Erlang-China

关于SMP Erlang的一些事实

• Author: jackyz
• September 17, 2008
• Category: misc

ICFP2008 召开在即，好东西纷纷出笼。下面这一篇是 Erlang/OTP Team 的 Kenneth 为 ICFP2008 准备的 SMP Erlang 相关 Topic 的主要内容。透过这样一篇文档，我们能够了解到 SMP Erlang 的一些内情、策略以及展望，其中对于 SMP Erlang 的不足以及潜在的性能陷阱，也提供了相当富有价值的绕路指引。

Here are some short facts about how the Erlang SMP implementation works and how it relates to performance and scalability.

There will be a more detailed description of how multi-core works and on the future plans available in a couple of weeks. I plan to include some of this in my presentation at the ICFP2008, Erlang Workshop in Victoria BC, September 27

The Erlang VM without SMP support has 1 scheduler which runs in the main process thread. The scheduler picks runnable Erlang processes and IO-jobs from the run-queue and there is no need to lock data structures since
there is only one thread accessing them.

The Erlang VM with SMP support can have 1 to many schedulers which are run in 1 thread each. The schedulers pick runnable Erlang processes and IO-jobs from one common run-queue. In the SMP VM all shared data structures are protected with locks, the run-queue is one example of a data structure protected with locks.

From OTP R12B the SMP version of the VM is automatically started as default if the OS reports more than 1 CPU (or Core) and with the same number of schedulers as CPU’s or Cores.

You can see what was chosen at the first line of printout from the “erl” command. E.g.

Erlang (BEAM) emulator version 5.6.4 [source] [smp:4] [asynch-threads:0] …..

The “[smp:4]” above tells that the SMP VM is run and with 4 schedulers.

The default behaviour can be overridden with the “-smp [enable|disable|auto]” auto is default and to set the number of schedulers, if smp is set to enable or auto “+S Number” where Number is the number of schedulers (1..1024)

Note ! that it is normally nothing to gain from running with more schedulers than the number of CPU’s or Cores.

Note2 ! On some operating systems the number of CPU’s or Cores to be used by a process can be restricted with commands. For example on Linux the command “taskset” can be used for this. The Erlang VM will currently only detect number of available CPU’s or Cores and will not take the mask set by “taskset” into account. Because of this it can happen and has happened that e.g. only 2 Cores are used even if the Erlang VM runs with 4 schedulers. It is the OS that limits this because it take the mask from “taskset” into account.

The schedulers in the Erlang VM are run on one OS-thread each and it is the OS that decides if the threads are
executed on different Cores. Normally the OS will do this just fine and will also keep the thread on the same Core throughout the execution.

The Erlang processes will be run by different schedulers because they are picked from a common run-queue by
the first scheduler that becomes available.

Performance and scalability
————————————

- The SMP VM with only one scheduler is slightly slower than the non SMP VM. The SMP VM need to to use all the locks inside but as long as there are no lock-conflicts the overhead caused by locking is not significant (it is the lock conflicts that takes time). This explains why it in some cases can be more efficient to run several SMP VM’s
with one scheduler each instead on one SMP VM with several schedulers. Of course the running of several VM’s require that the application can run in many parallel tasks which has no or very little communication with each other.

- If a program scale well with the SMP VM over many cores depends very much on the characteristics of the program, some programs scale linearly up to 8 and even 16 cores while other programs barely scale at all even on 2 cores. This might sound bad, but in practice many real programs scale well on the number of cores that are common on the market today, see below.

- Real telecoms products supporting a massive number if simultaneously ongoing “calls” represented as one or several Erlang processes per core have shown very good scalability on dual and quad core processors. Note, that these products was written in the normal Erlang style long before the SMP VM and multi core processors where available and they could benefit from the Erlang SMP VM without changes and even without need to recompile the code.

SMP performance is continually improved
——————————————————

The SMP implementation is continually improved in order to get better performance and scalability. In each service release R12B-1, 2, 3, 4, 5 , …, R13B etc. you will find new optimizations.

Some known bottlenecks
———————————

- The single common run-queue will become a dominant bottleneck when the number of CPU’s or Cores increase.
Will be visible from 4 cores and upwards, but 4 cores will probably still give ok performance for many applications. We are working on a solution with one run-queue per scheduler as the most important improvement right now.

- Ets tables involves locking. Before R12B-4 there was 2 locks involved in every access to an ets-table, but in R12B-4 the locking of the meta-table is optimized to reduce the conflicts significantly (as mentioned earlier it is the conflicts that are expensive). If many Erlang processes access the same table there will be a lot of lock conflicts causing bad performance especially if these processes spend a majority of their work accessing ets-tables. The locking is on table-level not on record level. Note! that this will have impact on Mnesia as well since Mnesia is a
heavy user of ets-tables.

…

Our strategy with SMP
—————————–

Already from the beginning when we started implementation of the SMP VM we decided on the strategy: “First make it work, then measure, then optimize”. We are still following this strategy consistently since the first stable working SMP VM that we released in May 2006 (R11B).

There are more known things to improve and we address them one by one taking the one we think gives most performance per implementation effort first and so on.

We are putting most focus on getting consistent better scaling on many cores (more than 4).

Best in class
—————–

Even if there are a number of known bottlenecks the SMP system already has good overall performance and scalability and I believe we are best in class when it comes to letting the programmer utilize multi-core machines
in an easy productive way.

by.Kenneth Erlang/OTP team, Ericsson

呼唤达人同学出来贡献此文的翻译版本。

update: 出去玩了几天回来，不得不惊呼“Erlang社区的达人真的很多啊！”。竟然有两份中文译版(都翻得不错哦，下下来研究下吧)：

luoyi同学的：http://www.luoyilinux.cn/erlang_smp_zh.pdf
ShiningRay同学的：http://shiningray.cn/some-facts-about-erlang-and-smp.html

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