Hi Greg,
thx for the explanations - now it is much clearer to me, especially why
it does not happen after a restart ( I guess the locking "situation" is
different then). We are using always only a local file system - nfs
should not be involved. Some of the machines which failed were virtual
machines, some of them nodes of a cluster. The screenshot I showed was
on our big server, which has 128GB ram, so the rise didn't cause a crash.
Next time I will monitor this more closely and will try stopping of the
child processes, as you suggested to get more insights. At the moment we
have finished all renderings (and they are great in Okulus ), but I'm
sure there will come more in some weeks.
thx for the help!
best
Jan
On 04/25/2018 07:14 PM, Gregory J. Ward wrote:
Hi Jan,
This could be an unexpected "hang" condition with one of the rtrace processes,
where a single ray evaluation is blocked waiting for access to the ambient file, while
the other processes continue computing away, filling up the queue with results after the
blocked one. I could see this becoming a runaway memory condition, but I don't know why
a process would be blocked. NFS file locking is used on the ambient file, and this has
been known to fail on some Linux builds, but I haven't seen it fail by refusing to
unlock. (The problem in the past has been unlocking when it shouldn't.)
If you can monitor your processes, watching for when the parent becomes large,
then stop all the child processes (kill -stop pid1 pid2 ...), restarting them
one by one using (kill -continue pidN). If the parent process starts to shrink
after that, or at least doesn't continue to grow, then this would support my
hypothesis.
The other thing to look for is a child process with 0% CPU time. If none of
the child processes are hung, then I'm not sure why memory would be growing in
the parent.
There's no sense trying to fix such an unusual problem until we have a firmer
idea of the cause.
Cheers,
-Greg
From: Jan Wienold <[email protected]>
Date: April 25, 2018 1:28:44 AM PDT
Hi Greg,
While doing the renderings for the VR of Kynthia we encountered "sometimes"
(that means not 100% reproducible) problems with the memory.
We rendered 4 images at the same time sharing an ambient file, each rtrace was
using the -n 2 or -n 3 option.
I made a screenshot of top and some of the processes. If you look at id 88263 it seems like the
"mother-process" uses in total 41 GB (virt) !! - since some of our machines don't have a
large swap space, some of these processes failed with "cannot allocate memory". I know
that the Virt mem is not a real indicator for what is ever used, but from our 400 jobs we had
around 10 failing with this issue.
The "children" use around 800-900mb, so this is fine and what we expected. But
we dont know how to estimate to total memory usage (lets say a single rtrace would need
500mb, I would have expected running -n 2 uses 1GB, but at least there is also the mother
process, which size a bit unpredictable and sometimes exploding.
This "growth" of the mother process happens always at the end of the images
(lets say 90% finished).
Interestingly when restarting the processes the fail never happened again (but
I have to admit I didn't restart the simulation explicitly on the same machine,
since I had a fully automized process, where the failed ones were automatically
restarted on one of the 50 machines we had available.)
Finally we finished all 400(!) renderings with a very good quality.
So this is not an urgent issue, but we wanted to report this. Maybe you have
some rules of thumb to calculate the memory usage when applying the -n option
when the usage of a single process is known?
best
Jan
--
Dr.-Ing. Jan Wienold
--
Dr.-Ing. Jan Wienold
Ecole Polytechnique Fédérale de Lausanne (EPFL)
EPFL ENAC IA LIPID
http://people.epfl.ch/jan.wienold
LE 1 111 (Office)
Phone +41 21 69 30849
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