index-pack: avoid excessive re-reading of pack directory

Since 45e8a74 (has_sha1_file: re-check pack directory before
giving up, 2013-08-30), we spend extra effort for
has_sha1_file to give the right answer when somebody else is
repacking. Usually this effort does not matter, because
after finding that the object does not exist, the next step
is usually to die().

However, some code paths make a large number of
has_sha1_file checks which are _not_ expected to return 1.
The collision test in index-pack.c is such a case. On a
local system, this can cause a performance slowdown of
around 5%. But on a system with high-latency system calls
(like NFS), it can be much worse.

This patch introduces a "quick" flag to has_sha1_file which
callers can use when they would prefer high performance at
the cost of false negatives during repacks. There may be
other code paths that can use this, but the index-pack one
is the most obviously critical, so we'll start with
switching that one.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 9ca0203922ba41139ad8daa9672acd22602bc1b5..96b110445d0820d27f8681c26be75a475fec1ff9 100644 (file)
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -721,7 +721,7 @@ static void sha1_object(const void *data, struct object_entry *obj_entry,
        assert(data || obj_entry);
 
        read_lock();
-       collision_test_needed = has_sha1_file(sha1);
+       collision_test_needed = has_sha1_file_with_flags(sha1, HAS_SHA1_QUICK);
        read_unlock();
 
        if (collision_test_needed && !data) {

diff --git a/cache.h b/cache.h
index f23fdbee96fc9ad14e3a16e77d277293207dd6c3..3fb75706715f44b341f3471d6c78888271c296b6 100644 (file)
--- a/cache.h
+++ b/cache.h
@@ -873,8 +873,17 @@ extern int has_sha1_pack(const unsigned char *sha1);
  * Return true iff we have an object named sha1, whether local or in
  * an alternate object database, and whether packed or loose.  This
  * function does not respect replace references.
+ *
+ * If the QUICK flag is set, do not re-check the pack directory
+ * when we cannot find the object (this means we may give a false
+ * negative answer if another process is simultaneously repacking).
  */
-extern int has_sha1_file(const unsigned char *sha1);
+#define HAS_SHA1_QUICK 0x1
+extern int has_sha1_file_with_flags(const unsigned char *sha1, int flags);
+static inline int has_sha1_file(const unsigned char *sha1)
+{
+       return has_sha1_file_with_flags(sha1, 0);
+}
 
 /*
  * Return true iff an alternate object database has a loose object

diff --git a/sha1_file.c b/sha1_file.c
index a38854ce553c1e59294d5542a37a5404ccd66dc5..18d0bfb8aca038b6387449efdd5e0ca46749cebf 100644 (file)
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -3017,7 +3017,7 @@ int has_sha1_pack(const unsigned char *sha1)
        return find_pack_entry(sha1, &e);
 }
 
-int has_sha1_file(const unsigned char *sha1)
+int has_sha1_file_with_flags(const unsigned char *sha1, int flags)
 {
        struct pack_entry e;
 
@@ -3025,6 +3025,8 @@ int has_sha1_file(const unsigned char *sha1)
                return 1;
        if (has_loose_object(sha1))
                return 1;
+       if (flags & HAS_SHA1_QUICK)
+               return 0;
        reprepare_packed_git();
        return find_pack_entry(sha1, &e);
 }