[erlang-questions] SMP for IO-bound applications

Erisa Dervishi <>
Fri Jun 22 13:51:52 CEST 2012


Hey, thank you for your reply, but I use the same mysql driver even for my
evaluations in  Erlang OTP R12 and do not get the same CPU usage (for R12
is up to 45%). And the tests are exactly the same (same load). I have also
checked the message queue of the process that supervises the connections,
and it looks like there isn't a bottleneck in there (the queue length is
most of the time 0 and up to 4 messages at most)
And even for Mnesia the CPU usage (though low), doubles from R12 to R15
with smp enabled from 6% to 12% for write tests


On Fri, Jun 22, 2012 at 10:22 AM, 
<>wrote:

> I think that much of CPU usage is created by your MySQL driver.
> You should have a look at driver details. There may be copying of the
> query you send to MySQL between your benchmarking process and the real
> process that owns the socket. The same with result - parsing response and
> sending result back to benchmarking process. There may be a bottleneck in
> choosing MySQL connection from connection pool if it is made through
> supervisor.
> Try to use etop to see what functions are at the bottlenecks.
> I mean your io-intensive test may be just the test showing driver
> bottlenecks and not the problems in smp.
>
>
>
>
> ----- Reply message -----
> From: "Erisa Dervishi" <>
> To: <>
> Subject: [erlang-questions] SMP for IO-bound applications
> Date: Thu, Jun 21, 2012 23:28
>
>
> 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 <>
> 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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