[GE users] Cannot make virtual_free consumable?

Piotr Domagalski piotr.domagalski at gmail.com
Thu Nov 20 20:37:19 GMT 2008


On Thu, Nov 20, 2008 at 8:19 PM, reuti <reuti at staff.uni-marburg.de> wrote:
>> 1. When a job has -l vf=3G it is not run -- the value from load sensor
>> (2G) is used.
>
> Correct.
>
>> 2. The system happily allows me to run 4 jobs each having -l vf=1G
>
> Well, not really. If your job would really consume 1GB of memory,
> then the load sensor would reflect this. Hence also only two would be
> possible. For now you used just a "sleep 60" or alike in the jobscript?

Yes, I used sleep. But even if I used jobs that really allocate 1GB
but do it slowly, I could submit 4 of them and consequently
over-subscribe the memory. That's what worries me.

This means that the load sensor value if lower than the one kept by
SGE is only used to verify whether the job requirements can be met.
The debiting itself always happens to value kept by SGE.

>> 3. The Advance Reservation doesn't consider the value from load
>> sensors at all (i.e. I can have 4 ARs on this host each requiring 1G).
>
> This is the same as above in 2. ?!?
>
> It can't know, what the value of the load sensor will be in the
> future.

I forgot to add that the case with AR is rather understandable to me
;-) I just wanted to be sure that the AR actually always ignores the
values from load sensors as this is what I've observed...

Unfortunately this may result in AR successfully being created but the
job not being able to run within this AR even when it requests the
same amount of memory as was reserved. But of course, taking into
account the load sensor values would not change much.

You say that one should set mem_free (or virtual_free, but let's use
mem_free in this example) to the amount of installed RAM. But isn't it
safer to set it rather lower than e.g. the mem_free reported by load
sensor "normally" at the machine? If I set it to 4GB on a 4GB machine
it means that I can create an AR for 4GB but then my jobs will not be
run if they require 4GB within this AR because the load sensor will
always report a lower value.

-- 
Piotr Domagalski

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