Improve file unpacking order

This commit moves the file unpacking order & job scheduling to a libfstree
function. The ordering is improved by making sure fragment blocks are not
extracted more than once and files with data blocks are extracted in order.

This way, serial unpacking of a 2GiB Debian live image could be reduced
from ~5' on my test machine to ~3.5', whereas parallel unpacking stays
roughly the same (~3' for -j 4).

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

2 files changed, 162 insertions, 0 deletions

diff --git a/lib/Makemodule.am b/lib/Makemodule.am
index f49ff91..c3451f4 100644
--- a/lib/Makemodule.am
+++ b/lib/Makemodule.am
@@ -5,6 +5,7 @@ libfstree_a_SOURCES += lib/fstree/node_stat.c lib/fstree/mknode.c
 libfstree_a_SOURCES += lib/fstree/add_by_path.c lib/fstree/xattr.c
 libfstree_a_SOURCES += lib/fstree/node_from_path.c include/fstree.h
 libfstree_a_SOURCES += lib/fstree/gen_file_list.c lib/fstree/deduplicate.c
+libfstree_a_SOURCES += lib/fstree/optimize_unpack_order.c
 libfstree_a_CFLAGS = $(AM_CFLAGS)
 libfstree_a_CPPFLAGS = $(AM_CPPFLAGS)
 
diff --git a/lib/fstree/optimize_unpack_order.c b/lib/fstree/optimize_unpack_order.c
new file mode 100644
index 0000000..ad036e8
--- /dev/null
+++ b/lib/fstree/optimize_unpack_order.c
@@ -0,0 +1,161 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+/*
+ * optimize_unpack_order.c
+ *
+ * Copyright (C) 2019 David Oberhollenzer <goliath@infraroot.at>
+ */
+#include "config.h"
+#include "fstree.h"
+
+static int compare_files(const fstree_t *fs, const file_info_t *lhs,
+			 const file_info_t *rhs)
+{
+	/* NOOP < everything else */
+	if (lhs->input_file == NULL)
+		return rhs->input_file == NULL ? 0 : -1;
+
+	if (rhs->input_file == NULL)
+		return 1;
+
+	/* Files with fragments come first, ordered by ID.
+	   In case of tie, files without data blocks come first,
+	   and the others are ordered by start block. */
+	if (lhs->flags & FILE_FLAG_HAS_FRAGMENT) {
+		if (!(rhs->flags & FILE_FLAG_HAS_FRAGMENT))
+			return -1;
+
+		if (lhs->fragment < rhs->fragment)
+			return -1;
+		if (lhs->fragment > rhs->fragment)
+			return 1;
+
+		if (lhs->size < fs->block_size)
+			return (rhs->size < fs->block_size) ? 0 : -1;
+		if (rhs->size < fs->block_size)
+			return 1;
+		goto order_by_start;
+	}
+
+	if (rhs->flags & FILE_FLAG_HAS_FRAGMENT)
+		return 1;
+
+	/* order the rest by start block */
+order_by_start:
+	return lhs->startblock < rhs->startblock ? -1 :
+		lhs->startblock > rhs->startblock ? 1 : 0;
+}
+
+/* TODO: unify ad-hoc merge sort with the one in fstree_sort */
+static file_info_t *merge(const fstree_t *fs, file_info_t *lhs,
+			  file_info_t *rhs)
+{
+	file_info_t *it;
+	file_info_t *head = NULL;
+	file_info_t **next_ptr = &head;
+
+	while (lhs != NULL && rhs != NULL) {
+		if (compare_files(fs, lhs, rhs) <= 0) {
+			it = lhs;
+			lhs = lhs->next;
+		} else {
+			it = rhs;
+			rhs = rhs->next;
+		}
+
+		*next_ptr = it;
+		next_ptr = &it->next;
+	}
+
+	it = (lhs != NULL ? lhs : rhs);
+	*next_ptr = it;
+	return head;
+}
+
+static file_info_t *list_sort(const fstree_t *fs, file_info_t *head)
+{
+	file_info_t *it, *half, *prev;
+
+	it = half = prev = head;
+	while (it != NULL) {
+		prev = half;
+		half = half->next;
+		it = it->next;
+		if (it != NULL) {
+			it = it->next;
+		}
+	}
+
+	// half refers to the (count/2)'th element ROUNDED UP.
+	// It will be null therefore only in the empty and the
+	// single element list
+	if (half == NULL) {
+		return head;
+	}
+
+	prev->next = NULL;
+
+	return merge(fs, list_sort(fs, head), list_sort(fs, half));
+}
+
+static file_info_t *split_list(file_info_t *list, uint64_t threashold)
+{
+	file_info_t *it, *new = NULL;
+	uint64_t size = 0;
+
+	for (it = list; it != NULL; it = it->next) {
+		size += it->size - it->sparse;
+
+		if (size >= threashold) {
+			new = it->next;
+			it->next = NULL;
+			break;
+		}
+	}
+
+	return new;
+}
+
+static uint64_t total_size(file_info_t *list)
+{
+	uint64_t size = 0;
+	file_info_t *it;
+
+	for (it = list; it != NULL; it = it->next)
+		size += it->size - it->sparse;
+
+	return size;
+}
+
+void optimize_unpack_order(fstree_t *fs, size_t num_jobs,
+			   file_info_t *out[num_jobs])
+{
+	file_info_t *file_list;
+	uint64_t threshold;
+	size_t i;
+
+	if (num_jobs < 1)
+		return;
+
+	for (i = 0; i < num_jobs; ++i)
+		out[i] = NULL;
+
+	file_list = list_sort(fs, fs->files);
+	while (file_list != NULL && file_list->input_file == NULL)
+		file_list = file_list->next;
+
+	fs->files = NULL;
+
+	if (num_jobs < 2) {
+		out[0] = file_list;
+		return;
+	}
+
+	threshold = total_size(file_list) / num_jobs;
+
+	for (i = 0; i < (num_jobs - 1); ++i) {
+		out[i] = file_list;
+		file_list = split_list(file_list, threshold);
+	}
+
+	out[i] = file_list;
+}