[PATCH v2 10/11] grep: re-enable threads in non-worktree case

From: Matheus Tavares <matheus.bernardino@usp.br>
Date: Sep 30, 2019, 01:50 UTC
Message-ID: <c1fe6545a90563419731932b0bb17869dab1cda7.1569808052.git.matheus.bernardino@usp.br>
In-Reply-To: <cover.1569808052.git.matheus.bernardino@usp.br>

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 detais), 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@usp.br>
---
 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 2d27969057..00fc59d565 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -330,6 +330,17 @@ 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 0947596bcd..163f14b60d 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;

-- 
2.23.0