Hi all,
I've got a program that runs under Xenomai 2.5.3 .... its process contains a
Xenomai real-time thread, a few dozen non-real-time threads (used for disk
I/O), and the main() thread which uses Linux's SCHED_RR at a high priority but
is not, of course, a Xenomai real-time thread.
In general this all works quite well... there is one problem that I am seeing,
though. Occasionally the main() thread needs to spawn a sub-process to handle
certain non-real-time chores (for example, it might spawn an rsync process to
back up some files to a remote machine). Depending on the feature being
exercised, does this via the standard fork()/exec() calling sequence, or via
forkpty(), or in some instances it just calls system().
The problem is that when the program has a lot of RAM allocated (e.g. a
gigabyte or so) and/or a lot of I/O threads present, the spawning of the
sub-process becomes very resource-intensive. For example, in my testing I can
do a system("/bin/sleep 1") and see the computer's RAM usage briefly shoot up
from ~55% to ~95%, while the CPU usage spikes to 100% for a second or two,
before the CPU usage and RAM usage both come back down again. If there is a
bit more RAM allocated by my program, this can even run the computer completely
out of memory, causing my program and/or various other processes to report
errors, crash, or exit.
I suspect that is has something to do with mlockall() and/or some other aspect
of Xenomai partially or fully defeating the fork() call's Copy-On-Write
feature, meaning that when the sub-process is spawned, most or all of the RAM
pages allocated by the parent process are being duplicated, even though they
are all going to be thrown away again immediately because I'm going to call
exec() right afterwards.
My question is, is there some recommended way to launch a sub-process from a
large Xenomai process that will avoid massive amounts of overhead? I suppose I
could launch a separate, persistent, non-Xenomai process that would act as a
"non-Xenomai process server", and connect to it via TCP and ask it to spawn
processes on my behalf.... but that seems like an awful lot of extra work. Is
there an easier way around this problem?
Thanks,
Jeremy
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