read_index_from(): speed index loading by skipping verification of the entry order

There is code in post_read_index_from() to catch out of order
entries when reading an index file.  This order verification is ~13%
of the cost of every call to read_index_from().

Update check_ce_order() so that it skips this verification unless
the "verify_ce_order" global variable is set.

Teach fsck to force this verification.

The effect can be seen using t/perf/p0002-read-cache.sh:

Test                                          HEAD              HEAD~1
--------------------------------------------------------------------------------------
0002.1: read_cache/discard_cache 1000 times   0.41(0.04+0.04)   0.50(0.00+0.10) +22.0%

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/fsck.c b/builtin/fsck.c
index d18244a..81cc5ad 100644 (file)
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -763,6 +763,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 
        if (keep_cache_objects) {
                verify_index_checksum = 1;
+               verify_ce_order = 1;
                read_cache();
                for (i = 0; i < active_nr; i++) {
                        unsigned int mode;

diff --git a/cache.h b/cache.h
index 5cc116b..177f5eb 100644 (file)
--- a/cache.h
+++ b/cache.h
@@ -720,6 +720,7 @@ extern int hold_locked_index(struct lock_file *, int);
 extern void set_alternate_index_output(const char *);
 
 extern int verify_index_checksum;
+extern int verify_ce_order;
 
 /* Environment bits from configuration mechanism */
 extern int trust_executable_bit;

diff --git a/read-cache.c b/read-cache.c
index 5e6d24d..945a377 100644 (file)
--- a/read-cache.c
+++ b/read-cache.c
@@ -1509,6 +1509,9 @@ struct ondisk_cache_entry_extended {
 /* Allow fsck to force verification of the index checksum. */
 int verify_index_checksum;
 
+/* Allow fsck to force verification of the cache entry order. */
+int verify_ce_order;
+
 static int verify_hdr(struct cache_header *hdr, unsigned long size)
 {
        git_SHA_CTX c;
@@ -1666,6 +1669,9 @@ static void check_ce_order(struct index_state *istate)
 {
        unsigned int i;
 
+       if (!verify_ce_order)
+               return;
+
        for (i = 1; i < istate->cache_nr; i++) {
                struct cache_entry *ce = istate->cache[i - 1];
                struct cache_entry *next_ce = istate->cache[i];