[PATCH v2] read_index_from(): Skip verification of the cache entry order to speed index loading

[Ben Peart <benpeart@microsoft.com>]

There is code in post_read_index_from() to detect out of order cache
entries when reading an index file.  This order verification adds cost
to read_index_from() that grows with the size of the index.

Put this on-disk data-structure validation code behind an #ifdef DEBUG
so only debug builds have to pay the cost.

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

Test w/git repo                       HEAD            HEAD~1
----------------------------------------------------------------------------
read_cache/discard_cache 1000 times   0.42(0.01+0.09) 0.48(0.01+0.09) +14.3%
read_cache/discard_cache 1000 times   0.41(0.03+0.04) 0.49(0.00+0.10) +19.5%
read_cache/discard_cache 1000 times   0.42(0.03+0.06) 0.49(0.06+0.04) +16.7%

Test w/10K files                      HEAD            HEAD~1
---------------------------------------------------------------------------
read_cache/discard_cache 1000 times   1.58(0.04+0.00) 1.71(0.00+0.07) +8.2%
read_cache/discard_cache 1000 times   1.64(0.01+0.07) 1.76(0.01+0.09) +7.3%
read_cache/discard_cache 1000 times   1.62(0.03+0.04) 1.71(0.00+0.04) +5.6%

Test w/100K files                     HEAD             HEAD~1
-----------------------------------------------------------------------------
read_cache/discard_cache 1000 times   25.85(0.00+0.06) 27.35(0.01+0.06) +5.8%
read_cache/discard_cache 1000 times   25.82(0.01+0.07) 27.25(0.01+0.07) +5.5%
read_cache/discard_cache 1000 times   26.00(0.01+0.07) 27.36(0.06+0.03) +5.2%

Test with 1,000K files                HEAD              HEAD~1
-------------------------------------------------------------------------------
read_cache/discard_cache 1000 times   200.61(0.01+0.07) 218.23(0.03+0.06) +8.8%
read_cache/discard_cache 1000 times   201.62(0.03+0.06) 217.86(0.03+0.06) +8.1%
read_cache/discard_cache 1000 times   201.64(0.01+0.09) 217.89(0.03+0.07) +8.1%

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---

Notes:
    Base Ref: master
    Web-Diff: https://github.com/benpeart/git/commit/95e20f17ff
    Checkout: git fetch https://github.com/benpeart/git no_ce_order-v2 && git checkout 95e20f17ff
    
    ### Interdiff (v1..v2):
    
    ### Patches

 read-cache.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/read-cache.c b/read-cache.c
index 65f4fe8375..fc90ec0fce 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1664,6 +1664,7 @@ static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk,
 	return ce;
 }
 
+#ifdef DEBUG
 static void check_ce_order(struct index_state *istate)
 {
 	unsigned int i;
@@ -1685,6 +1686,7 @@ static void check_ce_order(struct index_state *istate)
 		}
 	}
 }
+#endif
 
 static void tweak_untracked_cache(struct index_state *istate)
 {
@@ -1720,7 +1722,9 @@ static void tweak_split_index(struct index_state *istate)
 
 static void post_read_index_from(struct index_state *istate)
 {
+#ifdef DEBUG
 	check_ce_order(istate);
+#endif
 	tweak_untracked_cache(istate);
 	tweak_split_index(istate);
 }

base-commit: c52ca88430e6ec7c834af38720295070d8a1e330
-- 
2.14.1.windows.1.1034.g0776750557