[PATCH v2 2/5] fast-import: introduce mem_pool type

[Jameson Miller <jamill@microsoft.com>]

Introduce the mem_pool type and wrap the existing mp_block in this new
type. The new mem_pool type will allow for the memory pool logic to be
reused outside of fast-import. This type will be moved into its own file
in a future commit.

Signed-off-by: Jameson Miller <jamill@microsoft.com>
---
 fast-import.c | 108 +++++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 89 insertions(+), 19 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index 6c3215d7c3..1262d9e6be 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -216,6 +216,19 @@ struct mp_block {
 	uintmax_t space[FLEX_ARRAY]; /* more */
 };
 
+struct mem_pool {
+	struct mp_block *mp_block;
+
+	/*
+	 * The amount of available memory to grow the pool by.
+	 * This size does not include the overhead for the mp_block.
+	 */
+	size_t block_alloc;
+
+	/* The total amount of memory allocated by the pool. */
+	size_t pool_alloc;
+};
+
 struct atom_str {
 	struct atom_str *next_atom;
 	unsigned short str_len;
@@ -304,9 +317,7 @@ static int global_argc;
 static const char **global_argv;
 
 /* Memory pools */
-static size_t mem_pool_alloc = 2*1024*1024 - sizeof(struct mp_block);
-static size_t total_allocd;
-static struct mp_block *mp_block_head;
+static struct mem_pool fi_mem_pool =  {0, 2*1024*1024 - sizeof(struct mp_block), 0 };
 
 /* Atom management */
 static unsigned int atom_table_sz = 4451;
@@ -324,6 +335,7 @@ static off_t pack_size;
 /* Table of objects we've written. */
 static unsigned int object_entry_alloc = 5000;
 static struct object_entry_pool *blocks;
+static size_t total_allocd = 0;
 static struct object_entry *object_table[1 << 16];
 static struct mark_set *marks;
 static const char *export_marks_file;
@@ -634,6 +646,60 @@ static unsigned int hc_str(const char *s, size_t len)
 	return r;
 }
 
+static struct mp_block *pool_alloc_block()
+{
+	struct mp_block *p;
+
+	fi_mem_pool.pool_alloc += sizeof(struct mp_block) + fi_mem_pool.block_alloc;
+	p = xmalloc(st_add(sizeof(struct mp_block), fi_mem_pool.block_alloc));
+	p->next_block = fi_mem_pool.mp_block;
+	p->next_free = (char *)p->space;
+	p->end = p->next_free + fi_mem_pool.block_alloc;
+	fi_mem_pool.mp_block = p;
+
+	return p;
+}
+
+/*
+ * Allocates a block of memory with a specific size and
+ * appends it to the memory pool's list of blocks.
+ *
+ * This function is used to allocate blocks with sizes
+ * different than the default "block_alloc" size for the mem_pool.
+ *
+ * There are two use cases:
+ *  1) The initial block allocation for a memory pool.
+ *
+ *  2) large" blocks of a specific size, where the entire memory block
+ *     is going to be used. This means the block will not have any
+ *     available memory for future allocations. The block is placed at
+ *     the end of the list so that it will be the last block searched
+ *     for available space.
+ */
+static struct mp_block *pool_alloc_block_with_size(size_t block_alloc)
+{
+	struct mp_block *p, *block;
+
+	fi_mem_pool.pool_alloc += sizeof(struct mp_block) + block_alloc;
+	p = xmalloc(st_add(sizeof(struct mp_block), block_alloc));
+
+	block = fi_mem_pool.mp_block;
+	if (block) {
+		while (block->next_block)
+			block = block->next_block;
+
+		block->next_block = p;
+	} else {
+		fi_mem_pool.mp_block = p;
+	}
+
+	p->next_block = NULL;
+	p->next_free = (char *)p->space;
+	p->end = p->next_free + block_alloc;
+
+	return p;
+}
+
 static void *pool_alloc(size_t len)
 {
 	struct mp_block *p;
@@ -643,21 +709,25 @@ static void *pool_alloc(size_t len)
 	if (len & (sizeof(uintmax_t) - 1))
 		len += sizeof(uintmax_t) - (len & (sizeof(uintmax_t) - 1));
 
-	for (p = mp_block_head; p; p = p->next_block)
-		if ((p->end - p->next_free >= len))
-			break;
+	p = fi_mem_pool.mp_block;
+
+	/*
+	 * In performance profiling, there was a minor perf benefit to
+	 * check for available memory in the head block via the if
+	 * statement, and only going through the loop when needed.
+	 */
+	if (p &&
+	   (p->end - p->next_free < len)) {
+		for (p = p->next_block; p; p = p->next_block)
+			if (p->end - p->next_free >= len)
+				break;
+	}
 
 	if (!p) {
-		if (len >= (mem_pool_alloc/2)) {
-			total_allocd += len;
-			return xmalloc(len);
-		}
-		total_allocd += sizeof(struct mp_block) + mem_pool_alloc;
-		p = xmalloc(st_add(sizeof(struct mp_block), mem_pool_alloc));
-		p->next_block = mp_block_head;
-		p->next_free = (char *) p->space;
-		p->end = p->next_free + mem_pool_alloc;
-		mp_block_head = p;
+		if (len >= (fi_mem_pool.block_alloc / 2))
+			p = pool_alloc_block_with_size(len);
+		else
+			p = pool_alloc_block();
 	}
 
 	r = p->next_free;
@@ -667,7 +737,7 @@ static void *pool_alloc(size_t len)
 
 static void *pool_calloc(size_t count, size_t size)
 {
-	size_t len = count * size;
+	size_t len = st_mult(count, size);
 	void *r = pool_alloc(len);
 	memset(r, 0, len);
 	return r;
@@ -3541,8 +3611,8 @@ int cmd_main(int argc, const char **argv)
 		fprintf(stderr, "Total branches:  %10lu (%10lu loads     )\n", branch_count, branch_load_count);
 		fprintf(stderr, "      marks:     %10" PRIuMAX " (%10" PRIuMAX " unique    )\n", (((uintmax_t)1) << marks->shift) * 1024, marks_set_count);
 		fprintf(stderr, "      atoms:     %10u\n", atom_cnt);
-		fprintf(stderr, "Memory total:    %10" PRIuMAX " KiB\n", (total_allocd + alloc_count*sizeof(struct object_entry))/1024);
-		fprintf(stderr, "       pools:    %10lu KiB\n", (unsigned long)(total_allocd/1024));
+		fprintf(stderr, "Memory total:    %10" PRIuMAX " KiB\n", (total_allocd + fi_mem_pool.pool_alloc + alloc_count*sizeof(struct object_entry))/1024);
+		fprintf(stderr, "       pools:    %10lu KiB\n", (unsigned long)((total_allocd + fi_mem_pool.pool_alloc) /1024));
 		fprintf(stderr, "     objects:    %10" PRIuMAX " KiB\n", (alloc_count*sizeof(struct object_entry))/1024);
 		fprintf(stderr, "---------------------------------------------------------------------\n");
 		pack_report();
-- 
2.14.3