RE: OT - I/O and CPU load
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> running. However, our sysadmin insists that cpu-intensive jobs
> will have no effect on I/O intensive jobs. Is this generally true?
>
First of all, let me tell you I am no expert :)
Its strange that a machine designated as fileserver is running user
jobs at all, be they cpu- or io-intensive. A fileserver is supposed
to churn out files - fast, and jobs that compete with cpu, memory, or
bandwith resources don't belong there. (our fileservers are strictly
nologin, nobatch).
There is _some_ truth in what you sysadmin says though, in some
cases. If the fileserver has plenty of memory, and all jobs fit in
memory snugly, cpu-intensive processes should not slow down
io-intensive jobs significantly. Except if there are a lot more jobs
than CPU's: jobs all get a turn on the CPU but processes that often
have to wait for IO might miss out sometimes (not too bad though).
If memory is short, then there will be the overhead of swapping.
IO-job runs now, has to wait, part of it might be swapped to disk
while CPU job takes over for a while, then stuff needs to be reloaded
from disk if IO-job gets a turn (and maybe CPU-job gets swapped to
disk).
A heavily swapping system might slow down significantly - hence your
job will run slower.
People with long cpu-intensive jobs might also give their job a
loweer priority (nice it, or put it in a batch queue), which will
give io-restricted jobs a slightly better performance.
Anyway, as I said I am not an expert. But this is my understanding
how things work.
(if you really have to use the fileserver as a batchserver or
job-running machine, it might be better to set up proper batchqueues
which limit the number of jobs being run simultaneously - and
ofcourse give it low priority)
With kind regards,
- --
drs. Jean-Jack M. Riethoven
EMBL Outstation - Hinxton pow@ebi.ac.uk ICQ#: 3433929
European Bioinformatics Institute Phone: (+44) 1223 494635
Wellcome Trust Genome Campus Fax : (+44) 1223 494468
Hinxton, Cambridge CB10 1SD URL : http://industry.ebi.ac.uk/
UNITED KINGDOM
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