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

From: Daniel Jordan
Date: Mon May 04 2020 - 21:49:24 EST

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.


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)
( 1) -2.3% 4153.3 ( 2.5) -5.3% 1861.7 ( 5.5)
12% ( 6) 53.8% 2637.7 ( 38.7) 408.7% 346.7 ( 37.5)
25% ( 13) 62.4% 2497.3 ( 38.5) 739.7% 210.0 ( 41.8)
37% ( 19) 63.8% 2477.0 ( 19.0) 851.4% 185.3 ( 21.5)
50% ( 26) 64.1% 2471.7 ( 21.4) 881.4% 179.7 ( 25.8)
75% ( 39) 65.2% 2455.7 ( 33.2) 990.7% 161.7 ( 29.3)
100% ( 52) 66.5% 2436.7 ( 2.1) 1121.7% 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)
( 1) 1.4% 1930.7 ( 10.0) 3.8% 1053.3 ( 7.6)
12% ( 4) 70.0% 1151.7 ( 9.0) 292.5% 278.7 ( 0.6)
25% ( 8) 86.2% 1051.0 ( 7.8) 514.4% 178.0 ( 2.6)
37% ( 12) 95.1% 1003.3 ( 7.6) 672.0% 141.7 ( 3.8)
50% ( 16) 93.0% 1014.3 ( 20.0) 720.2% 133.3 ( 3.2)
75% ( 24) 97.8% 989.3 ( 6.7) 765.7% 126.3 ( 1.5)
100% ( 32) 96.5% 996.0 ( 7.2) 758.9% 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)
( 1) 1.0% 1649.7 ( 1.5) 3.0% 600.0 ( 1.0)
12% ( 4) 34.9% 1234.7 ( 21.4) 237.7% 183.0 ( 22.5)
25% ( 9) 42.0% 1173.0 ( 10.0) 417.9% 119.3 ( 9.6)
37% ( 13) 44.4% 1153.7 ( 17.0) 524.2% 99.0 ( 15.6)
50% ( 18) 44.8% 1150.3 ( 15.5) 534.9% 97.3 ( 16.2)
75% ( 27) 44.8% 1150.3 ( 2.5) 550.5% 95.0 ( 5.6)
100% ( 36) 45.5% 1145.3 ( 1.5) 594.4% 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)
( 1) 3.4% 995.3 ( 21.4) 4.5% 242.7 ( 5.5)
12% ( 2) 16.3% 885.7 ( 24.4) 86.5% 136.0 ( 5.2)
25% ( 4) 23.3% 835.0 ( 21.5) 195.0% 86.0 ( 1.7)
37% ( 6) 28.0% 804.7 ( 15.7) 249.1% 72.7 ( 2.1)
50% ( 8) 26.3% 815.3 ( 11.7) 290.3% 65.0 ( 3.5)
75% ( 12) 30.7% 787.7 ( 2.1) 284.3% 66.0 ( 3.6)
100% ( 16) 30.4% 789.3 ( 15.0) 322.8% 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) 3.6% 55.0 ( 0.0)
50% ( 2) 4.9% 722.3 ( 21.5) 74.5% 32.7 ( 4.6)
75% ( 3) 3.8% 729.7 ( 4.9) 119.2% 26.0 ( 0.0)
100% ( 4) 6.7% 710.3 ( 15.0) 171.4% 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) 7.0% 613.7 ( 5.1) 68.6% 34.0 ( 5.3)
75% ( 3) 7.4% 611.3 ( 6.7) 135.6% 24.3 ( 0.6)
100% ( 4) 9.4% 599.7 ( 5.9) 168.8% 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. ]