The relevant part is the use of atomic_fetch_add and atomic_fetch_sub in rwlock_* functions.
In short, clang-8 and earlier versions generate lock addl and lock subl for those atomics.
clang-9 generates lock incl and lock decl, and also lock xadd if adding 2 or other numbers.
The results show that lock addl and lock subl can be more efficient in some cases.
The rwlock implementation can be wrong. Any suggestions are welcome.
I have not identified the actual cause.
Right now I just personally hope to get the old compilation results with clang-9.
In this microbenchmark each thread repeatedly acquires and releases a randomly selected lock from a set. The number of threads and the number of locks are changed in the benchmark. Each thread is pinned to a unique core, assuming #threads <= #cores. The program reads the process' affinity settings to determine how many cores can be used.
Change the compiler with CC=x such as CC=clang-9. The default is clang.
If having multiple sockets, use numactl -N 0 ./rwlocktest.out to let it run on one socket.
Tested on both Broadwell (16-core) and Skylake (14-core) servers, with Archlinux (kernel 5.3.x) and Ubuntu 18.04, respectively.
Broadwell has HT turned off. Skylake has HT turned on (not on purpose) so I used the first 14 cores out of the 28 on a socket. Skylake results are omitted (similar to Broadwell's).
Results on Broadwell are listed below. Server specs: Archlinux, linux 5.3.6, Xeon E5-2697A v4, mitigations=off.
We observed that with more locks (128+), the binary produced by clang-9 yields 15-20 percent lower throughput.
However, with a _mm_pause() added in between the lock() and unlock(), the peak throughput (180+) can be reached by both of them.
But the throughput under high-contention (32 locks) is much lower (roughly 40 vs 60).
I have not exhausted all the possibilities by adding pauses, reordering the statements,
and writing inline assembly to force doing lock addl with clang-9.
Some are slightly better, but none has produced results as good as clang-8's.
#threads: 4 #locks: 32 clang-9: 48.66 million ops/sec clang-8: 48.60 million ops/sec
#threads: 6 #locks: 32 clang-9: 54.94 million ops/sec clang-8: 55.55 million ops/sec
#threads: 8 #locks: 32 clang-9: 57.63 million ops/sec clang-8: 58.30 million ops/sec
#threads: 10 #locks: 32 clang-9: 59.89 million ops/sec clang-8: 61.12 million ops/sec
#threads: 12 #locks: 32 clang-9: 61.24 million ops/sec clang-8: 62.14 million ops/sec
#threads: 14 #locks: 32 clang-9: 61.87 million ops/sec clang-8: 62.75 million ops/sec
#threads: 16 #locks: 32 clang-9: 61.41 million ops/sec clang-8: 62.77 million ops/sec
#threads: 4 #locks: 128 clang-9: 58.54 million ops/sec clang-8: 64.85 million ops/sec
#threads: 6 #locks: 128 clang-9: 77.23 million ops/sec clang-8: 86.35 million ops/sec
#threads: 8 #locks: 128 clang-9: 92.27 million ops/sec clang-8: 102.66 million ops/sec
#threads: 10 #locks: 128 clang-9: 103.32 million ops/sec clang-8: 117.78 million ops/sec
#threads: 12 #locks: 128 clang-9: 114.16 million ops/sec clang-8: 128.92 million ops/sec
#threads: 14 #locks: 128 clang-9: 121.44 million ops/sec clang-8: 137.28 million ops/sec
#threads: 16 #locks: 128 clang-9: 126.56 million ops/sec clang-8: 144.77 million ops/sec
#threads: 4 #locks: 512 clang-9: 55.80 million ops/sec clang-8: 68.21 million ops/sec
#threads: 6 #locks: 512 clang-9: 76.55 million ops/sec clang-8: 93.21 million ops/sec
#threads: 8 #locks: 512 clang-9: 95.32 million ops/sec clang-8: 116.38 million ops/sec
#threads: 10 #locks: 512 clang-9: 113.31 million ops/sec clang-8: 138.59 million ops/sec
#threads: 12 #locks: 512 clang-9: 128.26 million ops/sec clang-8: 157.17 million ops/sec
#threads: 14 #locks: 512 clang-9: 141.07 million ops/sec clang-8: 171.94 million ops/sec
#threads: 16 #locks: 512 clang-9: 150.64 million ops/sec clang-8: 185.40 million ops/sec
#threads: 4 #locks: 32 clang-9: 49.35 million ops/sec clang-8: 49.83 million ops/sec
#threads: 6 #locks: 32 clang-9: 54.67 million ops/sec clang-8: 55.24 million ops/sec
#threads: 8 #locks: 32 clang-9: 57.63 million ops/sec clang-8: 58.63 million ops/sec
#threads: 10 #locks: 32 clang-9: 60.32 million ops/sec clang-8: 61.11 million ops/sec
#threads: 12 #locks: 32 clang-9: 61.18 million ops/sec clang-8: 62.23 million ops/sec
#threads: 14 #locks: 32 clang-9: 61.82 million ops/sec clang-8: 62.03 million ops/sec
#threads: 16 #locks: 32 clang-9: 60.91 million ops/sec clang-8: 62.77 million ops/sec
#threads: 4 #locks: 128 clang-9: 58.78 million ops/sec clang-8: 64.83 million ops/sec
#threads: 6 #locks: 128 clang-9: 77.39 million ops/sec clang-8: 86.15 million ops/sec
#threads: 8 #locks: 128 clang-9: 92.70 million ops/sec clang-8: 103.39 million ops/sec
#threads: 10 #locks: 128 clang-9: 104.53 million ops/sec clang-8: 117.19 million ops/sec
#threads: 12 #locks: 128 clang-9: 114.24 million ops/sec clang-8: 128.67 million ops/sec
#threads: 14 #locks: 128 clang-9: 121.34 million ops/sec clang-8: 137.87 million ops/sec
#threads: 16 #locks: 128 clang-9: 126.83 million ops/sec clang-8: 144.52 million ops/sec
#threads: 4 #locks: 512 clang-9: 55.78 million ops/sec clang-8: 68.18 million ops/sec
#threads: 6 #locks: 512 clang-9: 76.47 million ops/sec clang-8: 92.97 million ops/sec
#threads: 8 #locks: 512 clang-9: 94.64 million ops/sec clang-8: 116.29 million ops/sec
#threads: 10 #locks: 512 clang-9: 113.06 million ops/sec clang-8: 138.39 million ops/sec
#threads: 12 #locks: 512 clang-9: 128.22 million ops/sec clang-8: 156.90 million ops/sec
#threads: 14 #locks: 512 clang-9: 140.87 million ops/sec clang-8: 172.90 million ops/sec
#threads: 16 #locks: 512 clang-9: 150.84 million ops/sec clang-8: 184.95 million ops/sec`
#threads: 4 #locks: 32 clang-9: 38.18 million ops/sec clang-8: 39.24 million ops/sec
#threads: 6 #locks: 32 clang-9: 40.24 million ops/sec clang-8: 40.11 million ops/sec
#threads: 8 #locks: 32 clang-9: 40.58 million ops/sec clang-8: 40.55 million ops/sec
#threads: 10 #locks: 32 clang-9: 39.76 million ops/sec clang-8: 40.95 million ops/sec
#threads: 12 #locks: 32 clang-9: 39.70 million ops/sec clang-8: 40.93 million ops/sec
#threads: 14 #locks: 32 clang-9: 39.65 million ops/sec clang-8: 40.56 million ops/sec
#threads: 16 #locks: 32 clang-9: 39.36 million ops/sec clang-8: 39.61 million ops/sec
#threads: 4 #locks: 128 clang-9: 63.01 million ops/sec clang-8: 63.58 million ops/sec
#threads: 6 #locks: 128 clang-9: 73.84 million ops/sec clang-8: 74.76 million ops/sec
#threads: 8 #locks: 128 clang-9: 82.95 million ops/sec clang-8: 84.40 million ops/sec
#threads: 10 #locks: 128 clang-9: 85.74 million ops/sec clang-8: 89.98 million ops/sec
#threads: 12 #locks: 128 clang-9: 90.47 million ops/sec clang-8: 93.14 million ops/sec
#threads: 14 #locks: 128 clang-9: 90.55 million ops/sec clang-8: 96.25 million ops/sec
#threads: 16 #locks: 128 clang-9: 92.44 million ops/sec clang-8: 98.99 million ops/sec
#threads: 4 #locks: 512 clang-9: 74.36 million ops/sec clang-8: 74.64 million ops/sec
#threads: 6 #locks: 512 clang-9: 100.40 million ops/sec clang-8: 100.49 million ops/sec
#threads: 8 #locks: 512 clang-9: 123.69 million ops/sec clang-8: 122.31 million ops/sec
#threads: 10 #locks: 512 clang-9: 140.79 million ops/sec clang-8: 143.30 million ops/sec
#threads: 12 #locks: 512 clang-9: 158.99 million ops/sec clang-8: 158.62 million ops/sec
#threads: 14 #locks: 512 clang-9: 172.64 million ops/sec clang-8: 172.19 million ops/sec
#threads: 16 #locks: 512 clang-9: 184.51 million ops/sec clang-8: 184.57 million ops/sec
#threads: 4 #locks: 32 clang-9: 38.36 million ops/sec clang-8: 39.67 million ops/sec
#threads: 6 #locks: 32 clang-9: 40.14 million ops/sec clang-8: 40.06 million ops/sec
#threads: 8 #locks: 32 clang-9: 40.62 million ops/sec clang-8: 40.85 million ops/sec
#threads: 10 #locks: 32 clang-9: 39.99 million ops/sec clang-8: 40.77 million ops/sec
#threads: 12 #locks: 32 clang-9: 39.58 million ops/sec clang-8: 40.88 million ops/sec
#threads: 14 #locks: 32 clang-9: 39.69 million ops/sec clang-8: 40.50 million ops/sec
#threads: 16 #locks: 32 clang-9: 39.17 million ops/sec clang-8: 40.54 million ops/sec
#threads: 4 #locks: 128 clang-9: 63.34 million ops/sec clang-8: 63.62 million ops/sec
#threads: 6 #locks: 128 clang-9: 73.35 million ops/sec clang-8: 75.12 million ops/sec
#threads: 8 #locks: 128 clang-9: 80.88 million ops/sec clang-8: 84.62 million ops/sec
#threads: 10 #locks: 128 clang-9: 85.64 million ops/sec clang-8: 89.63 million ops/sec
#threads: 12 #locks: 128 clang-9: 88.48 million ops/sec clang-8: 92.91 million ops/sec
#threads: 14 #locks: 128 clang-9: 92.44 million ops/sec clang-8: 95.29 million ops/sec
#threads: 16 #locks: 128 clang-9: 91.18 million ops/sec clang-8: 98.35 million ops/sec
#threads: 4 #locks: 512 clang-9: 74.33 million ops/sec clang-8: 74.70 million ops/sec
#threads: 6 #locks: 512 clang-9: 100.29 million ops/sec clang-8: 99.91 million ops/sec
#threads: 8 #locks: 512 clang-9: 123.39 million ops/sec clang-8: 124.08 million ops/sec
#threads: 10 #locks: 512 clang-9: 143.11 million ops/sec clang-8: 143.09 million ops/sec
#threads: 12 #locks: 512 clang-9: 160.06 million ops/sec clang-8: 158.95 million ops/sec
#threads: 14 #locks: 512 clang-9: 173.70 million ops/sec clang-8: 173.07 million ops/sec
#threads: 16 #locks: 512 clang-9: 185.78 million ops/sec clang-8: 184.45 million ops/sec
$ clang --version
clang version 9.0.0 (tags/RELEASE_900/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
$ clang --version
clang version 8.0.1 (tags/RELEASE_801/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
rwlock_trylock_read():
/home/x/work/rw/rwlocktest.c:284
if ((atomic_fetch_add(pvar, 1) >> RWLOCK_WSHIFT) == 0) {
2869: f0 83 04 f0 01 lock addl $0x1,(%rax,%rsi,8)
286e: 7e e0 jle 2850 <rwlock_worker+0x250>
rwlock_worker():
/home/x/work/rw/rwlocktest.c:373
rwlock_unlock_read(&(info->locks[x]));
2870: 48 8b 03 mov (%rbx),%rax
rwlock_unlock_read():
/home/x/work/rw/rwlocktest.c:307
atomic_fetch_sub(pvar, 1);
2873: f0 83 2c f0 01 lock subl $0x1,(%rax,%rsi,8)
rwlock_trylock_read():
283f: 90 nop
/home/x/work/rw/rwlocktest.c:284
if ((atomic_fetch_add(pvar, 1) >> RWLOCK_WSHIFT) == 0) {
2840: f0 ff 04 f0 lock incl (%rax,%rsi,8)
2844: 7f 1a jg 2860 <rwlock_worker+0x270>
/home/x/work/rw/rwlocktest.c:287
atomic_fetch_sub(pvar, 1);
2846: f0 ff 0c f0 lock decl (%rax,%rsi,8)
(benchmarking results omitted, not improved)
rwlock_trylock_read():
/home/x/work/rw/rwlocktest.c:284
if ((atomic_fetch_add(pvar, 3) >> RWLOCK_WSHIFT) == 0) {
2840: ba 03 00 00 00 mov $0x3,%edx
2845: f0 0f c1 14 f0 lock xadd %edx,(%rax,%rsi,8)
284a: 85 d2 test %edx,%edx
284c: 79 22 jns 2870 <rwlock_worker+0x280>
/home/x/work/rw/rwlocktest.c:287
atomic_fetch_sub(pvar, 3);
284e: f0 83 2c f0 03 lock subl $0x3,(%rax,%rsi,8)
https://archive.archlinux.org/packages/c/compiler-rt/
https://archive.archlinux.org/packages/l/llvm/
https://archive.archlinux.org/packages/l/llvm-libs/
https://archive.archlinux.org/packages/c/clang/