[erlang-questions] Non-blocking BEAM code loading?

Bob Ippolito <>
Sun Nov 6 05:02:48 CET 2011


Normally just a few hundred, purge isn't the slow part for us and I don't
believe that it blocks at all (not that I noticed).

On Saturday, November 5, 2011, Robert Virding <
> wrote:
> If you have many processes then code loading can take a noticeable time.
The code server must purge old versions of a module which it does by going
through all processes checking each one if it running the old code and if
so killing it. I don't know if this blocks all the schedulers and if so
why, but it can take a noticeable time to do.
>
> Robert
>
>
> ________________________________
>
> ETS is no good for our use case, we have ~60MB worth of uncompressed
serialized terms (nested gb_trees mostly) that we need live in a given
request. We traverse it very quickly and end up with a very small list of
terms as the result (essentially a filter on a nested structure). A no-copy
ets would work, but since the work is so short lived and code is tightly
associated to this structure I think that our current solution is
appropriate as long as we can fix the blocking.
>
> "declare constant" may also work, but I think it is more practical to
just make code loading better in the short term (which has other benefits).
You could implement "declare constant" on top of the code loader, we have a
mochiglobal module in mochiweb that basically serves that purpose.
>
> Using a module is a convenient way to give concurrent access to the data
to hundreds of simultaneous processes with minimal serialization.
>
> -bob
>
> On Saturday, November 5, 2011, Björn-Egil Dahlberg <
> wrote:
>> Yes, it is a simple (and currently only way) to push data to the
constant pool. You could use ETS instead. It would of course also remove
data from the heap and reduce GC copy strain but introduce copy on any read.
>> Björn Gustavsson talked about introducing a "declare constant" function
earlier but i don't know if he has done any work on it. The use case was
the same as for you, pushing lookup structures from gb_trees and
gb_sets. But, solving code loading would probably be a better
prioritization.
>> I would like to think that the garbage collector should solve this. Data
sets which are read only and live are tenured to a generational heap and
not included in minor gc phases. Putting it in a constant removes it all
together of course but i would like the garbage collector to identify and
handle this with generational strategies. The trade off is generational
heaps linger and may hold dead data longer than necessary.
>>
>>
>> Den 5 november 2011 21:30 skrev Bob Ippolito <>:
>>>
>>> We abuse code loading "upgrades" so that we can share memory and reduce
GC pressure for large data structures that do not change quickly (once
every few minutes). Works great except for all the blocking!
>>>
>>> On Saturday, November 5, 2011, Björn-Egil Dahlberg <
> wrote:
>>> > There is no other locking for code loading than blocking. This is an
optimization of course since locking mechanism overhead is removed from the
equation. Code loading is not used all that often in the normal cases
besides startups and upgrades.
>>> > That being said, there are plans to remove this "stop-the-world"
strategy since it is blocking other strategies and optimizations. Also, we
are well aware of that blocking does degrade performance when loading new
modules and does not agree with our concurrency policy.
>>> > I think we can lessen the time blocked in the current implementation
but the blocking strategy should (and probably will) be removed. Nothing
planned as of yet though.
>>> > Regards,
>>> > Björn-Egil
>>> >
>>> > 2011/11/5 Bob Ippolito <>
>>> >>
>>> >> We've found a bottleneck in some of our systems, when we load in
large
>>> >> new modules there is a noticeable pause (1+ seconds) that blocks all
>>> >> of the schedulers. It looks like this is because the
>>> >> erlang:load_binary/2 BIF blocks SMP before it does anything at all.
>>> >>
>>> >> It would be a big win for us if more of this happened without
blocking
>>> >> the VM, there's a lot of busy work in loading a module that shouldn't
>>> >> need any locking. For example, decompressing and decoding the literal
>>> >> table is probably where our code spends almost all of its time.
>>> >>
>>> >> There aren't a lot of comments for why it needs to lock the VM,
>>> >> especially for the whole of load_binary. Are there any hidden gotchas
>>> >> in here that I should know about before giving it a try? I'm unable
to
>>> >> find much where the block is actually necessary, but I am not very
>
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