RE: [PATCH v3] Add git-grep threads param

Hello Linus, 

quoted

According to several tests on systems with different number of CPU cores
the hard-coded number of 8 threads is not optimal for all systems:

Did you also compare cold-cache filesystem performance?

 One of the reasons for doing threaded grep is for CPU scaling. But another is for IO scaling. If your git tree is over NFS, doing grep eight threads at a time if likely going to make things much faster even if you are on a single CPU.

Yes, I have performed tests on cold-cache FS and it looks like number of threads affects performance. Here are the results for grepping linux kernel repo on a 4-core machine (similar test was conducted on 8-core machine):

Threads: 4 Time: 39.13
Threads: 8 Time: 34.39
Threads: 16 Time: 31.46
Threads: 32 Time: 27.40

Here is test scenario:

#!/bin/bash
TIMEFORMAT=%R
GIT=/home/del/git-dev/bin/git
TESTS=10
for n in 4 8 16 32; do
    echo -n "Threads: $n Time: "
    for i in $(seq 1 $TESTS); do
        echo 3 > /proc/sys/vm/drop_caches
        time $GIT grep --threads $n -e '#define' --and \( -e MAX_PATH -e PATH_MAX \)  >/dev/null
    done 2>&1 | awk -v ntests=${TESTS} '{sum+=$1} END{printf "%.2f\n", sum/ntests}'
done

Note: With hot-cache grepping with 4 threads gives fastest results on both 4-core and 8-core machines.

Thus I think it can be useful for users to be able to tune the threads number according to their needs.