[erlang-questions] VM using only 2 out of 8 schedulers available

Knut Nesheim knutin@REDACTED
Thu Feb 16 10:19:12 CET 2012

Thanks for a quick reply.

During the GMT night which is our low point, we see quite low traffic
on each machine with maybe 5% cpu utilization of each of the 8 cores.
This could explain why some schedulers are put as idle. When traffic
picks up, we're using around 25% of each core.

The problem for us is that when traffic picks up again, the VM did not
start using the idle schedulers again. erlang:statistics(run_queue)
reports that there almost always are processes waiting, with peaks of
around 20 processes in our case. This causes much higher latency than
when using all cores.

We will try pushing more load to this node to see if it starts using
the idle schedulers.


On Thu, Feb 16, 2012 at 8:01 AM, Kenneth Lundin
<kenneth.lundin@REDACTED> wrote:
> What is your real problem except that there is no reported cpu load on some
> of the cores?
> When there is low load on the system i.e. when there are too few runnable
> Erlang processes some of the schedulers will automatically be put as idle
> letting the remaining schedulers run all processes. When the load increases
> the idle schedulers will be taken into service again.
> Each scheduler normally runs on one processor core.
> Can this explain what you have observed?
> /Kenneth Erlang/OTP Ericsson
> Den 15 feb 2012 17:53 skrev "Knut Nesheim" <knutin@REDACTED>:
>> Dear list,
>> From time to time we see the VM using only some of the available
>> cores. Typically 2 or 4 out of 8 available cores will be busy, while
>> the others are not used at all. From 'top' and 'htop' we see only some
>> cores being used.
>> We are running the halfword emulator on 8 core machines and the VM is
>> correctly reporting the number of schedulers and the underlying
>> topology. The nodes are utilizing roughly 25% of each of the 8 cores
>> during normal operation.
>> This problem appeared again today and we still have the process
>> running if anyone has any ideas for how to debug the issue.
>> We are on Ubuntu 10.04 LTS, 2.6.32-342-ec2, 64bit, R14B04 with the
>> halfword emulator. The only NIF library we are using is jiffy.
>> Regards
>> Knut
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