On Thu, Jul 28, 2022 at 05:09:38PM -0400, Jeff King wrote:

quoted

Ditto TEST_NO_MALLOC_CHECK=1 & --no-bin-wrappers, but we can think about
all of those some other time....

I'd be surprised if the malloc checking itself is all that expensive,
though it does look like we call getconf and expr once per test there
for setup. We could almost certainly hoist that out and call it once per
script.

Nope, I was totally wrong here. Running with TEST_NO_MALLOC_CHECK=1 does
indeed make a big difference:

  Benchmark 1: make test
    Time (mean ± σ):     67.486 s ±  3.339 s    [User: 622.643 s, System: 409.951 s]
    Range (min … max):   63.634 s … 69.550 s    3 runs
   
  Benchmark 2: TEST_NO_MALLOC_CHECK=1 make test
    Time (mean ± σ):     57.596 s ±  0.899 s    [User: 577.273 s, System: 291.627 s]
    Range (min … max):   57.072 s … 58.634 s    3 runs

Eliminating the extra per-test processes with the patch below helps a
little, but most of the effort really is (presumably) going to the
actual malloc checking code:

  Benchmark 1: make test [nb: after applying patch...]
  Time (mean ± σ):     67.091 s ±  0.385 s    [User: 599.382 s, System: 410.781 s]
  Range (min … max):   66.648 s … 67.343 s    3 runs

Curious that most of the CPU time seems to go into system time. I'd have
thought it was extra internal malloc debugging bits, but maybe it is
allocating mmap/brk calls in a less efficient way. I dunno how any of it
actually works.

-Peff

---

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 55857af601..303fbe8ecc 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh

@@ -557,14 +557,20 @@ then
 		: nothing
 	}
 else
+	if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
+	   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
+	    expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
+	then
+		USE_LIBC_MALLOC_DEBUG=t
+	else
+		USE_LIBC_MALLOC_DEBUG=
+	fi
 	setup_malloc_check () {
 		local g
 		local t
 		MALLOC_CHECK_=3	MALLOC_PERTURB_=165
 		export MALLOC_CHECK_ MALLOC_PERTURB_
-		if _GLIBC_VERSION=$(getconf GNU_LIBC_VERSION 2>/dev/null) &&
-		   _GLIBC_VERSION=${_GLIBC_VERSION#"glibc "} &&
-		   expr 2.34 \<= "$_GLIBC_VERSION" >/dev/null
+		if test -n "$USE_LIBC_MALLOC_DEBUG"
 		then
 			g=
 			LD_PRELOAD="libc_malloc_debug.so.0"