diff --git a/.github/workflows/runtests.yml b/.github/workflows/runtests.yml
index 19c1e79e..6b27ab1c 100644
--- a/.github/workflows/runtests.yml
+++ b/.github/workflows/runtests.yml
@@ -10,29 +10,48 @@ jobs:
       name: "ubuntu-latest | g++"
       timeout-minutes: 60
       runs-on: ubuntu-latest
-      env:
-         CXX: "ccache g++"
-         CXXFLAGS: "-fdiagnostics-color"
-         CMAKE_CXX_COMPILER: "ccache g++"
+      strategy:
+            matrix:
+              sys:
+                - { compiler: 'gcc',   version: '6'}
+                - { compiler: 'gcc',   version: '7'}
+                - { compiler: 'gcc',   version: '8'}
+                - { compiler: 'gcc',   version: '9'}
+                - { compiler: 'gcc',   version: '10'}
+                - { compiler: 'gcc',   version: '11'}
+                # - { compiler: 'clang', version: '8'}
+                # - { compiler: 'clang', version: '10'}
+                # - { compiler: 'clang', version: '12'}
+                # - { compiler: 'clang', version: '13'}
+                # - { compiler: 'clang', version: '14'}
       steps:
          - name: "Checkout HPCombi repo . . ."
            uses: actions/checkout@v3
-         - name: "Setup ccache . . ."
-           uses: Chocobo1/setup-ccache-action@v1
-           with:
-             update_packager_index: false
-             override_cache_key: ${{ runner.os }}-${{ github.ref }}
-             override_cache_key_fallback: ${{ runner.os }}
+           if: ${{ matrix.sys.compiler == 'gcc' }}
+           run: |
+             GCC_VERSION=${{ matrix.sys.version }}
+             if [[ $GCC_VERSION == '6' || $GCC_VERSION == '7' || $GCC_VERSION == '8' ]]; then
+               sudo apt-get update
+               sudo apt-get --no-install-suggests --no-install-recommends install g++-$GCC_VERSION
+             fi
+             CC=gcc-$GCC_VERSION
+             echo "CC=$CC" >> $GITHUB_ENV
+             CXX=g++-$GCC_VERSION
+             echo "CXX=$CXX" >> $GITHUB_ENV
          - name: "Install dependencies . . ."
            run: |
-                sudo apt-get --yes update
-                sudo apt-get install --yes ccache
-                sudo apt-get install --yes libbenchmark-dev
-         - name: "Build + run HPCombi tests . . ."
+            sudo apt-get --yes update
+            sudo apt-get install --yes ccache
+            sudo apt-get install --yes libbenchmark-dev
+         - name: "Build HPCombi tests . . ."
+           env:
+                CC: ${{ env.CC }}
+                CXX: ${{ env.CXX }}
            run: |
-              mkdir build
-              cd build
-              cmake -DBUILD_TESTING=1 -DCMAKE_BUILD_TYPE=Release ..
-              cd tests
-              make
+            mkdir build
+            cd build
+            cmake -DBUILD_TESTS=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=$CC -DCMAKE_CXX_COMPILER=$CXX ..
+            cd tests
+            make -j4
+         - name: "Run HPCombi tests . . ."
               ./test_all