Limit the size of the new delta_base_cache

The new configuration variable core.deltaBaseCacheLimit allows the
user to control how much memory they are willing to give to Git for
caching base objects of deltas.  This is not normally meant to be
a user tweakable knob; the "out of the box" settings are meant to
be suitable for almost all workloads.

We default to 16 MiB under the assumption that the cache is not
meant to consume all of the user's available memory, and that the
cache's main purpose was to cache trees, for faster path limiters
during revision traversal.  Since trees tend to be relatively small
objects, this relatively small limit should still allow a large
number of objects.

On the other hand we don't want the cache to start storing 200
different versions of a 200 MiB blob, as this could easily blow
the entire address space of a 32 bit process.

We evict OBJ_BLOB from the cache first (credit goes to Junio) as
we want to favor OBJ_TREE within the cache.  These are the objects
that have the highest inflate() startup penalty, as they tend to
be small and thus don't have that much of a chance to ammortize
that penalty over the entire data.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 26 insertions, 4 deletions

diff --git a/sha1_file.c b/sha1_file.c
index ee64865b6..b0b21776e 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1354,6 +1354,7 @@ static void *unpack_compressed_entry(struct packed_git *p,
 
 #define MAX_DELTA_CACHE (256)
 
+static size_t delta_base_cached;
 static struct delta_base_cache_entry {
 	struct packed_git *p;
 	off_t base_offset;
@@ -1384,8 +1385,10 @@ static void *cache_or_unpack_entry(struct packed_git *p, off_t base_offset,
 	return unpack_entry(p, base_offset, type, base_size);
 
 found_cache_entry:
-	if (!keep_cache)
+	if (!keep_cache) {
 		ent->data = NULL;
+		delta_base_cached -= ent->size;
+	}
 	else {
 		ret = xmalloc(ent->size + 1);
 		memcpy(ret, ent->data, ent->size);
@@ -1396,14 +1399,33 @@ found_cache_entry:
 	return ret;
 }
 
+static inline void release_delta_base_cache(struct delta_base_cache_entry *ent)
+{
+	if (ent->data) {
+		free(ent->data);
+		ent->data = NULL;
+		delta_base_cached -= ent->size;
+	}
+}
+
 static void add_delta_base_cache(struct packed_git *p, off_t base_offset,
 	void *base, unsigned long base_size, enum object_type type)
 {
-	unsigned long hash = pack_entry_hash(p, base_offset);
+	unsigned long i, hash = pack_entry_hash(p, base_offset);
 	struct delta_base_cache_entry *ent = delta_base_cache + hash;
 
-	if (ent->data)
-		free(ent->data);
+	release_delta_base_cache(ent);
+	delta_base_cached += base_size;
+	for (i = 0; delta_base_cached > delta_base_cache_limit
+		&& i < ARRAY_SIZE(delta_base_cache); i++) {
+		struct delta_base_cache_entry *f = delta_base_cache + i;
+		if (f->type == OBJ_BLOB)
+			release_delta_base_cache(f);
+	}
+	for (i = 0; delta_base_cached > delta_base_cache_limit
+		&& i < ARRAY_SIZE(delta_base_cache); i++)
+		release_delta_base_cache(delta_base_cache + i);
+
 	ent->p = p;
 	ent->base_offset = base_offset;
 	ent->type = type;