Startvation of realtime piority threads

[Konstantin Belousov]

On Wed, Apr 04, 2012 at 06:54:06PM -0700, Sushanth Rai wrote:
> I have a multithreaded user space program that basically runs at realtime priority. Synchronization between threads are done using spinlock. When running this program on a SMP system under heavy memory pressure I see that thread holding the spinlock is starved out of cpu. The cpus are effectively consumed by other threads that are spinning for lock to become available. 
> 
> After instrumenting the kernel a little bit what I found was that under memory pressure, when the user thread holding the spinlock traps into the kernel due to page fault, that thread sleeps until the free pages are available. The thread sleeps PUSER priority (within vm_waitpfault()). When it is ready to run, it is queued at PUSER priority even thought it's base priority is realtime. The other siblings threads that are spinning at realtime priority to acquire the spinlock starves the owner of spinlock. 
> 
> I was wondering if the sleep in vm_waitpfault() should be a MAX(td_user_pri, PUSER) instead of just PUSER. I'm running on 7.2 and it looks like this logic is the same in the trunk.

It just so happen that your program stumbles upon a single sleep point in
the kernel. If for whatever reason the thread in kernel is put off CPU
due to failure to acquire any resource without priority propagation,
you would get the same effect. Only blockable primitives do priority
propagation, that are mutexes and rwlocks, AFAIR. In other words, any
sx/lockmgr/sleep points are vulnerable to the same issue.

Speaking of exactly your problem, did you considered wiring the memory
of your realtime process ? This is a common practice, used e.g. by ntpd.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 196 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-hackers/attachments/20120405/20efe70c/attachment.pgp