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

On Fri, May 17, 2019 at 5:26 AM Chapman Flack <chap(at)anastigmatix(dot)net> wrote:
> On 5/16/19 12:24 PM, Albert Cervera i Areny wrote:
> > Missatge de Thomas Munro <thomas(dot)munro(at)gmail(dot)com> del dia dj., 16 de
> > maig 2019 a les 13:09:
> >> With all three mitigations activated, my little dev machine has gone
> >> from being able to do ~11.8 million baseline syscalls per second to
> >
> > Did you mean "1.8"?
>
> Not in what I thought I saw:
>
> >> ~1.6 million, or ~1.4 million ...
> >>
> >> 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
> ^^^^^^^ ^^^^^^^

Right. Actually it's worse than that -- after I posted I realised
that I had some debug stuff enabled in my kernel that was slowing
things down a bit, so I reran the tests overnight with a production
kernel and here is what I see this morning. It's actually ~17.8
million syscalls/sec -> ~1.7 million syscalls/sec, if you go from all
mitigations off to all mitigations on, or -> ~3.2 million for just PTI
+ MDS. And the loss of TPS is ~5% for the case I was most interested
in, just turning on MDS=VERW if you already had PTI on and IBRS off.

Raw getuid() syscalls per second:

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 17771744 5372032 3575035
off on 3060923 2166527 1817052
on off 5622591 3150883 2463934
on on 2213190 1687748 1475605

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

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 22414 22103 21571
off on 21298 20817 20418
on off 22473 22080 21550
on on 21286 20850 20386

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

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 43508 42476 41123
off on 40729 39483 38555
on off 44110 42989 42012
on on 41143 39990 38798

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

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 100735 97689 96662
off on 80142 77804 77064
on off 100540 97010 95827
on on 79492 76976 76226

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

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 161015 152978 152556
off on 145605 139438 139179
on off 155359 147691 146987
on on 140976 134978 134177

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

PTI IBRS MDS=off MDS=VERW MDS=AVX
===== ===== ======== ======== ========
off off 157986 150132 149436
off on 142618 136220 135901
on off 153482 146214 145839
on on 138650 133074 132142

--
Thomas Munro
https://enterprisedb.com