[PATCH v2] erofs-utils: use qsort() to sort dir->i_subdirs

[Hu Weiwen]

Original implementation use insertion sort, and its time complexity is
O(n^2). This patch use qsort instead. When I create a directory with
100k entries, this reduces the user space time from around 3 mins to
0.5s.

Create such a large directory for benchmark with:
mkdir large; cd large; touch $(seq 100000);

Signed-off-by: Hu Weiwen <seh...@mail.scut.edu.cn>
---
 lib/inode.c | 53 +++++++++++++++++++++++++++++++++--------------------
 1 file changed, 33 insertions(+), 20 deletions(-)

diff --git a/lib/inode.c b/lib/inode.c
index d52facf..ef55e88 100644
--- a/lib/inode.c
+++ b/lib/inode.c
@@ -96,21 +96,6 @@ unsigned int erofs_iput(struct erofs_inode *inode)
        return 0;
 }

-static int dentry_add_sorted(struct erofs_dentry *d, struct list_head *head)
-{
-       struct list_head *pos;
-
-       list_for_each(pos, head) {
-               struct erofs_dentry *d2 =
-                       container_of(pos, struct erofs_dentry, d_child);
-
-               if (strcmp(d->name, d2->name) < 0)
-                       break;
-       }
-       list_add_tail(&d->d_child, pos);
-       return 0;
-}
-
 struct erofs_dentry *erofs_d_alloc(struct erofs_inode *parent,
                                   const char *name)
 {
@@ -122,7 +107,7 @@ struct erofs_dentry *erofs_d_alloc(struct erofs_inode 
*parent,
        strncpy(d->name, name, EROFS_NAME_LEN - 1);
        d->name[EROFS_NAME_LEN - 1] = '\0';

-       dentry_add_sorted(d, &parent->i_subdirs);
+       list_add_tail(&d->d_child, &parent->i_subdirs);
        return d;
 }

@@ -156,10 +141,19 @@ static int __allocate_inode_bh_data(struct erofs_inode 
*inode,
        return 0;
 }

+static int comp_subdir(const void *a, const void *b)
+{
+       const struct erofs_dentry *da, *db;
+
+       da = *((const struct erofs_dentry **)a);
+       db = *((const struct erofs_dentry **)b);
+       return strcmp(da->name, db->name);
+}
+
-int erofs_prepare_dir_file(struct erofs_inode *dir)
+int erofs_prepare_dir_file(struct erofs_inode *dir, unsigned int nr_subdirs)
 {
-       struct erofs_dentry *d;
-       unsigned int d_size, i_nlink;
+       struct erofs_dentry *d, *n, **sorted_d;
+       unsigned int d_size, i_nlink, i;
        int ret;

        /* dot is pointed to the current dir inode */
@@ -172,6 +166,22 @@ int erofs_prepare_dir_file(struct erofs_inode *dir)
        d->inode = erofs_igrab(dir->i_parent);
        d->type = EROFS_FT_DIR;

+       /* sort subdirs */
+       nr_subdirs += 2;
+       sorted_d = malloc(nr_subdirs * sizeof(d));
+       if (!sorted_d)
+               return -ENOMEM;
+       i = 0;
+       list_for_each_entry_safe(d, n, &dir->i_subdirs, d_child) {
+               list_del(&d->d_child);
+               sorted_d[i++] = d;
+       }
+       DBG_BUGON(i != nr_subdirs);
+       qsort(sorted_d, nr_subdirs, sizeof(d), comp_subdir);
+       for (i = 0; i < nr_subdirs; i++)
+               list_add_tail(&sorted_d[i]->d_child, &dir->i_subdirs);
+       free(sorted_d);
+
        /* let's calculate dir size and update i_nlink */
        d_size = 0;
        i_nlink = 0;
@@ -922,6 +932,7 @@ struct erofs_inode *erofs_mkfs_build_tree(struct 
erofs_inode *dir)
        DIR *_dir;
        struct dirent *dp;
        struct erofs_dentry *d;
+       unsigned int nr_subdirs;

        ret = erofs_prepare_xattr_ibody(dir);
        if (ret < 0)
@@ -961,6 +972,7 @@ struct erofs_inode *erofs_mkfs_build_tree(struct 
erofs_inode *dir)
                return ERR_PTR(-errno);
        }

+       nr_subdirs = 0;
        while (1) {
                /*
                 * set errno to 0 before calling readdir() in order to
@@ -984,6 +996,7 @@ struct erofs_inode *erofs_mkfs_build_tree(struct 
erofs_inode *dir)
                        ret = PTR_ERR(d);
                        goto err_closedir;
                }
+               nr_subdirs++;

                /* to count i_nlink for directories */
                d->type = (dp->d_type == DT_DIR ?
@@ -996,7 +1009,7 @@ struct erofs_inode *erofs_mkfs_build_tree(struct 
erofs_inode *dir)
        }
        closedir(_dir);

-       ret = erofs_prepare_dir_file(dir);
+       ret = erofs_prepare_dir_file(dir, nr_subdirs);
        if (ret)
                goto err;

--
2.25.1