jeffhost@xxxxxxxxxxxxx>

Very nice, especially the perf test!  But can we unbundle the different
optimizations into separate patches with their own performance numbers?
Candidates IMHO: The change in add_index_entry_with_check(), the first
hunk in has_dir_name(), the loop shortcuts.  That might also help find
the reason for the slight slowdown of 0006.3 for the kernel repository.

Let me take a look at this and see if it helps.

diff --git a/read-cache.c b/read-cache.c
index 97f13a1..ba95fbb 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -910,6 +910,9 @@ int strcmp_offset(const char *s1, const char *s2,
size_t *first_change)
  /*
   * Do we have another file with a pathname that is a proper
   * subset of the name we're trying to add?
+ *
+ * That is, is there another file in the index with a path
+ * that matches a sub-directory in the given entry?
   */
  static int has_dir_name(struct index_state *istate,
              const struct cache_entry *ce, int pos, int ok_to_replace)
@@ -918,9 +921,51 @@ static int has_dir_name(struct index_state *istate,
      int stage = ce_stage(ce);
      const char *name = ce->name;
      const char *slash = name + ce_namelen(ce);
+    size_t len_eq_last;
+    int cmp_last = 0;
+
+    /*
+     * We are frequently called during an iteration on a sorted
+     * list of pathnames and while building a new index.  Therefore,
+     * there is a high probability that this entry will eventually
+     * be appended to the index, rather than inserted in the middle.
+     * If we can confirm that, we can avoid binary searches on the
+     * components of the pathname.
+     *
+     * Compare the entry's full path with the last path in the index.
+     */
+    if (istate->cache_nr > 0) {
+        cmp_last = strcmp_offset(name,
+            istate->cache[istate->cache_nr - 1]->name,
+            &len_eq_last);
+        if (cmp_last > 0) {
+            if (len_eq_last == 0) {
+                /*
+                 * The entry sorts AFTER the last one in the
+                 * index and their paths have no common prefix,
+                 * so there cannot be a F/D conflict.
+                 */
+                return retval;
+            } else {
+                /*
+                 * The entry sorts AFTER the last one in the
+                 * index, but has a common prefix.  Fall through
+                 * to the loop below to disect the entry's path
+                 * and see where the difference is.
+                 */
+            }
+        } else if (cmp_last == 0) {
+            /*
+             * The entry exactly matches the last one in the
+             * index, but because of multiple stage and CE_REMOVE
+             * items, we fall through and let the regular search
+             * code handle it.
+             */
+        }
+    }
        for (;;) {
-        int len;
+        size_t len;
            for (;;) {
              if (*--slash == '/')
@@ -930,6 +975,66 @@ static int has_dir_name(struct index_state *istate,
          }
          len = slash - name;
  +        if (cmp_last > 0) {
+            /*
+             * (len + 1) is a directory boundary (including
+             * the trailing slash).  And since the loop is
+             * decrementing "slash", the first iteration is
+             * the longest directory prefix; subsequent
+             * iterations consider parent directories.
+             */
+
+            if (len + 1 <= len_eq_last) {
+                /*
+                 * The directory prefix (including the trailing
+                 * slash) also appears as a prefix in the last
+                 * entry, so the remainder cannot collide (because
+                 * strcmp said the whole path was greater).
+                 *
+                 * EQ: last: xxx/A
+                 *     this: xxx/B
+                 *
+                 * LT: last: xxx/file_A
+                 *     this: xxx/file_B
+                 */
+                return retval;
+            }
+
+            if (len > len_eq_last) {
+                /*
+                 * This part of the directory prefix (excluding
+                 * the trailing slash) is longer than the known
+                 * equal portions, so this sub-directory cannot
+                 * collide with a file.
+                 *
+                 * GT: last: xxxA
+                 *     this: xxxB/file
+                 */
+                return retval;
+            }
+

At this point len and len_eq_last are equal -- otherwise one of the two
previous conditions would have triggered.  Silly question: Is this
necessary for the following shortcut to work?  Removing the two checks
above doesn't seem to affect performance very much, and at least the
test suite still passes..

Both of these are highly dependent on the shape of the
pathnames in the tree.  Let me try to quantify this.


+            if (ce_namelen(istate->cache[istate->cache_nr - 1]) > len) {
+                /*
+                 * The directory prefix lines up with part of
+                 * a longer file or directory name, but sorts
+                 * after it, so this sub-directory cannot
+                 * collide with a file.
+                 *
+                 * last: xxx/yy-file (because '-' sorts before '/')
+                 * this: xxx/yy/abc
+                 */
+                return retval;
+            }

istate->cache_nr can be zero if remove_index_entry_at() had been called
in a previous iteration, resulting in a segfault.  Checking right here
is probably the easiest way out; not sure if exiting early when the
index becomes empty would be better.

yeah, i guess remove_...() could delete them all and this test
would be ill-defined.  Let me fix that.


+
+            /*
+             * This is a possible collision. Fall through and
+             * let the regular search code handle it.
+             *
+             * last: xxx
+             * this: xxx/file
+             */
+        }
+
          pos = index_name_stage_pos(istate, name, len, stage);
          if (pos >= 0) {
              /*
@@ -1021,7 +1126,16 @@ static int add_index_entry_with_check(struct
index_state *istate, struct cache_e
        if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
          cache_tree_invalidate_path(istate, ce->name);
-    pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce),
ce_stage(ce));
+
+    /*
+     * If this entry's path sorts after the last entry in the index,
+     * we can avoid searching for it.
+     */
+    if (istate->cache_nr > 0 &&
+        strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
+        pos = -istate->cache_nr - 1;
+    else
+        pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce),
ce_stage(ce));
        /* existing match? Just replace it. */
      if (pos >= 0) {