Re: PSA: New intel MDS vulnerability mitigations cause measurable slowdown

On Wed, May 15, 2019 at 1:13 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > I've run a quick pgbench benchmark:
> >
> > *Without* disabling SMT, for readonly pgbench, I'm seeing regressions
> > between 7-11%, depending on the size of shared_buffers (and some runtime
> > variations). That's just on my laptop, with an i7-6820HQ / Haswell CPU.
> > I'd be surprised if there weren't adversarial loads with bigger
> > slowdowns - what gets more expensive with the mitigations is syscalls.

This stuff landed in my FreeBSD 13.0-CURRENT kernel, so I was curious
to measure it with and without the earlier mitigations. On my humble
i7-8550U laptop with the new 1.22 microcode installed, with my usual
settings of PTI=on and IBRS=off, so far MDS=VERW gives me ~1.5% loss
of TPS with a single client, up to 4.3% loss of TPS for 16 clients,
but it didn't go higher when I tried 32 clients. This was a tiny
scale 10 database, though in a quick test it didn't look like it was
worse with scale 100.

With all three mitigations activated, my little dev machine has gone
from being able to do ~11.8 million baseline syscalls per second to
~1.6 million, or ~1.4 million with the AVX variant of the mitigation.

Raw getuid() syscalls per second:

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 11798658 4764159 3274043
off on 2652564 1941606 1655356
on off 4973053 2932906 2339779
on on 1988527 1556922 1378798

pgbench read-only transactions per second, 1 client thread:

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 19393 18949 18615
off on 17946 17586 17323
on off 19381 19015 18696
on on 18045 17709 17418

pgbench -M prepared read-only transactions per second, 1 client thread:

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 35020 34049 33200
off on 31658 30902 30229
on off 35445 34353 33415
on on 32415 31599 30712

pgbench -M prepared read-only transactions per second, 4 client threads:

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 79515 76898 76465
off on 63608 62220 61952
on off 77863 75431 74847
on on 62709 60790 60575

pgbench -M prepared read-only transactions per second, 16 client threads:

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 125984 121164 120468
off on 112884 108346 107984
on off 121032 116156 115462
on on 108889 104636 104027

time gmake -s check:

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 16.78 16.85 17.03
off on 18.19 18.81 19.08
on off 16.67 16.86 17.33
on on 18.58 18.83 18.99

--
Thomas Munro
https://enterprisedb.com