There has been some discussion on what versions of Erlang CouchDB should support, and what versions of Erlang are detrimental to use. Sadly there were some pretty substantial problems in the R15 line and even parts of R16 that are landmines for CouchDB. This post will describe the current state of things and make some potential recommendations on approach.
It was discovered by Basho that R15* and R16B are susceptible to scheduler collapse. There’s quite a bit of discussion and information in several threads     .
So what is scheduler collapse? Erlang schedulers can be put to sleep when there is not sufficient work to occupy all schedulers, which saves on CPU and power consumption. When the schedulers that are still running go through enough reductions to pass the work balancing threshold, they can trigger a rebalance of work that will wake up sleeping schedulers. The other mechanism for sharing scheduler load is work stealing. A scheduler that does not have any work to do can steal work from other schedulers. However a scheduler that has gone to sleep cannot steal work, it has to be woken up separately.
Now the real problem of scheduler collapse occurs when you take sleeping schedulers and long running NIFs and BIFs that do not report an appropriate amount of reductions. When you have NIFs and BIFs that don’t report an appropriate amount of reductions, you can get into a situation where a long running function call will only show up as taking one reduction, and never hit the work balance threshold, causing that scheduler to be blocked during the operation and no additional schedulers getting woken up.
I keep mentioning “NIFs and BIFs” because it’s important to note that
it is not just user defined NIFs that are problematic, but also a
number of Erlang BIFs that don’t properly report reductions.
Particularly relevant to CouchDB are the BIFs
binary_to_term which do not behave properly, and each report a
single reduction count, regardless of the size of the value passed to
them. Given that every write CouchDB makes goes through
term_to_binary, this is definitely not good.
This problem is systemic to all versions of R15 and R16B. In R16B01,
two changes were made to alleviate the problem. First, in
term_to_binary now uses an appropriate amount of reductions and will
yield back to the scheduler. The second important change was the
introduction of the
+sfwi (Scheduler Forced Wakeup Interval) flag
 which allows you to specify a time interval for a new watchdog
process to check scheduler run queues and wake up sleeping schedulers
if need be. These two changes help significantly, although from what I
understand, they do not fully eliminate scheduler collapse.
+sfwi is not enabled by default, you must specify a
greater than zero time interval to enable this. WE NEED TO ENABLE
THIS SETTING. We should figure out a way to conditionally add this
to vm.args or some such.
On a side note, Basho runs R15B01 because they backported the
feature to R15B01  . They recommend running with
+sfwi 500 for
a 500ms interval. It might be worth testing out different values, but
500 seems like a good starting point. For Riak 2.0, they will be
building against R16B03-1 and 17.0 as their set of patches to R16B02
landed in R16B03-1  .
R16B01 and the breaking of monitors
So R16B01 sorted out the scheduler collapse issues, but unfortunately
it also broke monitors, which immediately disqualifies this release as
something we should recommend to users. The issues was fixed in
OTP-11225 in R16B02.
R16B02 and R16B03*
I don’t know of any catastrophic problems on the order of those
described above in either of these releases. Basho fixed a number of
unrelated bugs in R16B02   that have since landed in R16B03-1,
which indicates we should probably prefer R16B03-1 over R16B02. R16B03
is also disqualified because it broke SSL and
in the patched R16B03-1.
R14B01, R14B03, and R14B04 are known good stable releases of Erlang,
and in my opinion the only known stable releases > R13 that don’t
present issues for CouchDB (I think R16B02/R16B03-1 are too new to
declare stable yet). As for R14B02, there are some bad
with that release.
It’s worth pointing out that there are two known bugs in R14B01, as Robert Newson explains:
There are two bugs in R14B01 that we do encounter, however. 1) Another
32/64 bit oops causes the vm to attempt to allocate huge amounts of
ram (terabytes, or more) if it ever tries to allocate more than 2gib
of ram at once. When this happens, the vm dies and is restarted. It’s
annoying, but infrequent. 2) Sometimes when closing a file, the
underlying file descriptor is *not* closed, though the erlang process
exits. This is rare but still quite annoying.
The 17.0 release brings in a number of interesting changes to help the
scheduler collapse situation.
OTP-11648 improves reduction cost and
term_to_binary. It also utilizes
allows for NIFs and BIFs to have more control over when and how they
are garbage collected (we should do some investigation on the
usefulness of this for NIFs like Jiffy).
The 17.0 release also updates
behave properly with reductions and yielding similar to
term_to_binary. This marks the 17.0 release as an important one for
CouchDB as now
binary_to_term both behave
One other interesting item introduced in the 17.0 release is the concept of dirty schedulers  . This is an experimental feature providing CPU and I/O schedulers specifically for NIFs that are known to take longer that 1ms to run. In general, we want to make sure the NIFs we use will yield and report reductions properly, but for situations where that isn’t feasible, we may want to look into using dirty schedulers down the road when it’s a non experimental feature.
Recommendations for CouchDB
In my opinion we need to take the Erlang release issues more seriously than we currently do and provide strong recommendations to users on what versions of Erlang we support. I suggest we loosely take an approach similar to Debian, and make three recommendations:
- OldStable: [R14B01, R14B03, R14B04 (NOTE: not R14B02)]
- Unstable: [R16B03-1 recommended, R16B02 acceptable]
- Experimental: [17.0]
I’m not suggesting permanently having three Erlang releases recommended like this, but it currently seems appropriate. I think long term we should target 17.x as our preferred Erlang release, and then make a CouchDB 3.0 release that is backwards incompatible with anything less than 17.0 so that we can switch over to using maps.
The narrowness of the acceptable releases list is going to cause some
problems. Debian Wheezy runs R15B01, which as established above, is
not good to run with unless you have the
+sfwi patch, and I’m sure
there are many other distros running R15 and R16B or R16B01. I think
it would be useful to users to have a set of packages with a proper
Erlang CouchDB release allowing us to bless specific versions of
Erlang and bundle it together, but I know this idea goes against the
recent change in stance on working with distributions, and I don’t
know the ASF stance on this issue well enough to comment on the
legality of it. That said, it does seem like the logical approach
until we get a range of stable releases spread out through the
Work to be done
We need to make sure that all NIFs we use that could potentially take longer than 1ms to run properly yield and report reductions. For Jiffy, there is already a good start on this work . We’ll want to look into what needs to be done for the rest of the NIFs.
There’s quite a bit of information here, and plenty more in the footnotes, so I hope this gives a good overview of the current state of Erlang releases and helps us to make informed decisions on what approach to take with Erlang releases.