[erlang-questions] SMP for IO-bound applications

[Erisa Dervishi]

One additonal comment I forgot to mention:

For all Mnesia tests, the CPU usage is low in general. Only for the
transactional read test in R15 with SMP enabled i get a CPU usage of 70%.
>From the load generator tool, i call the read(N) function that is showed
below:

exec_read(0) -> done_reading;

exec_read(N)  ->

        Id=random:uniform(7000000),

        %%io:format("~p~n",[Id]),

        %%[_Row] = mnesia:dirty_read({subscriber,Id}),

        Fun = fun() ->

           mnesia:read({subscriber,Id})

        end,

        {atomic,[Row]}=mnesia:transaction(Fun),

        %io:format("~p~n",[Row]),

        exec_read(N-1).

read(N) ->

        {A1,A2,A3} = now(),

        random:seed(A1, A2, A3),

        exec_read(N).


I am using mpstat while my tests are running and beside the CPU usage,
another difference of this test from the other Mnesia tests is that the
syscl (system calls) column of mpstat has a value which is 20 times higher
than the other test cases.



On Thu, Jun 21, 2012 at 8:37 PM, Erisa Dervishi <erisa85d@REDACTED> wrote:

> Hi,
> As part of my studies, I have recently been doing some performance
> evaluations on Erlang SMP improvements  for IO-bound applications.
>
> The applications I considered for the evaluation were Emysql driver (
> https://github.com/Eonblast/Emysql) and Mnesia.
> I have created an Erlang module for performing reads/writes from/to Mysql
> DB through Emysql driver, and another Erlang module for communicating with
> Mnesia.
> Both the modules have two methods write(nr_records) which writes
> nr_records records to the database (MySQL, or Mnesia), and read(nr_records)
> which reads nr_records random records from the db. (both the reads and
> writes are consecutive, not in chunks)
> I have a load generator tool which simulates simultaneous requests to both
> the modules and gathers performance statistics. I generate as much load as
> needed to stress the applications (near the saturation point)
> The metrics I uses are the throughput (no. of sessions/sec), and the
> session duration. A session is a call to write(nr_records)
> or read(nr_records) function depending on the test case (read or write). So
> if I call write(500), I measure the duration of a session that does 500
> hundred inserts into the database.
>
> The tests I run have these parameters:
>
> A)Type of operations: 1- Reads 2-Writes
> I have just one table with 20 fields, and the reads and writes are just
> select and insert operations in that table
>
> B) I/O applications: 1- Emysql 2-Mnesia
>
> C) SMP parameters:
> 1- SMP enabled, no. of schedulers = no. of cpu cores
> 2-SMP  enabled, no. of schedulers = 1
> 3- SMP disabled
>
> D) Erlang OTP versions:
> 1- Erlang R12B (The SMP has only one run-queue and multiple schedulers)
> 2- Erlang R15B (Improved SMP, 1 run-queue per each scheduler)
>
> The tests included all possible combinations from A,B,C,D. They were run
> in Solaris 10 x86 (a 16 cores machine).
> In general these were the results I got:
>
> *Emysql driver:*
> 1- There is a a big difference in performance between SMP enabled and
> disabled in both the Erlang versions (R12 and R15)  for both read and write
> tests.  So I can say that SMP rocks! However, you have to have enough load
> to achieve that (for low load I could not see any difference, sometimes it
> was even better to disable SMP)
>
> 2- I was expecting a much better performance for SMP enabled (no. of cores
> = no. of schedulers) in R15 than in R12, since the schedulers' logic has
> changed a lot from R12 to R15. But the results were more or less the same.
>  I was thinking since I had a multithreaded db like MySql, a multiple
> db-connections driver as Emysql, and SMP with multiple schedulers with
> their own run queues, I could get better results than in R12 where there is
> only one run-queue and multiple schedulers (more lock contention).
> Is it maybe because the processes are just doing IO and nothing
> CPU-intensive?
>
> 3- I realized that CPU usage in R15 SMP enabled is twice higher than the
> CPU use in R12 SMP enable. All the cores have a CPU usage over 90%, and I
> can see that the beam process is using up to 75-80% of the CPU, the rest is
> the mysql daemon process. Is it because in R15 the scheduling algorithm has
> become more complex?
>
> *Mnesia*
> *
> *
> I have only one table (as I said before) created with the attribute
> disc_copies, and it is stored only in one node. The whole table fits in
> RAM, and I tried to keep it simple by controlling the size of the table
> during my tests, in order not exceed RAM capacity
>
> 4-  For the write tests in Mnesia I see the same behavior as in Mysql.
> However, since the data are all loaded in RAM, the session duration is
> shorter, and the gain in performance between SMP enabled and disabled is
> not as huge as in Mysql.
>
> 5- For the read tests, I have two observations. First, the dirty reads are
> 10 times faster than the reads that use transactions. Second, I have better
> results for SMP disabled (twice faster, though in both cases the session
> duration is in order of millisecods, 70 msec vs 30 msec).
>
> 6- Same as in Mysql, no difference in performance between R12 and R15 with
> SMP enabled, for both read and write tests.
>
> If you have read so far, and have a comment on my results, you are
> welcome. Especially about the no difference btw R12 and R15 and the high
> CPU usage in R15 when the applications are IO-bound.
>
> Thanks,
> Erisa
>
>
>
>
>
>
>
>
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