[erlang-questions] About Erlang SMP scheduler

[Rickard Green]

The work stealing is there to quickly distribute work between
schedulers, but it doesn't balance the load. check_balance() sets up
migration paths and migration limits that are used in order to balance
the load between schedulers. Without balancing, processes with the
same assigned priority can get very different effective priorities.

Example: With 4 schedulers and 400 cpu bound processes executing the
same code, each scheduler will eventually end up with 100 processes to
manage. That is, all processes will have the same access to the cpu.
If you disable check_balance(), you may end up with three schedulers
only having one process each, and one scheduler with 397 processes.
These 397 processes will effectively have a much lower priority than
the other three processes.

Regards,
Rickard Green, Erlang/OTP, Ericsson AB

2012/5/2 Siyao Zheng(郑思遥) <zhengsyao@REDACTED>:
> Hi,
>
> Yes, that's true. But the "try_steal_task()" afterwards is also controlled
> by ERTS_SMP macro. It doesn't make any sense to talk about workload balance
> outside multi-processor environment. I'm just wondering why "check_balance"
> is needed while there exists work stealing(try_steal_task).
>
> Cheers
>
> On May 2, 2012, at 12:04 PM, xu yu wrote:
>
> Hi,
>
> "check_balance" controlled by "ifdef ERTS_SMP", so...
>
>
> 2012/4/27 "Siyao Zheng(郑思遥)" <zhengsyao@REDACTED>
>>
>> Hi,
>>
>> schedule(), in which check_balance() is only called.
>>
>> In schedule(), when check_balance_reds reaches zero, scheduler decides to
>> "check_balance". But later in schedule(), when scheduler finds run queue
>> being empty, it will try to steal task from other schedulers (by calling
>> try_steal_task()), which is also a load balance mechanism. I'm just
>> wondering what the purpose of check_balance().
>>
>> Cheers
>> Siyao
>>
>> On Apr 27, 2012, at 9:40 PM, Zabrane Mickael wrote:
>>
>> > Hi Siyao,
>> >
>> > Which "check_balance" you've commented out?
>> >
>> > Here is what I found so far:
>> >
>> > $ find /otp_src_R15B01  -type f | xargs grep check_balance | grep -v
>> > matches
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:    int
>> > forced_check_balance;
>> >
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:check_balance(ErtsRunQueue
>> > *c_rq)
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:#  error
>> > check_balance() assumes ERTS_MAX_PROCESS < (1 << 27)
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > c_rq->check_balance_reds = INT_MAX;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > c_rq->check_balance_reds = INT_MAX;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > rq->check_balance_reds = ERTS_RUNQ_CALL_CHECK_BALANCE_REDS;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:     *
>> > check_balance() is never called in more threads
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:    forced =
>> > balance_info.forced_check_balance;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> >  balance_info.forced_check_balance = 0;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > c_rq->check_balance_reds = INT_MAX;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > rq->check_balance_reds = INT_MAX;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > rq->check_balance_reds = ERTS_RUNQ_CALL_CHECK_BALANCE_REDS;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > rq->check_balance_reds = ERTS_RUNQ_CALL_CHECK_BALANCE_REDS;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> >  balance_info.forced_check_balance = 0;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> >  (RQ)->check_balance_reds = ERTS_RUNQ_CALL_CHECK_BALANCE_REDS;        \
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > balance_info.forced_check_balance = 1;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > ERTS_RUNQ_IX(0)->check_balance_reds = 0;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c: if
>> > (rq->check_balance_reds <= 0)
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.c:
>> > check_balance(rq);
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.h:    int
>> > check_balance_reds;
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.h:
>> >  (RQ)->check_balance_reds -= (REDS);                          \
>> > /opt/otp_src_R15B01/erts/emulator/beam/erl_process.h:
>> >  (RQ)->check_balance_reds -= (REDS);
>> > [...]
>> >
>> > For OTP Team: is this dangerous ?
>> >
>> > Regards,
>> > Zabrane
>> >
>> >
>> > On Apr 27, 2012, at 1:23 PM, Siyao Zheng(郑思遥) wrote:
>> >
>> >> Hi,
>> >>
>> >> In SMP version of Erlang, every scheduler periodically call
>> >> "check_balance()" to check load balance among all schedulers, then it might
>> >> migrate some processes to balance load, or shut down some schedulers with
>> >> low load. Does anyone know why scheduler should do this? check_balance() is
>> >> quite a big one, and it has to lock every run queue when it checks the run
>> >> queue. I think it's quite a big cost here. The work stealing at each
>> >> schedule step afterwards actually does work load balance very well. Actually
>> >> after I comment out the check_balance() step and run BigBang and Hackbench
>> >> benchmarks, they are really faster!
>> >>
>> >> So, I wonder what is the purpose of check_balance() step here, if it is
>> >> related to scalability on many core processors or something, and if there is
>> >> any other benchmark I can run to prove its usefulness.
>> >>
>> >> Cheers!
>> >>
>> >> Siyao
>> >> _______________________________________________
>> >> erlang-questions mailing list
>> >> erlang-questions@REDACTED
>> >> http://erlang.org/mailman/listinfo/erlang-questions
>> >
>> >
>>
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