rpi2 hangup during poudriere build: lots of pfault wmseg status

Laurent Cimon laurent at nuxi.ca
Thu Dec 7 01:47:16 UTC 2017

> On Dec 6, 2017, at 20:01, Mark Millard <markmi at dsl-only.net> wrote:
> On 2017-Dec-6, at 1:54 PM, Laurent Cimon <laurent at nuxi.ca> wrote:
>>> On Dec 6, 2017, at 00:57, Mark Millard <markmi at dsl-only.net> wrote:
>>> I tried to build some ports on a rpi2
>>> (via poudriere) but it hung up:
>>> Ethernet and normal console use. (Note:
>>> the root file system is on a USB SSD
>>> and the swap partition is also on that
>>> USB SSD.)
>>> But ~^b worked for getting to the db>
>>> prompt on the console.
>>> From there a ps suggests that it got hung
>>> up in pfault activity. (Possibly insufficient
>>> RAM+swap-partition space?) But it is not
>>> clear to me that it should end up hung up
>>> vs. killing processes or other such.
>> Hi,
>> From what I know the raspberry pis use the same controller for ethernet and
>> the USB hub on which you’re hosting an SSD. It seems like you make very heavy
>> use of the USB ports, and all of the resources used by poudriere except for the
>> CPU and the (very limited) memory that’s not in swap is attached to them. If you
>> really didn’t have enough memory and swap, the linkers would’ve been stopped.
>> I think it might just be a swap death. Poudriere compiles and fetches in parallel
>> a lot, ethernet and disk I/O is slow because it’s very limited, so linking takes
>> longer. You end up linking a few very big binaries at the same time, and they
>> all fight for the memory, to get out of swap through page faults, but there
>> are too many page faults, all too big, requesting for more CPU time that’s
>> allowed to them.
>> This would explain why you have 3 linkers waiting on a page fault out of the 4
>> CPUs poudriere allows builds on, on top of the awk processes. It would also
>> explain why you had easy access to the debugger: it was in memory already with
>> the kernel.
>> I’d advise you to disable parallel builds and see if it happens again,
>> but it would make building much slower. Using makejobs would help if you
>> can afford watching the build. Otherwise be patient, it should resolve itself
>> eventually, but it will take a while and it will happen again.
> My post was more about how FreeBSD handled the
> heavy-use context and less about getting the
> builds to finish: it managed to to get to a
> state of no-progress for processes and a loss
> of normal control as far as I could tell.
> I did a "c" to ddb and left it until just before
> this note then did ~ ^B again. Things looked the
> same. [I've finally rebooted the rpi2.]
> PARALLEL_JOBS=1 was already in use but
> ALLOW_MAKE_JOBS=yes was also in use.
> USE_TMPFS=no was already in use.
> While an ssh session was monitoring the
> build, Ethernet was not in heavy use.
> (No nfs mounts to its disks, for example.)
> I may try without ALLOW_MAKE_JOBS=yes and
> with ALLOW_MAKE_JOBS_PACKAGES empty/undefined
> to see if it can complete for such a context
> without having the same sort of problem.
> Ultimately I can cross-build and install from
> those materials when I really want updates. I
> have the context for such. This was more about
> seeing how well the rpi2 did for self-hosted.
> Classically I've used a BPI-M3 with 2 GiBytes
> of RAM and a proportionally bigger swap partition
> instead (approximately).
> FYI (rpi2 after rebooting):
> # swapinfo
> Device          1K-blocks     Used    Avail Capacity
> /dev/label/RPI2swap   1572860        0  1572860     0%
> # df -m
> Filesystem           1M-blocks  Used  Avail Capacity  Mounted on
> /dev/ufs/RPI2rootfs     195378 30791 148957    17%    /
> devfs                        0     0      0   100%    /dev
> /dev/label/RPI2Aboot        49    12     37    25%    /boot/msdos
> An rpi3 (aarch64) with the same amount of RAM,
> same type of USB SSD, etc., but well more swap
> completed building basically the same set of
> ports for the same poudriere settings just
> fine.
> Interestingly for the default kern.maxswzone:
> (Just to show the reported recommended maximum
> figures for swap.)
> rpi2: . . . exceeds maximum recommended amount (411488 pages).
> rpi3: . . . exceeds maximum recommended amount (925680 pages).
> (I was running with somewhat under those maximums for
> the tests.)
> # swapinfo
> Device          1K-blocks     Used    Avail Capacity
> /dev/gpt/RPI3swap   3702784        0  3702784     0%
> # df -m
> Filesystem           1M-blocks  Used  Avail Capacity  Mounted on
> /dev/ufs/RPI3rootfs     195378 14937 164811     8%    /
> devfs                        0     0      0   100%    /dev
> /dev/label/RPI3Aboot        49     7     42    15%    /boot/efi
> If I restricted the rpi3 to somewhat under what the
> rpi2 allows for swap, I do not know if it would also
> hang up vs. not.
> If having more swap makes the difference, then it
> would not seem to be being I/O-bound that would
> explain the hangup.
> ===
> Mark Millard
> markmi at dsl-only.net

There are a few factors that could have prevented this on your raspberry pi 3.

It has a faster, 64 bit CPU instead of the raspberry pi 2’s 32 bit CPU and the
RAM is twice as fast. These make it less likely for this to happen, because it
makes both building and linking faster, which reduces the odds of linking 2
binaries at once, let alone 3. There are many things that could have gone
differently in the build that didn’t make it end up linking 3 big binaries at
the same time to cause the same behaviour.

What I think happened on your raspberry pi 2 is just likely bad luck that could
also happen on your raspberry pi 3. The odds of 3 parallel builds needing so
much ram to link at the exact same time are still very low, just less low on
faster hardware.

Keep in mind that this is still entirely theoretical, I don’t present it as an
absolute explanation. It’s simply what I understand from this.

I’d be curious seeing how a different operating system using a system similar to
poudriere where builds are done on one CPU but in parallel would be handled on
the rpi2. My understanding is that this is simply a mix of hardware limitation
and conceptual flaws with the swap. And by flaws I mean, your operating system
cannot save you when you try to do something that your hardware cannot possibly


More information about the freebsd-current mailing list