Re: [PATCH 6/7] mm: parallelize deferred_init_memmap()

From: Daniel Jordan
Date: Mon May 04 2020 - 22:12:45 EST

On Mon, May 04, 2020 at 09:48:44PM -0400, Daniel Jordan wrote:
> On Mon, May 04, 2020 at 05:40:19PM -0700, Alexander Duyck wrote:
> > On Mon, May 4, 2020 at 4:44 PM Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > On May 4, 2020 3:33:58 PM PDT, Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote:
> > > >On Thu, Apr 30, 2020 at 1:12 PM Daniel Jordan
> > > ><daniel.m.jordan@xxxxxxxxxx> wrote:
> > > >> /*
> > > >> - * Initialize and free pages in MAX_ORDER sized increments so
> > > >> - * that we can avoid introducing any issues with the buddy
> > > >> - * allocator.
> > > >> + * More CPUs always led to greater speedups on tested
> > > >systems, up to
> > > >> + * all the nodes' CPUs. Use all since the system is
> > > >otherwise idle now.
> > > >> */
> > > >
> > > >I would be curious about your data. That isn't what I have seen in the
> > > >past. Typically only up to about 8 or 10 CPUs gives you any benefit,
> > > >beyond that I was usually cache/memory bandwidth bound.
>
> On Skylake it took more than 8 or 10 CPUs, though on other machines the benefit
> of using all versus half or 3/4 of the CPUs is less significant.
>
> Given that the rest of the system is idle at this point, my main concern is
> whether other archs regress past a certain thread count.

Reposting the data to be consistent with the way the percentages are reported
in the changelog.


Intel(R) Xeon(R) Platinum 8167M CPU @ 2.00GHz (Skylake, bare metal)
2 nodes * 26 cores * 2 threads = 104 CPUs
384G/node = 768G memory

kernel boot deferred init
------------------------ ------------------------
node% (thr) speedup time_ms (stdev) speedup time_ms (stdev)
( 0) -- 4056.7 ( 5.5) -- 1763.3 ( 4.2)
2% ( 1) -2.4% 4153.3 ( 2.5) -5.6% 1861.7 ( 5.5)
12% ( 6) 35.0% 2637.7 ( 38.7) 80.3% 346.7 ( 37.5)
25% ( 13) 38.4% 2497.3 ( 38.5) 88.1% 210.0 ( 41.8)
37% ( 19) 38.9% 2477.0 ( 19.0) 89.5% 185.3 ( 21.5)
50% ( 26) 39.1% 2471.7 ( 21.4) 89.8% 179.7 ( 25.8)
75% ( 39) 39.5% 2455.7 ( 33.2) 90.8% 161.7 ( 29.3)
100% ( 52) 39.9% 2436.7 ( 2.1) 91.8% 144.3 ( 5.9)


Intel(R) Xeon(R) CPU E5-2699C v4 @ 2.20GHz (Broadwell, bare metal)
1 node * 16 cores * 2 threads = 32 CPUs
192G/node = 192G memory

kernel boot deferred init
------------------------ ------------------------
node% (thr) speedup time_ms (stdev) speedup time_ms (stdev)
( 0) -- 1957.3 ( 14.0) -- 1093.7 ( 12.9)
3% ( 1) 1.4% 1930.7 ( 10.0) 3.7% 1053.3 ( 7.6)
12% ( 4) 41.2% 1151.7 ( 9.0) 74.5% 278.7 ( 0.6)
25% ( 8) 46.3% 1051.0 ( 7.8) 83.7% 178.0 ( 2.6)
38% ( 12) 48.7% 1003.3 ( 7.6) 87.0% 141.7 ( 3.8)
50% ( 16) 48.2% 1014.3 ( 20.0) 87.8% 133.3 ( 3.2)
75% ( 24) 49.5% 989.3 ( 6.7) 88.4% 126.3 ( 1.5)
100% ( 32) 49.1% 996.0 ( 7.2) 88.4% 127.3 ( 5.1)


Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz (Haswell, bare metal)
2 nodes * 18 cores * 2 threads = 72 CPUs
128G/node = 256G memory

kernel boot deferred init
------------------------ ------------------------
node% (thr) speedup time_ms (stdev) speedup time_ms (stdev)
( 0) -- 1666.0 ( 3.5) -- 618.0 ( 3.5)
3% ( 1) 1.0% 1649.7 ( 1.5) 2.9% 600.0 ( 1.0)
11% ( 4) 25.9% 1234.7 ( 21.4) 70.4% 183.0 ( 22.5)
25% ( 9) 29.6% 1173.0 ( 10.0) 80.7% 119.3 ( 9.6)
36% ( 13) 30.8% 1153.7 ( 17.0) 84.0% 99.0 ( 15.6)
50% ( 18) 31.0% 1150.3 ( 15.5) 84.3% 97.3 ( 16.2)
75% ( 27) 31.0% 1150.3 ( 2.5) 84.6% 95.0 ( 5.6)
100% ( 36) 31.3% 1145.3 ( 1.5) 85.6% 89.0 ( 1.7)


AMD EPYC 7551 32-Core Processor (Zen, kvm guest)
1 node * 8 cores * 2 threads = 16 CPUs
64G/node = 64G memory

kernel boot deferred init
------------------------ ------------------------
node% (thr) speedup time_ms (stdev) speedup time_ms (stdev)
( 0) -- 1029.7 ( 42.3) -- 253.7 ( 3.1)
6% ( 1) 3.3% 995.3 ( 21.4) 4.3% 242.7 ( 5.5)
12% ( 2) 14.0% 885.7 ( 24.4) 46.4% 136.0 ( 5.2)
25% ( 4) 18.9% 835.0 ( 21.5) 66.1% 86.0 ( 1.7)
38% ( 6) 21.9% 804.7 ( 15.7) 71.4% 72.7 ( 2.1)
50% ( 8) 20.8% 815.3 ( 11.7) 74.4% 65.0 ( 3.5)
75% ( 12) 23.5% 787.7 ( 2.1) 74.0% 66.0 ( 3.6)
100% ( 16) 23.3% 789.3 ( 15.0) 76.3% 60.0 ( 5.6)


AMD EPYC 7551 32-Core Processor (Zen, kvm guest)
1 node * 2 cores * 2 threads = 4 CPUs
16G/node = 16G memory

kernel boot deferred init
------------------------ ------------------------
node% (thr) speedup time_ms (stdev) speedup time_ms (stdev)
( 0) -- 757.7 ( 17.1) -- 57.0 ( 0.0)
25% ( 1) -1.0% 765.3 ( 5.5) 3.5% 55.0 ( 0.0)
50% ( 2) 4.7% 722.3 ( 21.5) 42.7% 32.7 ( 4.6)
75% ( 3) 3.7% 729.7 ( 4.9) 54.4% 26.0 ( 0.0)
100% ( 4) 6.2% 710.3 ( 15.0) 63.2% 21.0 ( 0.0)


Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz (Haswell, kvm guest)
1 node * 2 cores * 2 threads = 4 CPUs
14G/node = 14G memory

kernel boot deferred init
------------------------ ------------------------
node% (thr) speedup time_ms (stdev) speedup time_ms (stdev)
( 0) -- 656.3 ( 7.1) -- 57.3 ( 1.5)
25% ( 1) 1.8% 644.7 ( 3.1) 0.6% 57.0 ( 0.0)
50% ( 2) 6.5% 613.7 ( 5.1) 40.7% 34.0 ( 5.3)
75% ( 3) 6.9% 611.3 ( 6.7) 57.6% 24.3 ( 0.6)
100% ( 4) 8.6% 599.7 ( 5.9) 62.8% 21.3 ( 1.2)


> > > I've found pretty much linear performance up to memory bandwidth, and on the systems I was testing, I didn't saturate memory bandwidth until about the full number of physical cores. From number of cores up to number of threads, the performance stayed about flat; it didn't get any better or worse.
> >
> > That doesn't sound right though based on the numbers you provided. The
> > system you had was 192GB spread over 2 nodes with 48thread/24core per
> > node, correct? Your numbers went from ~290ms to ~28ms so a 10x
> > decrease, that doesn't sound linear when you spread the work over 24
> > cores to get there. I agree that the numbers largely stay flat once
> > you hit the peak, I have seen similar behavior when I was working on
> > the deferred init code previously. One concern I have though is that
> > we may end up seeing better performance with a subset of cores instead
> > of running all of the cores/threads, especially if features such as
> > turbo come into play. In addition we are talking x86 only so far. I
> > would be interested in seeing if this has benefits or not for other
> > architectures.
> >
> > Also what is the penalty that is being paid in order to break up the
> > work before-hand and set it up for the parallel work? I would be
> > interested in seeing what the cost is on a system with fewer cores per
> > node, maybe even down to 1. That would tell us how much additional
> > overhead is being added to set things up to run in parallel.
>
> The numbers above have the 1-thread case. It seems close to the noise.
>
> > If I get
> > a chance tomorrow I might try applying the patches and doing some
> > testing myself.
>
> If you end up doing that, you might find this helpful:
> https://oss.oracle.com/git/gitweb.cgi?p=linux-dmjordan.git;a=patch;h=afc72bf8478b95a1d6d174c269ff3693c60630e0
>
> and maybe this:
> https://oss.oracle.com/git/gitweb.cgi?p=linux-dmjordan.git;a=patch;h=dff6537eab281e5a9917682c4adf9059c0574223
>
> Thanks for looking this over.
>
> [ By the way, I'm going to be out Tuesday but back the rest of the week. ]
>