lcnt – The Lock Profiler
Internally in the Erlang runtime system locks are used to protect resources from being updated from multiple threads in a fatal way. Locks are necessary to ensure that the runtime system works properly but it also introduces a couple of limitations. Lock contention and locking overhead.
With lock contention we mean when one thread locks a resource and another thread, or threads, tries to acquire the same resource at the same time. The lock will deny the other thread access to the resource and the thread will be blocked from continuing its execution. The second thread has to wait until the first thread has completed its access to the resource and unlocked it. The lcnt tool measures these lock conflicts.
Locks has an inherent cost in execution time and memory space. It takes time initialize, destroy, aquiring or releasing locks. To decrease lock contention it some times necessary to use finer grained locking strategies. This will usually also increase the locking overhead and hence there is a tradeoff between lock contention and overhead. In general, lock contention increases with the number of threads running concurrently. The lcnt tool does not measure locking overhead.
5.1 Enabling lock-counting
For investigation of locks in the emulator we use an internal tool called lcnt (short for lock-count). The VM needs to be compiled with this option enabled. To enable this, use:
cd $ERL_TOP ./configure --enable-lock-counter
Another way to enable this alongside a normal VM is to compile it at emulator directory level, much like a debug build. To compile it this way do the following,
cd $ERL_TOP/erts/emulator make lcnt FLAVOR=smp
and then starting Erlang with,
To verify that you lock-counting enabled check that [lock-counting] appears in the status text when the VM is started.
Erlang R13B03 (erts-5.7.4) [64-bit] [smp:8:8] [rq:8] [async-threads:0] [hipe]
5.2 Getting started
Once you have a lock counting enabled VM the module lcnt can be used. The module is intended to be used from the current running nodes shell. To access remote nodes use lcnt:clear(Node) and lcnt:collect(Node).
All locks are continuously monitored and its statistics updated. Use lcnt:clear/0 to initially clear all counters before running any specific tests. This command will also reset the duration timer internally.
To retrieve lock statistics information use, lcnt:collect/0,1. The collect operation will start a lcnt server if it not already started. All collected data will be built into an erlang term and uploaded to the server and a duration time will also be uploaded. This duration is the time between lcnt:clear/0,1 and lcnt:collect/0,1.
Once the data is collected to the server it can be filtered, sorted and printed in many different ways.
See the reference manual for a description of each function.
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