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, 21 insertions, 0 deletions

diff --git a/cache.h b/cache.h
index 5aadaeb..c1c7e9e 100644
--- a/cache.h
+++ b/cache.h
@@ -15,6 +15,7 @@
 #include "path.h"
 #include "sha1-array.h"
 #include "repository.h"
+#include "mem-pool.h"
 
 #include <zlib.h>
 typedef struct git_zstream {
@@ -156,6 +157,7 @@ struct cache_entry {
 	struct stat_data ce_stat_data;
 	unsigned int ce_mode;
 	unsigned int ce_flags;
+	unsigned int mem_pool_allocated;
 	unsigned int ce_namelen;
 	unsigned int index;	/* for link extension */
 	struct object_id oid;
@@ -227,6 +229,7 @@ static inline void copy_cache_entry(struct cache_entry *dst,
 				    const struct cache_entry *src)
 {
 	unsigned int state = dst->ce_flags & CE_HASHED;
+	int mem_pool_allocated = dst->mem_pool_allocated;
 
 	/* Don't copy hash chain and name */
 	memcpy(&dst->ce_stat_data, &src->ce_stat_data,
@@ -235,6 +238,9 @@ static inline void copy_cache_entry(struct cache_entry *dst,
 
 	/* Restore the hash state */
 	dst->ce_flags = (dst->ce_flags & ~CE_HASHED) | state;
+
+	/* Restore the mem_pool_allocated flag */
+	dst->mem_pool_allocated = mem_pool_allocated;
 }
 
 static inline unsigned create_ce_flags(unsigned stage)
@@ -328,6 +334,7 @@ struct index_state {
 	struct untracked_cache *untracked;
 	uint64_t fsmonitor_last_update;
 	struct ewah_bitmap *fsmonitor_dirty;
+	struct mem_pool *ce_mem_pool;
 };
 
 extern struct index_state the_index;
@@ -373,6 +380,20 @@ struct cache_entry *make_empty_transient_cache_entry(size_t name_len);
  */
 void discard_cache_entry(struct cache_entry *ce);
 
+/*
+ * Duplicate a cache_entry. Allocate memory for the new entry from a
+ * memory_pool. Takes into account cache_entry fields that are meant
+ * for managing the underlying memory allocation of the cache_entry.
+ */
+struct cache_entry *dup_cache_entry(const struct cache_entry *ce, struct index_state *istate);
+
+/*
+ * Validate the cache entries in the index.  This is an internal
+ * consistency check that the cache_entry structs are allocated from
+ * the expected memory pool.
+ */
+void validate_cache_entries(const struct index_state *istate);
+
 #ifndef NO_THE_INDEX_COMPATIBILITY_MACROS
 #define active_cache (the_index.cache)
 #define active_nr (the_index.cache_nr)