They were disabled at 53b8d93 ("grep: disable threading in non-worktree
case", 12-12-2011), due to observable performance drops. But now that
zlib inflation can be performed in parallel, for some of git-grep's
options, we can regain the speedup.
Grepping 'abcd[02]' ("Regex 1") and '(static|extern) (int|double) \*'
("Regex 2") at chromium's repository[1] I got:
Threads | Regex 1 | Regex 2
---------|------------|-----------
1 | 17.3557s | 20.8410s
2 | 9.7170s | 11.2415s
8 | 6.1723s | 6.9378s
These are all means of 30 executions after 2 warmup runs. All tests were
executed on an i7-7700HQ with 16GB of RAM and SSD. But to make sure the
optimization also performs well on HDD, the tests were repeated on an
AMD Turion 64 X2 TL-62 (dual-core) with 4GB of RAM and HDD (SATA-150,
5400 rpm):
Threads | Regex 1 | Regex 2
---------|------------|-----------
1 | 40.3347s | 47.6173s
2 | 27.6547s | 35.1797s
Unfortunately, textconv and submodules' operations remain thread-unsafe,
needing locks to be safely executed when threaded. Because of that, it's
not currently worthy to grep in parallel with them. So, when --textconv
or --recurse-submodules are given for a non-worktree case, threads are
kept disabled. In order to clarify this behavior, let's also add a
"NOTES" section to Documentation/git-grep.txt explaining the thread
usage details.
[1]: chromium’s repo at commit 03ae96f (“Add filters testing at DSF=2”,
04-06-2019), after a 'git gc' execution.
Signed-off-by: Matheus Tavares <matheus.bernardino@xxxxxx>
---
Documentation/git-grep.txt | 12 ++++++++++++
builtin/grep.c | 3 ++-
2 files changed, 14 insertions(+), 1 deletion(-)
diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
index 2d27969057..9686875fbc 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -330,6 +330,18 @@ EXAMPLES
`git grep solution -- :^Documentation`::
Looks for `solution`, excluding files in `Documentation`.
+NOTES
+-----
+
+The --threads option (and grep.threads configuration) will be ignored when
+--open-files-in-pager is used, forcing a single-threaded execution.
+
+When grepping the index file (with --cached or giving tree objects), the
+following options will also suppress thread creation:
+
+ --recurse_submodules
+ --textconv
+
GIT
---
Part of the linkgit:git[1] suite
diff --git a/builtin/grep.c b/builtin/grep.c
index fa51392222..e5a9da471a 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -1073,7 +1073,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
pathspec.recursive = 1;
pathspec.recurse_submodules = !!recurse_submodules;
- if (list.nr || cached || show_in_pager) {
+ if (show_in_pager ||
+ ((list.nr || cached) && (recurse_submodules || opt.allow_textconv))) {
if (num_threads > 1)
warning(_("invalid option combination, ignoring --threads"));
num_threads = 1;
--
2.22.0
