To demonstrate the need for the core.enableSHA1DC knob, this test
compares the performance of the SHA-1 algorithms with collision
detection vs OpenSSL's (that does not detect attempted collision
attacks).
The payload size of 300MB was actually not concocted from thin air, but
is based on the massive Windows monorepo, whose index file weighs in
with roughly that size (and which is SHA-1'ed upon every single read and
write).
On this developer's machine, this comparison shows a hefty difference:
0013.1: calculate SHA-1 for 300MB (SHA1DC) 3.03(0.03+0.17)
0013.2: calculate SHA-1 for 300MB (OpenSSL) 0.58(0.06+0.16)
It is not only that ~6x slower performance is a pretty tall order, the
absolute numbers themselves speak a very clear language: having to wait
one second every time a file is `git add`ed is noticeable, but one can
handwave it away. Having to wait six seconds (3 to read the index, a
fraction of a millisecond to hash the new contents and update the
in-memory index, then 3 seconds to write out the index) is outright
annoying. And unnecessary, too: the content of the index is never
crafted to cause SHA-1 collisions.
Obviously, this test requires that Git was built with the new
DC_AND_OPENSSL_SHA1 make flag; it is skipped otherwise.
Signed-off-by: Johannes Schindelin <johannes.schindelin@xxxxxx>
---
t/perf/p0013-sha1dc.sh | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
create mode 100644 t/perf/p0013-sha1dc.sh
diff --git a/t/perf/p0013-sha1dc.sh b/t/perf/p0013-sha1dc.sh
new file mode 100644
index 00000000000..e08473ac969
--- /dev/null
+++ b/t/perf/p0013-sha1dc.sh
@@ -0,0 +1,23 @@
+#!/bin/sh
+
+test_description="Tests performance of SHA1DC vs OpenSSL"
+
+. ./perf-lib.sh
+
+test -n "$DC_AND_OPENSSL_SHA1" || {
+ skip_all='DC_AND_OPENSSL_SHA1 required for this test'
+ test_done
+}
+
+test_perf 'calculate SHA-1 for 300MB (SHA1DC)' '
+ dd if=/dev/zero bs=1M count=300 |
+ test-sha1
+'
+
+test_perf 'calculate SHA-1 for 300MB (OpenSSL)' '
+ dd if=/dev/zero bs=1M count=300 |
+ test-sha1 --disable-sha1dc
+'
+
+test_done
+
--
2.12.1.windows.1