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

On Wed, May 15, 2019 at 1:13 PM Andres Freund <andres@anarazel.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