Avoid hammerring the Code server #12
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Additional details and impacted files@@ Coverage Diff @@
## main #12 +/- ##
==========================================
- Coverage 86.05% 86.05% -0.01%
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Files 3 3
Lines 918 932 +14
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+ Hits 790 802 +12
- Misses 128 130 +2
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[Why] When we are about to execute a standalone function, we first need to load the module. This is a synchronous call to the Code server. So far, we checked that the module was already loaded using `code:is_loaded/1` before doing that to avoid too many useless calls to the Code server. Unfortunately, up to Erlang/OTP 25, `code:is_loaded/1` is a synchronous call too! It is changed to an ETS query in Erlang/OTP 26, but before that, it remains an expansive and contentious call. If the same standalone function is executed concurrently a lot of time simultaneously, we end up spamming the Code server with a boat load of "is loaded" queries. [What] To solve the problem, we introduce a `horus_utils:is_module_loaded/1` helper. On Erlang/OTP 26+, we call `code:is_loaded/1`. However on Erlang/OTP 25 and before, we replace that with a test of `erlang:get_module_info/2`, the function underneath any `Module:module_info/1` calls. This way, it's a relatively cheap way of checking that the module is loaded. At the same time, we acquire a lock around that module load code to make sure there is only a single attempt to load the module and concurrent processes will stop at "is loaded".
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Why
When we are about to execute a standalone function, we first need to load the module. This is a synchronous call to the Code server.
So far, we checked that the module was already loaded using
code:is_loaded/1before doing that to avoid too many useless calls to the Code server.Unfortunately, up to Erlang/OTP 25,
code:is_loaded/1is a synchronous call too! It is changed to an ETS query in Erlang/OTP 26, but before that, it remains an expansive and contentious call. If the same standalone function is executed concurrently a lot of time simultaneously, we end up spamming the Code server with a boat load of "is loaded" queries.What
To solve the problem, we introduce a
horus_utils:is_module_loaded/1helper. On Erlang/OTP 26+, we callcode:is_loaded/1. However on Erlang/OTP 25 and before, we replace that with a test oferlang:get_module_info/2, the function underneath anyModule:module_info/1calls.This way, it's a relatively cheap way of checking that the module is loaded.
At the same time, we acquire a lock around that module load code to make sure there is only a single attempt to load the module and concurrent processes will stop at "is loaded".