block alloc: allocate cache entries from mem_pool

When reading large indexes from disk, a portion of the time is
dominated in malloc() calls. This can be mitigated by allocating a
large block of memory and manage it ourselves via memory pools.

This change moves the cache entry allocation to be on top of memory
pools.

Design:

The index_state struct will gain a notion of an associated memory_pool
from which cache_entries will be allocated from. When reading in the
index from disk, we have information on the number of entries and
their size, which can guide us in deciding how large our initial
memory allocation should be. When an index is discarded, the
associated memory_pool will be discarded as well - so the lifetime of
a cache_entry is tied to the lifetime of the index_state that it was
allocated for.

In the case of a Split Index, the following rules are followed. 1st,
some terminology is defined:

Terminology:
  - 'the_index': represents the logical view of the index

  - 'split_index': represents the "base" cache entries. Read from the
    split index file.

'the_index' can reference a single split_index, as well as
cache_entries from the split_index. `the_index` will be discarded
before the `split_index` is.  This means that when we are allocating
cache_entries in the presence of a split index, we need to allocate
the entries from the `split_index`'s memory pool.  This allows us to
follow the pattern that `the_index` can reference cache_entries from
the `split_index`, and that the cache_entries will not be freed while
they are still being referenced.

Managing transient cache_entry structs:
Cache entries are usually allocated for an index, but this is not always
the case. Cache entries are sometimes allocated because this is the
type that the existing checkout_entry function works with. Because of
this, the existing code needs to handle cache entries associated with an
index / memory pool, and those that only exist transiently. Several
strategies were contemplated around how to handle this:

Chosen approach:
An extra field was added to the cache_entry type to track whether the
cache_entry was allocated from a memory pool or not. This is currently
an int field, as there are no more available bits in the existing
ce_flags bit field. If / when more bits are needed, this new field can
be turned into a proper bit field.

Alternatives:

1) Do not include any information about how the cache_entry was
allocated. Calling code would be responsible for tracking whether the
cache_entry needed to be freed or not.
  Pro: No extra memory overhead to track this state
  Con: Extra complexity in callers to handle this correctly.

The extra complexity and burden to not regress this behavior in the
future was more than we wanted.

2) cache_entry would gain knowledge about which mem_pool allocated it
  Pro: Could (potentially) do extra logic to know when a mem_pool no
       longer had references to any cache_entry
  Con: cache_entry would grow heavier by a pointer, instead of int

We didn't see a tangible benefit to this approach

3) Do not add any extra information to a cache_entry, but when freeing a
   cache entry, check if the memory exists in a region managed by existing
   mem_pools.
  Pro: No extra memory overhead to track state
  Con: Extra computation is performed when freeing cache entries

We decided tracking and iterating over known memory pool regions was
less desirable than adding an extra field to track this stae.

Signed-off-by: Jameson Miller <jamill@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 100 insertions, 19 deletions

diff --git a/read-cache.c b/read-cache.c
index 41e4d0e..b073696 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -46,6 +46,48 @@
 		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
 		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)
 
+
+/*
+ * This is an estimate of the pathname length in the index.  We use
+ * this for V4 index files to guess the un-deltafied size of the index
+ * in memory because of pathname deltafication.  This is not required
+ * for V2/V3 index formats because their pathnames are not compressed.
+ * If the initial amount of memory set aside is not sufficient, the
+ * mem pool will allocate extra memory.
+ */
+#define CACHE_ENTRY_PATH_LENGTH 80
+
+static inline struct cache_entry *mem_pool__ce_alloc(struct mem_pool *mem_pool, size_t len)
+{
+	struct cache_entry *ce;
+	ce = mem_pool_alloc(mem_pool, cache_entry_size(len));
+	ce->mem_pool_allocated = 1;
+	return ce;
+}
+
+static inline struct cache_entry *mem_pool__ce_calloc(struct mem_pool *mem_pool, size_t len)
+{
+	struct cache_entry * ce;
+	ce = mem_pool_calloc(mem_pool, 1, cache_entry_size(len));
+	ce->mem_pool_allocated = 1;
+	return ce;
+}
+
+static struct mem_pool *find_mem_pool(struct index_state *istate)
+{
+	struct mem_pool **pool_ptr;
+
+	if (istate->split_index && istate->split_index->base)
+		pool_ptr = &istate->split_index->base->ce_mem_pool;
+	else
+		pool_ptr = &istate->ce_mem_pool;
+
+	if (!*pool_ptr)
+		mem_pool_init(pool_ptr, 0);
+
+	return *pool_ptr;
+}
+
 struct index_state the_index;
 static const char *alternate_index_output;
 
@@ -746,7 +788,7 @@ int add_file_to_index(struct index_state *istate, const char *path, int flags)
 
 struct cache_entry *make_empty_cache_entry(struct index_state *istate, size_t len)
 {
-	return xcalloc(1, cache_entry_size(len));
+	return mem_pool__ce_calloc(find_mem_pool(istate), len);
 }
 
 struct cache_entry *make_empty_transient_cache_entry(size_t len)
@@ -1668,13 +1710,13 @@ int read_index(struct index_state *istate)
 	return read_index_from(istate, get_index_file(), get_git_dir());
 }
 
-static struct cache_entry *cache_entry_from_ondisk(struct index_state *istate,
+static struct cache_entry *cache_entry_from_ondisk(struct mem_pool *mem_pool,
 						   struct ondisk_cache_entry *ondisk,
 						   unsigned int flags,
 						   const char *name,
 						   size_t len)
 {
-	struct cache_entry *ce = make_empty_cache_entry(istate, len);
+	struct cache_entry *ce = mem_pool__ce_alloc(mem_pool, len);
 
 	ce->ce_stat_data.sd_ctime.sec = get_be32(&ondisk->ctime.sec);
 	ce->ce_stat_data.sd_mtime.sec = get_be32(&ondisk->mtime.sec);
@@ -1716,7 +1758,7 @@ static unsigned long expand_name_field(struct strbuf *name, const char *cp_)
 	return (const char *)ep + 1 - cp_;
 }
 
-static struct cache_entry *create_from_disk(struct index_state *istate,
+static struct cache_entry *create_from_disk(struct mem_pool *mem_pool,
 					    struct ondisk_cache_entry *ondisk,
 					    unsigned long *ent_size,
 					    struct strbuf *previous_name)
@@ -1748,13 +1790,13 @@ static struct cache_entry *create_from_disk(struct index_state *istate,
 		/* v3 and earlier */
 		if (len == CE_NAMEMASK)
 			len = strlen(name);
-		ce = cache_entry_from_ondisk(istate, ondisk, flags, name, len);
+		ce = cache_entry_from_ondisk(mem_pool, ondisk, flags, name, len);
 
 		*ent_size = ondisk_ce_size(ce);
 	} else {
 		unsigned long consumed;
 		consumed = expand_name_field(previous_name, name);
-		ce = cache_entry_from_ondisk(istate, ondisk, flags,
+		ce = cache_entry_from_ondisk(mem_pool, ondisk, flags,
 					     previous_name->buf,
 					     previous_name->len);
 
@@ -1828,6 +1870,22 @@ static void post_read_index_from(struct index_state *istate)
 	tweak_fsmonitor(istate);
 }
 
+static size_t estimate_cache_size_from_compressed(unsigned int entries)
+{
+	return entries * (sizeof(struct cache_entry) + CACHE_ENTRY_PATH_LENGTH);
+}
+
+static size_t estimate_cache_size(size_t ondisk_size, unsigned int entries)
+{
+	long per_entry = sizeof(struct cache_entry) - sizeof(struct ondisk_cache_entry);
+
+	/*
+	 * Account for potential alignment differences.
+	 */
+	per_entry += align_padding_size(sizeof(struct cache_entry), -sizeof(struct ondisk_cache_entry));
+	return ondisk_size + entries * per_entry;
+}
+
 /* remember to discard_cache() before reading a different cache! */
 int do_read_index(struct index_state *istate, const char *path, int must_exist)
 {
@@ -1874,10 +1932,15 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	istate->cache = xcalloc(istate->cache_alloc, sizeof(*istate->cache));
 	istate->initialized = 1;
 
-	if (istate->version == 4)
+	if (istate->version == 4) {
 		previous_name = &previous_name_buf;
-	else
+		mem_pool_init(&istate->ce_mem_pool,
+			      estimate_cache_size_from_compressed(istate->cache_nr));
+	} else {
 		previous_name = NULL;
+		mem_pool_init(&istate->ce_mem_pool,
+			      estimate_cache_size(mmap_size, istate->cache_nr));
+	}
 
 	src_offset = sizeof(*hdr);
 	for (i = 0; i < istate->cache_nr; i++) {
@@ -1886,7 +1949,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 		unsigned long consumed;
 
 		disk_ce = (struct ondisk_cache_entry *)((char *)mmap + src_offset);
-		ce = create_from_disk(istate, disk_ce, &consumed, previous_name);
+		ce = create_from_disk(istate->ce_mem_pool, disk_ce, &consumed, previous_name);
 		set_index_entry(istate, i, ce);
 
 		src_offset += consumed;
@@ -1983,17 +2046,13 @@ int is_index_unborn(struct index_state *istate)
 
 int discard_index(struct index_state *istate)
 {
-	int i;
+	/*
+	 * Cache entries in istate->cache[] should have been allocated
+	 * from the memory pool associated with this index, or from an
+	 * associated split_index. There is no need to free individual
+	 * cache entries.
+	 */
 
-	for (i = 0; i < istate->cache_nr; i++) {
-		if (istate->cache[i]->index &&
-		    istate->split_index &&
-		    istate->split_index->base &&
-		    istate->cache[i]->index <= istate->split_index->base->cache_nr &&
-		    istate->cache[i] == istate->split_index->base->cache[istate->cache[i]->index - 1])
-			continue;
-		discard_cache_entry(istate->cache[i]);
-	}
 	resolve_undo_clear_index(istate);
 	istate->cache_nr = 0;
 	istate->cache_changed = 0;
@@ -2007,6 +2066,12 @@ int discard_index(struct index_state *istate)
 	discard_split_index(istate);
 	free_untracked_cache(istate->untracked);
 	istate->untracked = NULL;
+
+	if (istate->ce_mem_pool) {
+		mem_pool_discard(istate->ce_mem_pool);
+		istate->ce_mem_pool = NULL;
+	}
+
 	return 0;
 }
 
@@ -2798,7 +2863,23 @@ void move_index_extensions(struct index_state *dst, struct index_state *src)
 	src->untracked = NULL;
 }
 
+struct cache_entry *dup_cache_entry(const struct cache_entry *ce,
+				    struct index_state *istate)
+{
+	unsigned int size = ce_size(ce);
+	int mem_pool_allocated;
+	struct cache_entry *new_entry = make_empty_cache_entry(istate, ce_namelen(ce));
+	mem_pool_allocated = new_entry->mem_pool_allocated;
+
+	memcpy(new_entry, ce, size);
+	new_entry->mem_pool_allocated = mem_pool_allocated;
+	return new_entry;
+}
+
 void discard_cache_entry(struct cache_entry *ce)
 {
+	if (ce && ce->mem_pool_allocated)
+		return;
+
 	free(ce);
 }