Speed up running of compiler test suites in coverage mode

I have spent too much time lately waiting for 'cover' to finish,
so now its time to optimize the running time of the tests suite
in coverage mode.

Basically, when 'cover' is running, the test suites would not
run any tests in parallel. The reason is that using too many
parallel processes when running 'cover' would be slower than
running them sequentially. But those measurements were made
several years ago, and many improvements have been made to
improve the parallelism of the run-time system.

Experimenting with the test_lib:p_run/2 function, I found that
increasing the number of parallel processes would speed up the
self_compile tests cases in compilation_SUITE. The difference
between using 3 processes or 4 processes was slight, though,
so it seems that we should not use more than 4 processes when
running 'cover'.

We don't want to change test_lib:parallel/0, because there is
no way to limit the number of test cases that will be run in
parallel by common_test. However, there as test suites (such as
andor_SUITE) that don't invoke the compiler at run-time. We can
run the cases in such test suites in parallel even if 'cover'
is running.

1 files changed, 13 insertions, 4 deletions

diff --git a/lib/compiler/test/test_lib.erl b/lib/compiler/test/test_lib.erl
index a8befbecd9..e06e42276a 100644
--- a/lib/compiler/test/test_lib.erl
+++ b/lib/compiler/test/test_lib.erl
@@ -44,6 +44,10 @@ smoke_disasm(File) when is_list(File) ->
     Res = beam_disasm:file(File),
     {beam_file,_Mod} = {element(1, Res),element(2, Res)}.
 
+%% If we are running cover, we don't want to run test cases that
+%% invokes the compiler in parallel, as doing so would probably
+%% be slower than running them sequentially.
+
 parallel() ->
     case ?t:is_cover() orelse erlang:system_info(schedulers) =:= 1 of
 	true -> [];
@@ -90,13 +94,18 @@ get_data_dir(Config) ->
 %%  Will fail the test case if there were any errors.
 
 p_run(Test, List) ->
+    S = erlang:system_info(schedulers),
     N = case ?t:is_cover() of
 	    false ->
-		erlang:system_info(schedulers);
+		S + 1;
 	    true ->
-		%% Cover is running. Using more than one process
-		%% will probably only slow down compilation.
-		1
+		%% Cover is running. Using too many processes
+		%% could slow us down. Measurements on my computer
+		%% showed that using 4 parallel processes was
+		%% slightly faster than using 3. Using more than
+		%% 4 would not buy us much and could actually be
+		%% slower.
+		max(S, 4)
 	end,
     p_run_loop(Test, List, N, [], 0, 0).