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SubjectRe: [RFC PATCH v2 00/17] Core scheduling v2
On Thu, Apr 25, 2019 at 11:55:08AM +0200, Ingo Molnar wrote:
> > > Would it be possible to post the results with HT off as well ?
> >
> > What's the point here to turn HT off? The latency is sensitive to the
> > relationship
> > between the task number and CPU number. Usually less CPU number, more run
> > queue wait time, and worse result.
>
> HT-off numbers are mandatory: turning HT off is by far the simplest way
> to solve the security bugs in these CPUs.
>
> Any core-scheduling solution *must* perform better than HT-off for all
> relevant workloads, otherwise what's the point?
>

I agree. Not only should HT-off be evaluated but it should properly
evaluate for different levels of machine utilisation to get a complete
picture.

Around the same time this was first posted and because of kernel
warnings from L1TF, I did a preliminary evaluation of HT On vs HT Off
using nosmt -- this is sub-optimal in itself but it was convenient. The
conventional wisdom that HT gets a 30% boost appears to be primarily based
on academic papers evaluating HPC workloads on a Pentium 4 with a focus
on embarassingly parallel problems which is the ideal case for HT but not
the universal case. The conventional wisdom is questionable at best. The
only modern comparisons I could find were focused on games primarily
which I think hit scaling limits before HT is a factor in some cases.

I don't have the data in a format that can be present everything in a clear
format but here is an attempt anyway. This is long but the central point
that when when a machine is lightly loaded, HT Off generally performs
better than HT On and even when heavily utilised, it's still not a
guaranteed loss. I only suggest reading after this if you have coffee
and time. Ideally all this would be updated with a comparison to core
scheduling but I may not get it queued on my test grid before I leave
for LSF/MM and besides, the authors pushing this feature should be able
to provide supporting data justifying the complexity of the series.

Here is a tbench comparison scaling from a low thread count to a high
thread count. I picked tbench because it's relatively uncomplicated and
tends to be reasonable at spotting scheduler regressions. The kernel
version is old but for the purposes of this discussion, it doesn't matter

1-socket Skylake (8 logical CPUs HT On, 4 logical CPUs HT Off)
smt nosmt
Hmean 1 484.00 ( 0.00%) 519.95 * 7.43%*
Hmean 2 925.02 ( 0.00%) 1022.28 * 10.51%*
Hmean 4 1730.34 ( 0.00%) 2029.81 * 17.31%*
Hmean 8 2883.57 ( 0.00%) 2040.89 * -29.22%*
Hmean 16 2830.61 ( 0.00%) 2039.74 * -27.94%*
Hmean 32 2855.54 ( 0.00%) 2042.70 * -28.47%*
Stddev 1 1.16 ( 0.00%) 0.62 ( 46.43%)
Stddev 2 1.31 ( 0.00%) 1.00 ( 23.32%)
Stddev 4 4.89 ( 0.00%) 12.86 (-163.14%)
Stddev 8 4.30 ( 0.00%) 2.53 ( 40.99%)
Stddev 16 3.38 ( 0.00%) 5.92 ( -75.08%)
Stddev 32 5.47 ( 0.00%) 14.28 (-160.77%)

Note that disabling HT performs better when cores are available but hits
scaling limits past 4 CPUs when the machine is saturated with HT off.
It's similar with 2 sockets

2-socket Broadwell (80 logical CPUs HT On, 40 logical CPUs HT Off)

smt nosmt
Hmean 1 514.28 ( 0.00%) 540.90 * 5.18%*
Hmean 2 982.19 ( 0.00%) 1042.98 * 6.19%*
Hmean 4 1820.02 ( 0.00%) 1943.38 * 6.78%*
Hmean 8 3356.73 ( 0.00%) 3655.92 * 8.91%*
Hmean 16 6240.53 ( 0.00%) 7057.57 * 13.09%*
Hmean 32 10584.60 ( 0.00%) 15934.82 * 50.55%*
Hmean 64 24967.92 ( 0.00%) 21103.79 * -15.48%*
Hmean 128 27106.28 ( 0.00%) 20822.46 * -23.18%*
Hmean 256 28345.15 ( 0.00%) 21625.67 * -23.71%*
Hmean 320 28358.54 ( 0.00%) 21768.70 * -23.24%*
Stddev 1 2.10 ( 0.00%) 3.44 ( -63.59%)
Stddev 2 2.46 ( 0.00%) 4.83 ( -95.91%)
Stddev 4 7.57 ( 0.00%) 6.14 ( 18.86%)
Stddev 8 6.53 ( 0.00%) 11.80 ( -80.79%)
Stddev 16 11.23 ( 0.00%) 16.03 ( -42.74%)
Stddev 32 18.99 ( 0.00%) 22.04 ( -16.10%)
Stddev 64 10.86 ( 0.00%) 14.31 ( -31.71%)
Stddev 128 25.10 ( 0.00%) 16.08 ( 35.93%)
Stddev 256 29.95 ( 0.00%) 71.39 (-138.36%)

Same -- performance is better until the machine gets saturated and
disabling HT hits scaling limits earlier.

The workload "mutilate" is a load generator for memcached that is meant
to simulate a workload interesting to Facebook.

1-socket
Hmean 1 28570.67 ( 0.00%) 31632.92 * 10.72%*
Hmean 3 76904.93 ( 0.00%) 89644.73 * 16.57%*
Hmean 5 107487.40 ( 0.00%) 93418.09 * -13.09%*
Hmean 7 103066.62 ( 0.00%) 79843.72 * -22.53%*
Hmean 8 103921.65 ( 0.00%) 76378.18 * -26.50%*
Stddev 1 112.37 ( 0.00%) 261.61 (-132.82%)
Stddev 3 272.29 ( 0.00%) 641.41 (-135.56%)
Stddev 5 406.75 ( 0.00%) 1240.15 (-204.89%)
Stddev 7 2402.02 ( 0.00%) 1336.68 ( 44.35%)
Stddev 8 1139.90 ( 0.00%) 393.56 ( 65.47%)

2-socket
Hmean 1 24571.95 ( 0.00%) 24891.45 ( 1.30%)
Hmean 4 106963.43 ( 0.00%) 103955.79 ( -2.81%)
Hmean 7 154328.47 ( 0.00%) 169782.56 * 10.01%*
Hmean 12 235108.36 ( 0.00%) 236544.96 ( 0.61%)
Hmean 21 238619.16 ( 0.00%) 234542.88 * -1.71%*
Hmean 30 240198.02 ( 0.00%) 237758.38 ( -1.02%)
Hmean 48 212573.72 ( 0.00%) 172633.74 * -18.79%*
Hmean 79 140937.97 ( 0.00%) 112915.07 * -19.88%*
Hmean 80 134204.84 ( 0.00%) 116904.93 ( -12.89%)
Stddev 1 40.95 ( 0.00%) 284.57 (-594.84%)
Stddev 4 7556.84 ( 0.00%) 2176.60 ( 71.20%)
Stddev 7 10279.89 ( 0.00%) 3510.15 ( 65.85%)
Stddev 12 2534.03 ( 0.00%) 1513.61 ( 40.27%)
Stddev 21 1118.59 ( 0.00%) 1662.31 ( -48.61%)
Stddev 30 3540.20 ( 0.00%) 2056.37 ( 41.91%)
Stddev 48 24206.00 ( 0.00%) 6247.74 ( 74.19%)
Stddev 79 21650.80 ( 0.00%) 5395.35 ( 75.08%)
Stddev 80 26769.15 ( 0.00%) 5665.14 ( 78.84%)

Less clear-cut. Performance is better with HT off on Skylake but similar
until the machine is saturated on Broadwell.

With pgbench running a read-only workload we see

2-socket
Hmean 1 13226.78 ( 0.00%) 14971.99 * 13.19%*
Hmean 6 39820.61 ( 0.00%) 35036.50 * -12.01%*
Hmean 12 66707.55 ( 0.00%) 61403.63 * -7.95%*
Hmean 22 108748.16 ( 0.00%) 110223.97 * 1.36%*
Hmean 30 121964.05 ( 0.00%) 121837.03 ( -0.10%)
Hmean 48 121530.97 ( 0.00%) 117855.86 * -3.02%*
Hmean 80 116034.43 ( 0.00%) 121826.25 * 4.99%*
Hmean 110 125441.59 ( 0.00%) 122180.19 * -2.60%*
Hmean 142 117908.18 ( 0.00%) 117531.41 ( -0.32%)
Hmean 160 119343.50 ( 0.00%) 115725.11 * -3.03%*

Mix of results -- single client is better, 6 and 12 clients regressed for
some reason and after that, it's mostly flat. Hence, HT for this database
load makes very little difference because the performance limits are not
based on CPUs being immediately available.

SpecJBB 2005 is ancient but it does lend itself to easily scaling the
number of active tasks so here is a sample of the performance as
utilisation ramped up to saturation

2-socket
Hmean tput-1 48655.00 ( 0.00%) 48762.00 * 0.22%*
Hmean tput-8 387341.00 ( 0.00%) 390062.00 * 0.70%*
Hmean tput-15 660993.00 ( 0.00%) 659832.00 * -0.18%*
Hmean tput-22 916898.00 ( 0.00%) 913570.00 * -0.36%*
Hmean tput-29 1178601.00 ( 0.00%) 1169843.00 * -0.74%*
Hmean tput-36 1292377.00 ( 0.00%) 1387003.00 * 7.32%*
Hmean tput-43 1458913.00 ( 0.00%) 1508172.00 * 3.38%*
Hmean tput-50 1411975.00 ( 0.00%) 1513536.00 * 7.19%*
Hmean tput-57 1417937.00 ( 0.00%) 1495513.00 * 5.47%*
Hmean tput-64 1396242.00 ( 0.00%) 1477433.00 * 5.81%*
Hmean tput-71 1349055.00 ( 0.00%) 1472856.00 * 9.18%*
Hmean tput-78 1265738.00 ( 0.00%) 1453846.00 * 14.86%*
Hmean tput-79 1307367.00 ( 0.00%) 1446572.00 * 10.65%*
Hmean tput-80 1309718.00 ( 0.00%) 1449384.00 * 10.66%*

This was the most surprising result -- HT off was generally a benefit
even when the counts were higher than the available CPUs and I'm not
sure why. It's also interesting with HT off that the chances of keeping
a workload local to a node are reduced as a socket gets saturated earlier
but the load balancer is generally moving tasks around and NUMA Balancing
is also in play. Still, it shows that disabling HT is not a universal loss.

netperf is inherently about two tasks. For UDP_STREAM, it shows almost
no difference and it's within noise. TCP_STREAM was interesting

Hmean 64 1154.23 ( 0.00%) 1162.69 * 0.73%*
Hmean 128 2194.67 ( 0.00%) 2230.90 * 1.65%*
Hmean 256 3867.89 ( 0.00%) 3929.99 * 1.61%*
Hmean 1024 12714.52 ( 0.00%) 12913.81 * 1.57%*
Hmean 2048 21141.11 ( 0.00%) 21266.89 ( 0.59%)
Hmean 3312 27945.71 ( 0.00%) 28354.82 ( 1.46%)
Hmean 4096 30594.24 ( 0.00%) 30666.15 ( 0.24%)
Hmean 8192 37462.58 ( 0.00%) 36901.45 ( -1.50%)
Hmean 16384 42947.02 ( 0.00%) 43565.98 * 1.44%*
Stddev 64 2.21 ( 0.00%) 4.02 ( -81.62%)
Stddev 128 18.45 ( 0.00%) 11.11 ( 39.79%)
Stddev 256 30.84 ( 0.00%) 22.10 ( 28.33%)
Stddev 1024 141.46 ( 0.00%) 56.54 ( 60.03%)
Stddev 2048 200.39 ( 0.00%) 75.56 ( 62.29%)
Stddev 3312 411.11 ( 0.00%) 286.97 ( 30.20%)
Stddev 4096 299.86 ( 0.00%) 322.44 ( -7.53%)
Stddev 8192 418.80 ( 0.00%) 635.63 ( -51.77%)
Stddev 16384 661.57 ( 0.00%) 206.73 ( 68.75%)

The performance difference is marginal but variance is much reduced
by disabling HT. Now, it's important to note that this particular test
did not control for c-states and it did not bind tasks so there are a
lot of potential sources of noise. I didn't control for them because
I don't think many normal users would properly take concerns like that
into account. MMtests is able to control for those factors so it could
be independently checked.

hackbench is the most obvious loser. This is for processes communicating
via pipes.

Amean 1 0.7343 ( 0.00%) 1.1377 * -54.93%*
Amean 4 1.1647 ( 0.00%) 2.1543 * -84.97%*
Amean 7 1.6770 ( 0.00%) 3.1300 * -86.64%*
Amean 12 2.4500 ( 0.00%) 4.6447 * -89.58%*
Amean 21 3.9927 ( 0.00%) 6.8250 * -70.94%*
Amean 30 5.5320 ( 0.00%) 8.6433 * -56.24%*
Amean 48 8.4723 ( 0.00%) 12.1890 * -43.87%*
Amean 79 12.3760 ( 0.00%) 17.8347 * -44.11%*
Amean 110 16.0257 ( 0.00%) 23.1373 * -44.38%*
Amean 141 20.7070 ( 0.00%) 29.8537 * -44.17%*
Amean 172 25.1507 ( 0.00%) 37.4830 * -49.03%*
Amean 203 28.5303 ( 0.00%) 43.5220 * -52.55%*
Amean 234 33.8233 ( 0.00%) 51.5403 * -52.38%*
Amean 265 37.8703 ( 0.00%) 58.1860 * -53.65%*
Amean 296 43.8303 ( 0.00%) 64.9223 * -48.12%*
Stddev 1 0.0040 ( 0.00%) 0.0117 (-189.97%)
Stddev 4 0.0046 ( 0.00%) 0.0766 (-1557.56%)
Stddev 7 0.0333 ( 0.00%) 0.0991 (-197.83%)
Stddev 12 0.0425 ( 0.00%) 0.1303 (-206.90%)
Stddev 21 0.0337 ( 0.00%) 0.4138 (-1127.60%)
Stddev 30 0.0295 ( 0.00%) 0.1551 (-424.94%)
Stddev 48 0.0445 ( 0.00%) 0.2056 (-361.71%)
Stddev 79 0.0350 ( 0.00%) 0.4118 (-1076.56%)
Stddev 110 0.0655 ( 0.00%) 0.3685 (-462.72%)
Stddev 141 0.3670 ( 0.00%) 0.5488 ( -49.55%)
Stddev 172 0.7375 ( 0.00%) 1.0806 ( -46.52%)
Stddev 203 0.0817 ( 0.00%) 1.6920 (-1970.11%)
Stddev 234 0.8210 ( 0.00%) 1.4036 ( -70.97%)
Stddev 265 0.9337 ( 0.00%) 1.1025 ( -18.08%)
Stddev 296 1.5688 ( 0.00%) 0.4154 ( 73.52%)

The problem with hackbench is that "1" above doesn't represent 1 task,
it represents 1 group and so the machine gets saturated relatively
quickly and it's super sensitive to cores being idle and available to
make quick progress.

Kernel building which is all anyone ever cares about is a mixed bag

1-socket
Amean elsp-2 420.45 ( 0.00%) 240.80 * 42.73%*
Amean elsp-4 363.54 ( 0.00%) 135.09 * 62.84%*
Amean elsp-8 105.40 ( 0.00%) 131.46 * -24.73%*
Amean elsp-16 106.61 ( 0.00%) 133.57 * -25.29%*

2-socket
Amean elsp-2 406.76 ( 0.00%) 448.57 ( -10.28%)
Amean elsp-4 235.22 ( 0.00%) 289.48 ( -23.07%)
Amean elsp-8 152.36 ( 0.00%) 116.76 ( 23.37%)
Amean elsp-16 64.50 ( 0.00%) 52.12 * 19.20%*
Amean elsp-32 30.28 ( 0.00%) 28.24 * 6.74%*
Amean elsp-64 21.67 ( 0.00%) 23.00 * -6.13%*
Amean elsp-128 20.57 ( 0.00%) 23.57 * -14.60%*
Amean elsp-160 20.64 ( 0.00%) 23.63 * -14.50%*
Stddev elsp-2 75.35 ( 0.00%) 35.00 ( 53.55%)
Stddev elsp-4 71.12 ( 0.00%) 86.09 ( -21.05%)
Stddev elsp-8 43.05 ( 0.00%) 10.67 ( 75.22%)
Stddev elsp-16 4.08 ( 0.00%) 2.31 ( 43.41%)
Stddev elsp-32 0.51 ( 0.00%) 0.76 ( -48.60%)
Stddev elsp-64 0.38 ( 0.00%) 0.61 ( -60.72%)
Stddev elsp-128 0.13 ( 0.00%) 0.41 (-207.53%)
Stddev elsp-160 0.08 ( 0.00%) 0.20 (-147.93%)

1-socket matches other patterns, the 2-socket was weird. Variability was
nuts for low number of jobs. It's also not universal. I had tested in a
2-socket Haswell machine and it showed different results

Amean elsp-2 447.91 ( 0.00%) 467.43 ( -4.36%)
Amean elsp-4 284.47 ( 0.00%) 248.37 ( 12.69%)
Amean elsp-8 166.20 ( 0.00%) 129.23 ( 22.24%)
Amean elsp-16 63.89 ( 0.00%) 55.63 * 12.93%*
Amean elsp-32 36.80 ( 0.00%) 35.87 * 2.54%*
Amean elsp-64 30.97 ( 0.00%) 36.94 * -19.28%*
Amean elsp-96 31.66 ( 0.00%) 37.32 * -17.89%*
Stddev elsp-2 58.08 ( 0.00%) 57.93 ( 0.25%)
Stddev elsp-4 65.31 ( 0.00%) 41.56 ( 36.36%)
Stddev elsp-8 68.32 ( 0.00%) 15.61 ( 77.15%)
Stddev elsp-16 3.68 ( 0.00%) 2.43 ( 33.87%)
Stddev elsp-32 0.29 ( 0.00%) 0.97 (-239.75%)
Stddev elsp-64 0.36 ( 0.00%) 0.24 ( 32.10%)
Stddev elsp-96 0.30 ( 0.00%) 0.31 ( -5.11%)

Still not a perfect match to the general pattern for 2 build jobs and a
bit variable but otherwise the pattern holds -- performs better until the
machine is saturated. Kernel builds (or compilation builds) are always a
bit off as a benchmark as it has a mix of parallel and serialised tasks
that are non-deterministic.

With the NASA Parallel Benchmark (NPB, aka NAS) it's trickier to do a
valid comparison. Over-saturating NAS decimates performance but there
are limits on the exact thread counts that can be used for MPI. OpenMP
is less restrictive but here is an MPI comparison anyway comparing a
fully loaded HT On with fully loaded HT Off -- this is crucial, HT Off
has half the level of parallelisation

Amean bt 771.15 ( 0.00%) 926.98 * -20.21%*
Amean cg 445.92 ( 0.00%) 465.65 * -4.42%*
Amean ep 70.01 ( 0.00%) 97.15 * -38.76%*
Amean is 16.75 ( 0.00%) 19.08 * -13.95%*
Amean lu 882.84 ( 0.00%) 902.60 * -2.24%*
Amean mg 84.10 ( 0.00%) 95.95 * -14.10%*
Amean sp 1353.88 ( 0.00%) 1372.23 * -1.36%*

ep is the embarassingly parallel problem and it shows with half the cores
with HT off, we take a 38.76% performance hit. However, even that is not
universally true as cg for example did not parallelise as well and only
performacne 4.42% worse even with HT off. I can show a comparison with
equal levels of parallelisation but with HT off, it is a completely broken
configuration and I do not think a comparison like that makes any sense.

I didn't do any comparison that could represent Cloud. However, I think
it's worth noting that HT may be popular there for packing lots of virtual
machines onto a single host and over-subscribing. HT would intuitively
have an advantage there *but* it depends heavily on the utilisation and
whether there is sustained VCPU activity where the number of active VCPUs
exceeds physical CPUs when HT is off. There is also the question whether
performance even matters on such configurations but anything cloud related
will be "how long is a piece of string" and "it depends".

So there you have it, HT Off is not a guaranteed loss and can be a gain
so it should be considered as an alternative to core scheduling. The case
where HT makes a big difference is when a workload is CPU or memory bound
and the number of active tasks exceeds the number of CPUs on a socket
and again when number of active tasks exceeds the number of CPUs in the
whole machine.

--
Mel Gorman
SUSE Labs

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