[Jack-Devel] Stack preallocation issues

Hi,

I ran into issues with Foo YC20 and the latest and greatest release of JACK
1. The organ synth dies immediately when running it with JACK 0.120.1 in
realtime (I tested both dummy and alsa drivers). It worked fine with
previous releases, so I dug into SVN history and found the critical change
in JACK to be commit r3996. The commit moved the stack touching code from
libjack/client.c to libjack/thread.c. Stack touching means that each JACK
thread reads & writes to nearly all stack memory. This forces the operating
system to map the stack memory and thus the application can use the memory
without page faults. This code has been used regardless of whether the
thread is realtime or not.

In r3996, not only was the code moved, but it was also uncommented. I do not
know why the code was not active before this release. The current
implementation touches 50000 bytes of memory (of 524288 bytes set as the
thread stack size), but leaves all of that memory allocated!

The DSP in Foo YC20 is Faust, which uses a lot of stack variables. The
remaining 24k memory just isn't enough and it dies due to a page fault /
running out of stack.

I've attached a proposed patch. Note that my tests showed that it was not
enough to have the buf variable go out of scope. The stack allocation had to
be put in a separate function for gcc (4.4.3) to release it before
continuing to the actual code the thread is supposed to run.


 Sampo

* Attachment: 'jack-stack-touch.patch'
content-type: text/x-patch

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