After doing a bit more research I stumbled upon this:

http://unix.stackexchange.com/questions/253903/creating-threads-fails-with-resource-temporarily-unavailable-with-4-3-kernel
https://news.ycombinator.com/item?id=11675129

Looks like systemd imposes another, smaller limit on the number of
processes that a user can run:

cat /sys/fs/cgroup/pids/user.slice/user-1000.slice/pids.max
12288

It would have been nice if the 'fork' man page mentioned that this could
be a cause for failure. :(

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https://bugs.launchpad.net/bugs/1624043

Title:
  Call to fork/clone fails with EAGAIN (before encountering resource
  limits)

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  I wrote a test program that forks processes until the fork calls start
  to fail.  It forks around 12000 processes and then the fork calls
  start failing with EAGAIN.  According to the fork man page, there are
  four conditions that could cause EAGAIN to be returned:

  - the  RLIMIT_NPROC soft resource limit, which limits the number of processes 
and threads for a real user ID, was reached
  - the kernel's system-wide limit on the number of processes and threads, 
/proc/sys/kernel/threads-max, was reached
  - the maximum number of PIDs, /proc/sys/kernel/pid_max, was reached
  - The caller is operating under the SCHED_DEADLINE scheduling policy and does 
not have the reset-on-fork flag set

  On my machine:
   - Before running the program, ~250 processes / ~500 threads are running (as 
determined by ps)
   - RLIMIT_NPROC (soft and hard) is 31616
   - threads-max is 63233
   - pid_max is 32768
   - the program runs with the SCHED_NORMAL scheduling policy (so, not 
SCHED_DEADLINE)

  It seems strange that the fork calls fail after ~12000 forks, (it
  should fail at 31616.)  Some more technical details:

   - Reproducible on Ubuntu 16.04.1 running with kernel 4.4.0-36-generic.
   - Reproducible when tested with mainline kernel 4.8.0-040800rc6-generic
   - Doesn't occur on Ubuntu 12.04 running with kernel 3.2.0-23-generic
   - Monitoring thread usage, it appears to fail at exactly the 12,500 thread 
mark
   - From using strace, it looks like clone is the syscall actually being used 
behind the scenes (should have the same EAGAIN error semantics, from the clone 
man page.)
   - From using systemtap and ftrace, it looks like copy_process in _do_fork 
returns an error when this case is hit.  Maybe from sched_trace?  It's hard to 
tell - the ftrace output doesn't seem complete.

  I'm attaching the test fork program I've been using, which has some
  code to also print the aforementioned values.

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: linux-image-4.4.0-36-generic 4.4.0-36.55
  ProcVersionSignature: Ubuntu 4.4.0-36.55-generic 4.4.16
  Uname: Linux 4.4.0-36-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.1
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC1:  shockwave   3454 F.... pulseaudio
   /dev/snd/controlC0:  shockwave   3454 F.... pulseaudio
  CurrentDesktop: GNOME-Flashback:Unity
  Date: Thu Sep 15 11:07:16 2016
  EcryptfsInUse: Yes
  HibernationDevice: RESUME=UUID=281b1d85-c94d-43ee-a37b-736e550f48e7
  InstallationDate: Installed on 2016-09-12 (3 days ago)
  InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 
(20160719)
  IwConfig:
   lo        no wireless extensions.
   
   eno1      no wireless extensions.
  MachineType: Dell Inc. Precision T1600
  ProcFB: 0 nouveaufb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-36-generic 
root=UUID=e15cb9c6-c1a4-4313-9067-340edb6098a1 ro quiet splash vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-4.4.0-36-generic N/A
   linux-backports-modules-4.4.0-36-generic  N/A
   linux-firmware                            1.157.3
  RfKill:
   
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 04/11/2011
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: A02
  dmi.board.name: 06NWYK
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A00
  dmi.chassis.type: 6
  dmi.chassis.vendor: Dell Inc.
  dmi.modalias: 
dmi:bvnDellInc.:bvrA02:bd04/11/2011:svnDellInc.:pnPrecisionT1600:pvr01:rvnDellInc.:rn06NWYK:rvrA00:cvnDellInc.:ct6:cvr:
  dmi.product.name: Precision T1600
  dmi.product.version: 01
  dmi.sys.vendor: Dell Inc.

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