系统技术非业余研究

高性能网络服务器通常会涉及大量和时间相关的场景和操作，比如定时器，读取事件的发生时间，日志等等。

erlang提供了二种方式来获取时间：
1. erlang:now()
2. os:timestamp()
获取取到时间后，我们通常用calendar:now_to_universal_time来格式化类似”{{2013,11,4},{8,46,20}}”这样人可读的时间。

由于时间调用非常的频繁，而且通常发生在关键路径上，所以效率和性能就非常值得深挖了。

我们先来看下这二个函数的说明：
erlang:now 参看这里

now() -> Timestamp

Types:

Timestamp = timestamp()
timestamp() =
{MegaSecs :: integer() >= 0,
Secs :: integer() >= 0,
MicroSecs :: integer() >= 0}
Returns the tuple {MegaSecs, Secs, MicroSecs} which is the elapsed time since 00:00 GMT, January 1, 1970 (zero hour) on the assumption that the underlying OS supports this. Otherwise, some other point in time is chosen. It is also guaranteed that subsequent calls to this BIF returns continuously increasing values. Hence, the return value from now() can be used to generate unique time-stamps, and if it is called in a tight loop on a fast machine the time of the node can become skewed.

It can only be used to check the local time of day if the time-zone info of the underlying operating system is properly configured.

If you do not need the return value to be unique and monotonically increasing, use os:timestamp/0 instead to avoid some overhead.

os:timestamp 参看这里

timestamp() -> Timestamp

Types:

Timestamp = erlang:timestamp()
Timestamp = {MegaSecs, Secs, MicroSecs}
Returns a tuple in the same format as erlang:now/0. The difference is that this function returns what the operating system thinks (a.k.a. the wall clock time) without any attempts at time correction. The result of two different calls to this function is not guaranteed to be different.

The most obvious use for this function is logging. The tuple can be used together with the function calendar:now_to_universal_time/1 or calendar:now_to_local_time/1 to get calendar time. Using the calendar time together with the MicroSecs part of the return tuple from this function allows you to log timestamps in high resolution and consistent with the time in the rest of the operating system.

但是事情没这么简单!

由于erlang支持时间纠正机制，简单的说在时间发生突变的时候，还能维持正常的时间逻辑，具体的实现参看这篇：服务器时间校正思考。

时间纠正机制让事情变得复杂，这个时间纠正机制如何禁止呢：

+c
Disable compensation for sudden changes of system time.

Normally, erlang:now/0 will not immediately reflect sudden changes in the system time, in order to keep timers (including receive-after) working. Instead, the time maintained by erlang:now/0 is slowly adjusted towards the new system time. (Slowly means in one percent adjustments; if the time is off by one minute, the time will be adjusted in 100 minutes.)

When the +c option is given, this slow adjustment will not take place. Instead erlang:now/0 will always reflect the current system time. Note that timers are based on erlang:now/0. If the system time jumps, timers then time out at the wrong time.

正是由于时间纠正机制的存在，所以服务器需要不时的修正时间，同一时刻可能还有很多线程在读取时间，为了维护时间的一致性，需要有个锁来保护。
我们来看下相关的代码实现：
Read more…

抄书:otp_doc_html_R13B04/lib/tools-2.6.5.1/doc/html /lcnt_chapter.html#id2252207

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,

$ERL_TOP/bin/cerl -lcnt

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]
[kernel-poll:false] [lock-counting]

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.

在调试版本下 这个工具可以直观的看出你的Erlang程序所用到的锁,对于诊断问题和提高性能非常有帮助.

OTP-8424 Add lock profiling tool.

The Lock profiling tool, lcnt, can make use of the internal lock
statistics when the runtime system is built with this feature
enabled.

This provides a mechanism to examine potential lock bottlenecks
within the runtime itself.

– Add erts_debug:lock_counters({copy_save, bool()}). This option
enables or disables statistics saving for destroyed processes and
ets-tables. Enabling this might consume a lot of memory.

– Add id-numbering for lock classes which is otherwise undefined.