They were disabled at 53b8d93 ("grep: disable threading in non-worktree
case", 12-12-2011), due to observable performance drops (to the point
that using a single thread would be faster than multiple threads). But
now that zlib inflation can be performed in parallel we can regain the
speedup, so let's re-enable threads in non-worktree grep.
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 | 17.2920s | 20.9624s
2 | 9.6512s | 11.3184s
4 | 6.7723s | 7.6268s
8** | 6.2886s | 6.9843s
These are all means of 30 executions after 2 warmup runs. All tests were
executed on an i7-7700HQ (quad-core w/ hyper-threading), 16GB of RAM and
SSD, running Manjaro Linux. But to make sure the optimization also
performs well on HDD, the tests were repeated on another machine with an
i5-4210U (dual-core w/ hyper-threading), 8GB of RAM and HDD (SATA III,
5400 rpm), also running Manjaro Linux:
Threads | Regex 1 | Regex 2
---------|------------|-----------
1 | 18.4035s | 22.5368s
2 | 12.5063s | 14.6409s
4** | 10.9136s | 12.7106s
** Note that in these cases we relied on hyper-threading, and that's
probably why we don't see a big difference in time.
Unfortunately, multithreaded git-grep might be slow in the non-worktree
case when --textconv is used and there're too many text conversions.
Probably the reason for this is that the object read lock is used to
protect fill_textconv() and therefore there is a mutual exclusion
between textconv execution and object reading. Because both are
time-consuming operations, not being able to perform them in parallel
can cause performance drops. To inform the users about this (and other
threading details), let's also add a "NOTES ON THREADS" section to
Documentation/git-grep.txt.
[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 | 11 +++++++++++
builtin/grep.c | 2 +-
2 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
index c89fb569e3..de628741fa 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -347,6 +347,17 @@ EXAMPLES
`git grep solution -- :^Documentation`::
Looks for `solution`, excluding files in `Documentation`.
+NOTES ON THREADS
+----------------
+
+The `--threads` option (and the grep.threads configuration) will be ignored when
+`--open-files-in-pager` is used, forcing a single-threaded execution.
+
+When grepping the object store (with `--cached` or giving tree objects), running
+with multiple threads might perform slower than single threaded if `--textconv`
+is given and there're too many text conversions. So if you experience low
+performance in this case, it might be desirable to use `--threads=1`.
+
GIT
---
Part of the linkgit:git[1] suite
diff --git a/builtin/grep.c b/builtin/grep.c
index 1535fd50f8..6aaa8d4406 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -1054,7 +1054,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
if (recurse_submodules && (!use_index || untracked))
die(_("option not supported with --recurse-submodules"));
- if (list.nr || cached || show_in_pager) {
+ if (show_in_pager) {
if (num_threads > 1)
warning(_("invalid option combination, ignoring --threads"));
num_threads = 1;
--
2.24.1
