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

[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 <
wallentin.dahlberg@REDACTED> 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 <bob@REDACTED>
>>
>> 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
>> familiar with the BEAM implementation yet.
>>
>> I expect that the erts_export_consolidate, insert_new_code and
>> final_touch are really the only things that need so much
>> serialization, and maybe the set_default_trace_pattern… is there
>> anything big that I'm missing? It seems that breaking up
>> erts_load_module into two functions (one to do all the decoding
>> without the erts_smp_block_system(0), and the other to do the
>> integration work with the block) would be straightforward.
>>
>> -bob
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>
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