[Cocci] [PATCH 0/2 v2] Improve Coccinelle Parallelisation

[Cocci] [PATCH 0/2 v2] Improve Coccinelle Parallelisation

[Sumera Priyadarsini]

Coccinelle utilises all available threads to implement parallelisation.
However, this results in a decrease in performance.

This patchset aims to improve performance by modifying cocciccheck to
use at most one thread per core by default.

Sumera Priyadarsini (2):
  scripts: coccicheck: Change default value for parallelism
  Documentation: Coccinelle: Modify parallelisation information in docs

 Documentation/dev-tools/coccinelle.rst | 4 ++--
 scripts/coccicheck                     | 5 +++++
 2 files changed, 7 insertions(+), 2 deletions(-)

-- 
2.25.1

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[Cocci] [PATCH 1/2 V2] scripts: coccicheck: Change default value for parallelism

[Sumera Priyadarsini]

By default, coccicheck utilizes all available threads to implement
parallelisation. However, when all available threads are used,
a decrease in performance is noted. The elapsed time is  minimum
when at most one thread per core is used.

For example, on benchmarking the semantic patch kfree.cocci for
usb/serial using hyperfine, the outputs obtained for J=5 and J=2
are 1.32 and 1.90 times faster than those for J=10 and J=9
respectively for two separate runs. For the larger drivers/staging
directory, minimium elapsed time is obtained for J=3 which is 1.86
times faster than that for J=12. The optimal J value does not
exceed 6 in any of the test runs. The benchmarks are run on a machine
with 6 cores, with 2 threads per core, i.e, 12 hyperthreads in all.

To improve performance, modify coccicheck to use at most only
one thread per core by default.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>

---
Changes in V2:
	- Change commit message as suggested by Julia Lawall
Changes in V3:
	- Use J/2 as optimal value for machines with more
than 8 hyperthreads as well.
---
 scripts/coccicheck | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/scripts/coccicheck b/scripts/coccicheck
index e04d328210ac..a72aa6c037ff 100755
--- a/scripts/coccicheck
+++ b/scripts/coccicheck
@@ -75,8 +75,13 @@ else
         OPTIONS="--dir $KBUILD_EXTMOD $COCCIINCLUDE"
     fi
 
+    # Use only one thread per core by default if hyperthreading is enabled
+    THREADS_PER_CORE=$(lscpu | grep "Thread(s) per core: " | tr -cd [:digit:])
     if [ -z "$J" ]; then
         NPROC=$(getconf _NPROCESSORS_ONLN)
+	if [ $THREADS_PER_CORE -gt 1 -a $NPROC -gt 2 ] ; then
+		NPROC=$((NPROC/2))
+	fi
     else
         NPROC="$J"
     fi
-- 
2.25.1

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[Cocci] [PATCH 2/2 V2] Documentation: Coccinelle: Modify parallelisation information in docs

[Sumera Priyadarsini]

This patchset modifies coccicheck to use at most one thread per core by
default for optimal performance. Modify documentation in coccinelle.rst
to reflect the same.

Signed-off-by: Sumera Priyadarsini <sylphrenadin@gmail.com>
---
 Documentation/dev-tools/coccinelle.rst | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/dev-tools/coccinelle.rst b/Documentation/dev-tools/coccinelle.rst
index 74c5e6aeeff5..a27a4867018c 100644
--- a/Documentation/dev-tools/coccinelle.rst
+++ b/Documentation/dev-tools/coccinelle.rst
@@ -130,8 +130,8 @@ To enable verbose messages set the V= variable, for example::
 Coccinelle parallelization
 --------------------------
 
-By default, coccicheck tries to run as parallel as possible. To change
-the parallelism, set the J= variable. For example, to run across 4 CPUs::
+By default, coccicheck uses at most only one thread per core of the system.
+To change the parallelism, set the J= variable. For example, to run across 4 CPUs::
 
    make coccicheck MODE=report J=4
 
-- 
2.25.1

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